Carpentry & Joinery → Structural Carcassing → Roofs → Flat Roofs

Warm Deck Flat Roofs

When assembling the structure of a flat deck, the main prerequisite is a set of wall plates that have already been bedded to the masonry of the walls that the roof will sit on.

Plates and layout

The first step is to parallel the plates, though this is purely out of good practice, and won't actually affect the finished roof assembly as it would if the plates were out of parallel for a pitched roof. With the plates paralleled, they can be pinned in place with a few nails into the muck bed. Before the plate straps can be installed to hold the plates securely in place, the layout needs to be carried out so that the straps don't fall on the location of a joist. As with any roof, the layout will begin from one end of the structure. The layout of the joists needs to suit a 16” OC layout to accommodate the first layer of 8’ sheathing. When it comes to installing the plywood, the short edge of the first sheet will line up with the outside edge of the rim joist, and so the first step of the layout is to determine this location. Due to this, the layout of the joists is determined by the position of the outside rim joist.

When marrying into an existing fascia line during renovation works, a measurement for the width of the existing soffit needs to be determined. If this is not the case, consult the drawings for a soffit width. In both scenarios, this width is the distance that the structure of the flat roof will overhang the external skin by to accommodate the desired soffit width. In order to prevent any layout mistakes at this point, cutting a batten that is 8’ long is beneficial. On this batten, the soffit width can be marked in from one end. By placing this batten along the wall plate, with the marked end overhanging the external masonry, the position of the 8’ on centre joist can be found. Using a straightedge or spirit level placed against the external masonry, the soffit width mark on the batten can be lined up directly in line with the outside of the external skin. The location of this batten now displays the outside edge of the rim joist, as well as the centre of the joist that the other end of the plywood sheathing will land on. This location on the plate is important, as without a solid registration mark at the end of the plates, this mark will serve as the basis for the entire on centre layout. By measuring to one side of the 8’ centre point half the thickness of the joists, a starting point for the correct layout is determined.

The rest of the 16” OC layout can be pulled from this initial 8’ mark, with the location of each joist being denoted on the plate with clear markings. If the layout is accurate, each 8' sheet will fall halfway on top of a joist. To jump back to the initial process of finding the 8’ mark for the layout, this step is required due to the range of different specific roof styles. Some flat roofs feature an overhanging soffit, whilst others feature no soffit and a fascia capping that sits against the external masonry. Due to this factor, the position of the first joist is not set in stone, unlike a common 50mm inset for a joist within a pitched roof assembly. Without a set beginning point, the correct on centre layout is difficult without following that process we’ve looked at. In a scenario where there is a soffit overhang, as shown in the diagram, the joists do not suit the installation of full plasterboard sheets. Already there is a discrepancy in the layout in that imperial sheets of plywood and metric sheets of plasterboard are not the same size. The layout of the joists could be installed to suit the plasterboard, but this creates far more work in cutting the plywood to suit the irregular layout on top. Therefore, it's preferable to layout the joists for the plywood and cut the plasterboard underneath accordingly.

It should be noted here that the specific attention to detail in regard to supporting the installation of standard plywood sheets is only applicable if the furrings are installed parallel on top of each joist. In this instance the furrings follow the layout of the joists and must suit the layout for plywood. Alternatively, depending on the criteria of the roof and desired direction of flow for water, the furrings may be installed perpendicular to the run of the joists. In this instance, the joists can be installed to accommodate the installation of the underside plasterboard, with the actual on centre layout for the plywood being met with the layout of the furrings.

Transferring the layout and fitting straps

With the layout of one plate completed, the familiar batten trick can be employed to accurately carry the layout of the roof onto the other side of the building. Care should be taken to ensure the correct alignment of this layout batten with the same location on both sides of the roof. The outside of the exterior masonry skin is a good registration point for this layout. At this point, if the[5] re is a halfway beam or steel that splits the span of the flat roof, the layout should be accurately transferred onto this as well. With the layout complete, the plate straps can be installed. The plate straps should be installed approximately every 1.8-2m, making sure to fix them to the plate out of the way of the layout marks.

Cutting and installing the joists

If there is steel beam present in the roof structure, a pattern for the notch of the joists should be acquired as per the methods we’ve previously looked at. If the central beam is made of timber, joist hangers should be installed at this point on the layout marks to allow for easier installation of the joists. For smaller single span flat roofs, whole lengths of timber can be installed as joists and cut to length after installation. In other scenarios where the joists are supported by another structural member, the timber can also be left long and cut off after the fact to a line.

In some warm roof situations where the joists are particularly deep, the total depth of the roof assembly from the bottom of the joists to the top of the ply can cause the required fascia capping to be far too large. As such, we can reduce the width of the joists after they pass the wall plate, reducing the overall on show depth of the roof. This reduced width past the place is supported by the external masonry skin being built up higher than the internal skin. If we are reducing the depth of the joists, they should be cut on the bench marked from a pattern to maintain consistency.

With the joists cut and ready, the installation can begin. The first joist at each end and side of the roof that receives the soffit ladder should be screwed in location in case adjustments need to be made at a later date. The other standard joists can be fitted sequentially on the layout marks and nailed in place with a 4” nail skewed through each side into the plate.

If the joists are already cut to length prior to installation, then care should be taken to ensure that the ends of the joists as they project past the external masonry are consistent to maintain an even soffit line. If not, then they can be cut to length to a string line after installation. If a rim joist is present in the assembly, then the thickness of this timber should be accounted for when cutting the joints to length in either scenario.

Cutting off the fascia line and installing the rim joists

With the primary body of the joists installed, the ends of the joists can be trimmed if required, and the rim joists can be installed to form the soffit and fascia line of the structure. A straightedge or level can be used to pull a line vertically up from the outside of the external masonry, up the side of the end joists within the assembly. The total width of the soffit overhang can be measured off of this line at each end of the roof, with a deduction for the thickness of the rim joist if required. Transfer these lines to the top of the joists, with a chalk line being pulled between the two ends. A square or level can be used to transfer this line onto the side of each joist to provide a good registration mark to cut to. Using a circular saw or handsaw, cut these joist tails off in a neat line. They should now be long enough that when the rim joist is pinned on, the overall desired soffit width is achieved.

With the joists cut to length, the rim joists can be installed if required. These are just standard lengths of timber, nailed to the ends of the joists with 4” nails, flushed to the bottom of the timbers in order to allow for ease of installation of the soffit line. Once installed, these timbers provide stability to the ends of the joists - maintaining the correct on centre layout and preventing the timber from warping/distorting over time. The timbers can be cut half on the ends of joists or left overhanging and then blocked behind to prevent waste. These can be half lapped, though they don't have to be. The end of the rim joist at each end of the roof can be left long to provide a physical stop for the ladder to be installed to. The excess will then be cut off after the fact. To fix the rim joists to the ends of the joists, temporary screws can be installed to hold the component in the correct alignment, with 4” nails being used to more securely fasten the pieces together. It's important to ensure that the ends of the joists are pulled upright in relation to the wall plate before the rim joist is installed.

Assembling and fitting the ladder

With the primary joist system installed, the next step is to assemble the soffit ladder. If the length of this level ladder is made from 1 length of timber, it can be placed on the end plate of the wall and moved laterally into place in relation to the length of the other joists. Consistent 16” layout marks can then be made for the blocking of the ladder, additionally ensuring that a timber falls in the correct place at each corner of the roof to act as a plasterboard pickup. For larger roof structures that feature a ladder that is made from 2 or more lengths of timber timbers, half lap joints should be cut on the ends of the timbers relative to the requirement of the ladder. This will ensure positive support at the location of the joints and provides an overlapping surface to install fixings once the ladder is situated. With the appropriate joints cut, the timbers can be laid on the plates in the correct orientation and marked appropriately.

With the ladder timber marked out, the required length of blocking for the ladder can be calculated. This is achieved in the same way as calculating a regular pitched roof barge ladder rung. By holding a straight edge down the external masonry as before, a measurement from the outside edge of the first joist to the outside edge of the external skin can be taken. The distance of the overhang from the external masonry to the outside face of the ladder is then added to this measurement, with the thickness of the ladder timber being deducted. A test timber at this length can be held against the last joist to ensure the correct measurement has been calculated. The total number of blocking timbers can be counted from the layout marks of the ladder timber and cut with a chop saw or skill saw.

Using a nail gun the blocking pieces can be pinned to the ladder joist on the correct layout markings. This process is in accordance with the standard procedure for assembling timber ladders. Once all the pieces are attached, 4” nails should be installed through the face of the ladder timber to more securely attach the pieces. Once the ladder is assembled, they are ready to be attached to the structure of the roof. These ladders are often awkward to hold and install, and so temporary support battens can be used to support the weight of the ladder whilst it is installed.

Place the ladder on the outside plate with the ends of the blocking butted tight into the first joist. The lateral position of the ladder can be established by sliding it along the plate, either lined up with the ends of the installed joists or butted into the extended rim joist. Using temporary screws, a few of the blocking pieces can be fastened to the first joist to stabilise the assembly. Checks should be made to ensure that the ladder is in the correct position before permanent fixings are installed into the component. A square can be placed on the inside of the ladder to ensure that each blocking piece is perpendicular between the ladder timber and the first joist. The blocking pieces can be fixed to the first joist using 4” nails through the face of the joist. Where two half lap joints meet each other, coach screws can be used to pull the timber tight together, with nails being used to more permanently fasten the timber together.

A solid piece of blocking can be cut to width behind t he joint and nailed securely in place through the other pieces of blocking and through the face of the joint in order to allow for maximum support and stability in the joint. Using a level and a tape, check that the soffit line is a consistent measurement from the outside of the ladder to the outside of the exterior masonry, as well as ensuring that it is straight. Any adjustments can be made now by unscrewing and moving the last joist that we temporarily pinned. The middles of the ladders can also be pushed or pulled in or out and pinned with a temporary batten to straighten them.

Installing the solid blocking and straps

At this stage of the flat roof assembly process, the majority of the superstructure is assembled. Before the furrings and sheathing can be installed, solid blocking must be placed between the joists to straighten them out, lock them in place, and provide stability to the assembly. In other structures such as pitched roofs, installation of the blocking is not imperative before the main structure can be built, as the joists are still free to move and twist after the fact. This means that the blocking can be installed after the fact from the inside of the structure. In the case of a flat roof, once the sheathing is installed on top of the joists, they will no longer be able to move. Particularly bowed or twisted timbers can cause the ends of sheets of ply to not land half on a joist. As such, we must do our best to straighten all of the joists in accordance with the initial layout before the plywood is installed.

In order to make the installation of the solid blocking easier for ourselves, the joists can be straightened to some extent beforehand by fixing layout battens along the top of them at the centre of the span and at the ends. For a standard flat roof, the initial layout transference rod can be used, being pinned across the centre of the span of the joists. Each joist may need to be pushed or pulled onto the layout marks on the batten, due to either bow or twist in the length. Laying planks across the joists can make travelling across them much safer. With the ends of all the joists fixed to the plate and pinned together, the joists are unable to roll or fall over when walking on them, making the pinning process and installation of the rim joists crucial before attempting to travel out onto the joists. For flat roof assemblies that feature multiple spans of joists, battens should be tacked at the middle point of each run. This will help to upright the ends of joists as they receive into steels, making it easier to install blocking in the steels. Battens may also be required where joists receive into steels.

At this point, the blocking is ready to be cut and installed. This process is in accordance with the standard process for installing solid blocking that we’ve already looked at. From below, using benches and planks, make a halfway mark on the joists at each end of the run of joists, and ping a line between the two. This denotes the run of the blocking. Either scribing with offcuts or taking measurements from the top of the joists in between them, mark out and cut the blocking. Installing the blocking in a staggered fashion, using a nail gun, hammer, and 4” nails to bring the joists together and upright. Twisted joists may need to be screwed with long structural screws to pull them tight in the row of blocking. If blocking is required in the steel, use the same method to install it flush to the front of the steel. Once the rows of blocking are installed, the battens that are directly above them can be removed. If the roof features a steel or beam in the middle of its assembly, we can now install the strapping. Using tin snips, cut lengths of strap banding that are long enough to pass over the structural member, with 200-300mm of banding on the ends of each joist. Use angled 3” nails to pull the banding tight over the top and the bottom of the joists, and over the structural member. This will ensure that the joists are never able to pull out of the steel or structural member.

Installing the furrings

At this stage the main structure of the flat roof is assembled, and the furrings are ready to be installed on top of the joists. The first step is to determine the direction of fall, and how the furrings need to be laid out. If the roof is to slope in both directions sideways into a plastic gutter, then the furrings need to taper from the thick end to the thin end, starting with the thick ends of each side meeting in the middle of the roof. To do this, ping a line through the centre of the roof and line all the furrings for one side up with the line. Go along the roof and nail the furrings onto the joists with a nail gun, lining each one up as you go. Fully nail up each furring on the joist before moving to the next, to avoid missing any nails. This would be unsatisfactory for the quality of the work, but also dangerous to walk on. Once one side is fully nailed up, start with the other side, butting the thick end of the furring up to the fixed thick end of the other side.

For roofs that fall in one direction from one side to the other, simply line the thick end of the furring up with the highest desired edge of the roof, and nail the furrings on as before. After they are nailed on in both scenarios, cut off any excess thin ends of the furring with a hand saw.

For roofs that slope over a longer distance than the furring is long, custom furring extensions may need to be cut. For these scenarios, we can use the full length of the premade furring and cut the remainder out of treated material on site. Fix the thinnest end of the furring first at the desired termination point. Fully nail up the furrings before cutting the remaining short pieces. The easiest way to mark these furrings out is to measure the length of the custom piece that needs to be cut. Mark this length on a furring from the end to the desired length. Measure the difference in thickness over this length and take the measurement to the bench. The thinnest end of the custom furring will be the same thickness as the thickest end of the premade furring. Mark this on a timber and then mark the desired length of the furring. Add the determined difference in thickness to this measurement to mark the thickest end of the custom piece. With one cut, a template or jig can be made, as we’ve previously discussed, in order to cut the remaining furrings.

When installing the furrings in the same direction as the length of the joists, the furrings can be fixed directly above each joist. When a scenario occurs when the furrings are installed perpendicular to the length of the joists, they should be set out to accommodate the ply. In this situation, the underlying layout of the joists does not necessarily need to be accommodating for the ply, as the above furrings can be applied to allow for the easy and correct installation of the ply. In these situations, the thinner ends of the furrings may be required to have solid blocking installed underneath them between the joists to ensure that they remain supported.

Fixing the first layer of sheathing ply

With the furrings fixed in location the installation of the first layer of sheathing can be installed. For cold deck flat roofs, this layer of ply will be the final layers of ply, as the insulation will be installed under the ply. For warm deck roofs, this layer of ply will form the foundation for the rigid insulation to sit on, with another layer of ply typically being installed on top of the insulation.

Depending on the direction of fall of the furrings and the type of insulation being used on the deck, the ply can be started in a number of places. If the furrings fall from one side to the other, the ply can begin to be installed from one edge. If the furrings fall in both directions from the centre point, this bottom layer of plywood should be installed with a join in the boards where the thick ends of the furrings meet, as at this stage the plywood wont effectively bend over the sharp change in angles. The first sheet regardless of location can be placed on the deck in its entirety, with the short edge lining up with the outside of the rim joist horizontal to the run of the furrings. If the initial layout was correct, the other end of this sheet will land halfway on a joist. Line this ply up with the edges of the rim joists, and pin it in place with a few nail gun nails. Hold off on fully fixing the sheet at this point, as we may need to remove the nails and move the sheet into another alignment. Take another sheet and cut it directly in half - leaving a 4’x4’ piece of ply. This sheet will start the next row of ply, with the factory edge of the plywood also landing halfway on a joist.

From here, a few more full sheets can be laid out in relation to these initial sheets to begin to form the deck of the roof. Tap the sheets into alignment with one another, making sure that the seams in the boards are nice and tight. Once we’re certain that the sheets are falling correctly, and aligned perfectly square with the roof structure, each sheet can be nailed up fully using a nail gun. Using a level or a string line, or by eye if you feel confident, place nails at regular intervals in the ply along the lines of the furrings and joists. If the furrings run on top of the joists the nails can be placed anywhere along the run. If the furrings run perpendicular to the run of the joists, then care needs to be taken to mark out the intersection of the joists and furrings to ensure the fixing enters the joist, especially at the thinner end of the furrings. Long screws may be required in this scenario due to the fewer quantity of nails that can be fired into the sheets. Continue to cut and fit the play along the deck, making sure to alternate the sheets and stagger the joints. When we reach the other end of the run of the roof, the sheets can be fired on as whole sheets, and then cut off to a line after the fact.

Cold deck finishings

At this point, if the flat roof is a cold deck, the total structure is almost finished. If the water is to fall directly into a plastic gutter then a drip edge must be installed at the location where the water leaves the roof. A 2”x1” or 2”x2” can be pinned onto the edge where the water falls off the roof to allow an outstanding support for the felt or roof covering to land over above the location of the gutter. Without the drip edge, the water would flow directly over the hard edge of the roof and run down the fascia line behind the guttering. In addition to this, any edges that require a tilt fillet to prevent water from tracking over it must have one installed. This triangular piece of timber can be gunned directly onto the deck, and mitred at the corners if they run around.

Warm deck curb and insulation

If the flat roof construction is that of a warm deck, then the next step is the installation of the retaining curbs and rigid insulation. The curb is a timber component that is fixed vertically in line surrounding the edge of the initial ply deck. The appropriate vertical height of the curb should match the thickness of the rigid insulation. The purpose of the curb is to retain the insulation as well as allowing for the fascia capping to be fixed to it. With the height of the curb being the same measurement as the thickness of the insulation, the top layer of ply will be able to seamlessly sail over the top of this whole layer. As a general trend over the course of my carpentry career, the regulations for the thickness of the rigid insulation get larger every few years. When I began, 100mm of insulation was sufficient, with a 95x47mm(4”x2”) curb being suitable. A few years after I began the regulations changed to call for 120mm of insulation, making a 5”x2” the necessary choice of timber for a curb. As it stands at the time of my writing, the regulations call for 150mm of insulation, with a 6”x2” curb being required for the retention of this foam. Whilst the consistent increase in the size of the insulation is a step in the right direction for the improvement of energy efficient homes, the cost to build these warm deck flat roofs goes up with each change. Additionally, in terms of the actual construction, the required width of fascia capping becomes more as the makeup of the roof increases in height. Additionally, when are they going to stop increasing the required thickness of insulation? The thickness can't keep going up forever. Sooner or later there will be a complete change in the makeup of these roofs, possibly calling for another type of insulating material. That or they'll decide on a thickness to stay at. Anyway, once the required thickness of the curb is decided upon, we can either purchase this size from the merchant or rip it down from existing material that may be on site. The timber that we get from the merchant is never an absolute match to the thickness of the insulation, tending to be a few mm shorter. This causes the edge of the ply to curve slightly and fall a bit more aggressively as it makes the transition from the insulation to the curb. This isn't necessarily a bad thing, but ripping a timber down to the exact size of the insulation can help to remedy this. The curb should be screwed down into the rim joists with long structural screws for maximum stability. Note that as the curb is sat on the plywood that is also sat on the furrings, the curb is going to lean outwards ever so slightly, at the angle of the run of the furrings. Generally speaking this isn't going to cause a problem or even be noticeable when the plastic fascia capping is installed. That being said, if we are cutting custom curbs from larger stock, we can apply a small angle to the bottom of the curb to counteract this tilt and keep the curb perfectly upright and in line with the outside edge of the rim joists. The curb must completely surround the area that is going to contain the insulation, with the joins in the lengths of the curb being nailed together. When the curb is fully installed all the way around and fixed securely, it's imperative that we install a vapour barrier underneath the layer of insulation. This vapour barrier (DPM) must cover the entire surface under the insulation, wrapping up and over the sides of the curb and being taped along any seams. The purpose of this vapour barrier is to stop any potential condensation or trapped water from entering the habitable side of the building, ensuring that any moisture stays above the vapour barrier. With the vapour barrier installed, we can begin to install the insulation. Generally speaking, it's good practice to install the rows of insulation sheets at right angles to the run of the bottom layer of sheathing ply. Doing so helps to keep the top layer of ply nice and flat by keeping the majority of the joins in the plywood away from the joins in the insulation. The insulation should be cut in tight within the curb, making sure there are no gaps in the insulation layer. Once all of the insulation is installed, it's time for the final layer of sheathing ply to be installed.

Installing the drip edge and final layer of ply

With the insulation correctly installed, we can finish the final part of the warm deck flat roof assembly by installing the drip edge and top layer of sheathing. Before we install the layer of sheathing, we must install the drip edge on the sides of the roof that the water will be flowing over. Without a drip edge, the water will flow directly over the side of the roof and down behind the guttering. A drip edge is a batten component that is attached to the roof under the top layer of ply to extend the surface of the deck so that water will fall directly into the gutter. This component can either be a 2x2, 2 2x1, or a singular 2x1 on edge, really depending on how you're feeling in honesty. This component is not structural and simply allows the felt or roof covering to sail further over the gutter. In actuality, with the 18mm layer of ply sheathing on top, an actual drip batten is not completely necessary, provided we overhang the ply by 2 inches for the drip. I still prefer to back this ply overhang up with timber underneath though. This drip batten is gunned on the outside of the curb flush to the top, so that the top ply layer will run over the top of both. Now for the plywood. To install this layer, we can begin by aligning a few sheets in one corner of the roof to give us a point to start from. The rows in this layer will run in the same direction as the bottom layer, and at 90 degrees to the layer of insulation under it. We will also continue to stagger the joints of the plywood in the same fashion as before. This time however, ensuring that the ends of the boards lie above the joists is not imperative. Because of the layout of the joists, the ends of the ply will almost certainly still line up in this fashion, but as we’re not fixing into the joists this time, it doesn't really matter. Around the edge of the roof the sheets of plywood can be nailed directly into the top of the curb, helping to align the first few sheets. The rest of the boards will be fixed through the insulation and into the bottom layer of ply, using appropriately sized screws. These screws are part threaded, PZ3 flat countersunk screws, and need to be purchased in the correct size. Due to the part thread, if the screws are too long they will go all the way through the bottom layer of ply and not grab at all. If they're not long enough, they won't reach the bottom layer of ply and will be stuck in the insulation with nothing to grab onto. The perfect sized screw will end up with the thread in the bottom layer of ply with the head flush or ever so slightly below the surface of the top layer of ply. When screwing the top sheets, countersinking holes into the sheets before inserting the screws helps achieve a much more consistent and proper finish. Trying to wind the screws in so the head is flush with the ply without countersinking them causes either the screw thread to strip out of the bottom layer of ply, or pull the top layer too far down, compressing the insulation and still not going below the surface of the ply. The wide head of the screw combined with the relative surface hardness of the ply requires a countersunk hole. Good practice would see 35 screws per sheet, arranged in a grid 5x7. If working in a pair, one operator should go through ahead of the screwer and pre-drill the holes to allow for easy installation. The screws should be kept in around 2-3 inches from the edges and corner of the ply to prevent them from pulling the edges of the plywood into the insulation too far. An impact driver can be used, though a drill driver with a torque setting on them can help to achieve a better desired finish. The driver will drive the screw down into the countersunk hole, pulling the two layers of ply together and ever so slightly compressing the insulation for a tight fit, clutching out before the head is pulled too far under or the thread in the bottom layer spins out. One final thought, where we place a board in relation to where the thick ends of furrings change direction doesn't matter now that the insulation is down. The insulation smooths the joint, and whilst the board over the top of this will bend, it will be gradual as opposed to abrupt, and so it's okay.

How do flat roofs meet pitched roofs?

In the world of extensions and renovations, flat roofs that marry into pitched roofs are not uncommon. The intersection of these two different structures are not complicated, and so here is an overview of how they come together. In a scenario where the pitched roof is already in place and the flat roof is being built into it, the first step is to remove some rows of tiles at the fascia line of the pitched roof, and remove the fascia and soffit to expose the rafter tails. The tails need to be cut off in order to allow the full installation of the flat roof joists, keeping in mind the plasterboard pickups below. When cutting off the tails, we want to remove enough material so that the pitch of the roof comes down close to the first layer of ply, without removing too much material. We can now install the joists and furrings, as well as the bottom layer of ply. This gives us a solid deck to work off of to carry on the tying in operation. From the top of the bottom deck we can work out the horizontal height at which the top layer of ply will come into the pitched roof. From this point, we need at least 9” of solid lay board running up the rafters for the felt or other roof covering to be adhered to. The actual top of the layboard needs to fall in line with the accommodation of the tiles, coming up to the bottom of the batten that will hold the last row of tiles. This will ensure that the last row of tiles fully covers the top of the felt, keeping the roof waterproof even in extreme weather. The lay board in this scenario then will run from the bottom layer of ply all the way up to this marked out point, supporting the insulation and vapour barrier, and giving support to the roof covering. We can rip the lay board at that width and gun it to the rafters. This rip can be cut square, as the poor fit of the intersection where the bottom of the layboard meets the top of the bottom layer of ply really doesn't matter and will never be seen. Where the layboard meets a hip, the board should be cut to the angle of the hip, making sure the cut is made down the centre of the hip and plumb. The same is true for valleys. The rest of the flat roof assembly can then be put together, with the insulation being cut on the angle where it meets the lay board. The top layer of ply must also have the angle cut on it, to allow for a tidy and seamless transition from the top deck to the lay board. This will improve the quality of the roof covering by eliminating any air pockets in the deck. It also makes it much easier to properly nail the angled edge of the top ply into the angled lay board.

How do flat roofs meet vertical walls?

The other scenario in flat roof extensions sees the flat roof coming into a vertical wall. If the joists run parallel to the wall then no extra steps need to be taken when constructing the deck. If not, then the joists may be notched into a steel that is holding the above wall up, or hanging from a timber that is bolted to the wall underneath. Typically, if the flat roof is part of a large interior room extension, then a steel will be holding up the wall after having knocked out the wall below. In this case, the joists will be notched into the steel as normal. If the extension is more of an outbuilding or shed, then the existing interior will most likely be left as is, possibly being built around existing exterior doors. In this case the roof will most likely hang from a plate bolted onto the walla and be supported from it with hangers. Most of the substructure of the deck in either scenario is going to be relatively straight forward, with details on the drawings for any unusual components. The real thing to consider is the weather situation above where the roof meets the wall. If the extension is an out building built around an existing exterior door, then we can most likely get away with using flash banding or lead flashings to weather the felt into the roof, as the existing cavity still remains. However, if we have an RSJ and the flat roof covers a new internal habitable area, then lead cavity trays must be installed. Any water that makes its way into the cavity is now able to travel down onto the ceiling of the new room, and cause water issues. As such, lead trays need to be installed above the level of the top of the flat roof to adequately remove any water from the cavity before it reaches the lower levels.