What are common rafters?

Common rafters are structural timbers that form the bulk of the components in most pitched roof assemblies. In any regular pitched roof, these rafters are identical on both sides of the ridge board, and all the way down the length of the wall plates - being cut from the same pattern. These rafters are installed at regular intervals - typically 400mm centres, along the roof, often in relation to the layout of the roof joists. The rafters come together to form the structure of the roof, which is then felt and battened, and weathered in accordingly.  

For each roof assembly a pitch is required - whether specified on the drawings of a new build or determined from the pitch of an existing roof in the case of an extension. As mentioned, the pitch of a roof describes the angle by which the rafters rise upwards away from a level plane. The pitch of the roof in degrees determines the angle of the plumb cut at the top of the rafter. For example, a 35 degree pitch requires a 55 degree plumb cut at the top of the rafter in order to meet the ridge correctly, relative to the long edge of the timber. As previously mentioned, this angle can be cut at the pitch of the rafter on a mitre saw, as the pitch is the correct angle of the plumb cut relative to the perpendicular blade in its neutral position. The plumb cut is known as such because the cut is perfectly plumb when installed at the relevant angle. This plumb cut angle is also the same for the fascia line, as well as the back cut of the birdsmouth.  

The birdsmouth is the name given to the notch that is cut into the underside of a rafter where the rafter meets the wall plate. This notch allows for the rafter to perfectly receive over the wall plate, giving the rafter a solid bearing and a good fixing on the wall plate. The portion of the cut that sits on the wall plate is known as the “seat cut”. The seat cut presents itself at 90 degrees to the plumb cut of the rafter. The specific angle of the seat cut is 90, less the amount of the plumb cut - based on the simple principles of a right-angled triangle. In order to maintain the structural integrity of the rafter, the material removed to create the birdsmouth should not exceed more than ⅓ of the rafters’ width, with ⅔ of the width remaining as the backing. The measurement of the backing comes into play later when creating hips and valleys. The exact position of the birdsmouth in relation to the plumb cut can be calculated in a number of ways. With the principles of a common rafter explained, we’ll now look specifically at how they are set out.  

Creating the pattern

When pitching a roof, provided that the plates are correctly installed, the next step is to layout and cut an initial rafter in relation to the given dimensions and measurements of the roof. This first rafter is used as a pattern. This pattern is used to mark out all of the other rafters - ensuring consistency in the roof assembly. It is from this pattern that the success of the roof is determined, and so it's imperative that the time is taken to correctly set it out. Here we will look at the different methods that can be used to set out a common rafter pattern.

Calculating the length of the rafter

The most traditional method of calculating the length of the common rafter in relation to the plumb cut and birdsmouth cut is through the use of the rafter table. The rafter table is a small collection of calculated numbers that relate to the length of a rafter. This table can be commonly found engraved on framing squares, as well as in most roofing books, and can be purchased in both metric and imperial varieties.

The diagram shows the common imperial framing square. The table on this square is displayed in feet and inches and is designed for use with the US style ratio pitches that we’ve already looked at. We can see in the top row of the table an assortment of the decimalised fractions that are relevant to the lengths of common rafters. Each column of numbers is located directly underneath a number on the blade of the square. The number at the head of the column relates to the rise in inches of the rafters pitch, in accordance with the rise:run ratio. The number in the first row under the rise is the length of a rafter per foot of run. For example, for a 12:12 pitch (45 degrees), the length of a rafter per foot of run is 16.97”. In order to use this table to find the length of a rafter, we simply take the run of the roof in feet (decimalised), and multiply it by the decimalised number of inches in relation to the pitch. Again, for a 12:12(45 degree) pitch roof, the length of the rafter per foot of run is 16.97. On our roof, the run is 5’6”, or 5.5 feet. We multiply the run in feet by the length of rafter per foot of run to return the true length of the rafter. In this instance, that would be - 5.5’x16.97”=93.335”. Using a decimal calculator we can transform this final decimalised number into a fraction so that we can use a tape measure to mark it out. 93.335” as a fraction is 93 5/16”, or 7’ 9 5/16”. By using this calculation, we now have a true rafter length of 7’ 9 5/16”. The true rafter length is the distance from the back of the plate to the very apex of the roof. Seeing as most roof assemblies feature a central ridge board, we must account for this to determine the actual length of the rafter when we set the rafter out.  

A metric rafter table is typically associated with degrees of pitch, allowing for more variety in the calculations we can perform in this method. The metric rafter table is most often found in the “roofing ready reckoner”, a published book that focuses on the metric mathematics behind pitched roofs. The premise of the table is the same as the framing square table. Every page of the book is dedicated to a different angle of pitch in degrees, ranging from 5 to 75 degrees. On each page, a length of rafter per metre of run is specified in relation to the pitch. This number is used in exactly the same way as the imperial framing square table. For example, a 45 degree pitch roof features a rafter length of 1.142m per metre of run. Our roof features a 3.6m run. We simply multiply the run by the length of rafter per metre of run to return the true length of the rafter. For example - 3.6m x 1.142m = 4.111m.  

A variation of the metric rafter table is engraved on a patented metric framing square. This square is expensive relative to regular framing squares but is more versatile for use with degrees of pitch and metric operations. 

All of these formulas are present within a range of construction calculators that are available on the market, or through web/app-based programs. The raw mathematical formulas are phrased in construction terminology and can be used to easily calculate the lengths, run, rise, and pitch of common roofs given basic measurements. At their core these formulas utilise basic trigonometry and Pythagorean mathematics to return the values of right angled triangles. Understanding these formulas can be a little more complex than using a rafter table but is ultimately faster given practice.

The most practical of these formulas is H = run/cos(angle), where H represents the true rafter length, and the angle is the pitch of the roof in degrees. Cos, or cosine is a trigonomic function that can be employed to help us find the length of the rafter. The exact mathematics behind the function is irrelevant in this context, instead we must focus on understanding how to phrase the formula in construction terms. The diagram shows a roof with a 5 metre span, at a pitch of 25 degrees. The run of the roof at half the span is 2.5m. If we insert these values into the formula, it will appear as 2.5/cos(25). If we put this formula into a calculator exactly as written, making sure to use the cosine function, the calculator will return a value of 2.758m. This value is the true length of the rafter, from the back of the plate to the centre of the ridge and apex of the roof.   

If we know both the run and the rise exactly, we can use the Pythagorean formula “”. This is just a rearrangement of a² + b² = c². C, representing the hypotenuse of the triangle (True rafter length), can therefore be determined provided that we know the run and the rise of the roof. From here all we do is subtract half the thickness of the ridge. This is a less common approach though.  

Another formula, rise/sin(angle), also returns the same value as run/cos(angle),so we could theoretically calculate the same length based on the rise and the angle, though this is a far less likely scenario. 

Finding the angle of the plumb cut

The next step in setting out the pattern is finding the angle of the plumb cut. Of course, if we know the pitch of the roof, we know the angle of the plumb cut, but how do we physically mark this angle on the timber? Here is a selection of different methods for establishing the plumb cut.  

If the new roof is tying into an existing roof, then the pitch of the existing roof can be found through the use of an angle finder on the existing rafters. A plumb line can be struck anywhere on the side of a rafter, and a bevel set to this angle to establish the plumb cut of the existing and new rafters.  

A speed square can be used in conjunction with its pivot function to accurately mark a specific angle in degrees on the rafter. A framing square can also be used in a similar function.  

A mitre saw can be set to the specific angle of the desired pitch, and the plumb cut can be physically cut on the top of the rafter. A bevel can then be set to this angle to aid in the rest of the setting out process.  

Regardless of the method of finding the bevel angle, the plumb cut should be made on the top of the timber, with the timber being crowned away from oneself before the marking out begins.  

Marking out the main rafter

Once we’ve determined the true length of the rafter and are able to mark the plumb cut on the timber, we’re ready to mark out the rafter. The easiest method in modern construction is to simply measure the length of the rafter using a tape measure. With the plumb cut at the top, the tape can be hooked over the long edge of the bevel and pulled to the correct length down the timber. A small mark should be made here on the very top edge of the timber. From this point, the plumb cut angle can be made again, to represent the back edge of the wall plate. If two rafters of this length were pitched off of the wall plates, they would meet perfectly at the apex with no room to allow for the ridge board between them. As such, we must subtract half the thickness of the ridge board from each rafter either side to allow room for it to fit between them. To do this, we can measure backwards at 90 degrees from our plumb line, half the thickness of the ridge board. This new position represents the actual back of the wall plate in relation to the plumb cut, and the actual length of the rafter.  

The next step is to mark out the birdsmouth notch. This notch consists of a seat cut at 90 degrees to the plumb cut that sits on top of the wall plate, and a small plumb cut that registers against the back of the wall plate. The notch itself is ⅓ of the total width of the rafter, with ⅔ of the material remaining to provide stability and strength. We must divide the width of the board into thirds to mark this notch. Seeing as none of the dimensional timber used in modern construction is immediately divisible by three, the easiest method is to skew a tape measure across the face of the board until a number easily divisible by three lands across the top edge. A mark can then be made at ⅓ of the width of the board. The point at which this distance meets the plumb line is the very corner of the birdsmouth notch. From here, all we need to do is simply square a line away from the plumb line. This successfully marks the birdsmouth notch.

Stepping off is another method that can be used to accurately mark out the length of the rafter, without the necessity for the length of the rafter to have been calculated. This method utilises the framing square, and does not require the use of a tape measure. As such, this method was commonly used in the past in the UK and is still commonly used in the US. Before the modern implementation of tape measures, most framing operations were carried out with the use of a framing square. Once the angle of the plumb cut has been established through one of the many methods, the framing square can be set to this angle by using stair gauges, or simply pinching it in place by hand. Making sure to stick to either the inside or outside measurements, we begin at the top of the rafter. With the square in the correct orientation in relation to the plumb cut, the measurements displayed on the blade of the square represent the horizontal plane of the roof. This is the same plane in which the run/half span of the roof exists. By using the square to measure the distance of the run on the rafter, at the correct pitch of the roof, we can measure and mark the length of the rafter without any calculations. In this instance, the diagram shows the use of the outside of the scale.  

Starting at the top of the timber, with the square aligned with the plumb cut, or set to the appropriate pitch/degree angle (depending on how exactly we’re using the square) we can see that the tongue is oriented so that it is plumb when the rafter is pitched. When we follow the measurements along the blade of the square in this orientation, we are travelling horizontally in relation to the pitch of the rafter. With the run of the roof in mind, we can begin stepping out. This method works for both imperial and metric measurements.  

The span of this roof is 16’ 10”, with a run of 8’ 5”, and so we must travel that far horizontally along the rafter in increments that our square will allow for. The square is set to the 6:12 pitch, and so every full step that we make represents a foot of horizontal travel. The mark must be precise to avoid inaccuracy during the process, with the plumb line of the next step starting exactly where the seat line of the previous step ends. Seeing as the run is 8’ 5”, we need to make 8 full steps, and an allowance for the extra 5”.  If the run is not a perfect division of the increments, the last mark simply meets the requirement for the length of the run. This last mark, regardless of the length of the run, is the position for the back of the birdsmouth at the true rafter length. We must then simply account for the half-thickness of the ridge in the same manner as before.  

Marking out the fascia and soffit line

Marking out the fascia and soffit line on the pattern can aid in the assembly of the roof and is relatively straightforward. The easiest method is to continue stepping out with the rafter square from the back of the birdsmouth notch. When we look at the through section of the wall plate assembly, we know the distance from the back of the wall plate(back of birdsmouth) to the outside face of the external masonry (typically 200mm for a 100mm cavity). From the drawings or through reasonable decision we can also determine the distance of the overhang for the eaves, as the rafter tails project past the external masonry. In this instance, we want the overhang to accommodate a 150mm flat soffit board. Seeing as we know all of the horizontal distances - in the same way that we know the horizontal run - we can step out for the soffit and fascia line. Aligning the square with the back of the birdsmouth, we can measure out the distance of travel. 100mm for the cavity, plus 100mm for the external masonry, plus 150mm for plastic soffit, gives us a distance of 350mm. Travelling along the rafter horizontally, we can make a mark at this point. The very edge of the soffit will be pinned to the outside edge of the timber fascia back board, and so we must deduct this thickness to accurately mark the position for the fascia line. This new mark is the position at which the rafter tails will be cut off plumb. That being said it’s usually more accurate to pull a line across the installed rafters and make the plumb cuts in situ, as this ensures the fascia backer will be installed to a straight line. However, we must find this point during the stepping out phase to determine a height for the seat cut, which is certainly easier to cut on the pattern. Trying to cut a perfectly level seat cut on the rafter tails after installation is awkward and difficult.  

With the rafter tail plumb cut marked we can decide upon the width of the fascia to be installed. A 150mm fascia is appropriate in most instances, and so we will work with that. The backer board typically requires 1-2” of upstand above the top of the rafter to allow for the kick of the last row of roof tiles. With the tile vent installed on top of this, the last row is given adequate kick. Assuming an upstand of 1 ½ inches, a standard 6” backer board will sit with 4 ½” against the plumb cut of the rafter. Therefore, we can measure down from the top edge of the timber 4 ½”, and strike a seat cut at this point. This seat cut can be cut, as well as the birdsmouth, to create the pattern. Small battens can be nailed to the top of the pattern, overhanging the edges slightly, so that it can be hooked over other timbers and the marks can be accurately transferred.  

Fascia kick - slows water slightly to prevent it over-shooting the gutter, also keeps bottom rows of tiles at the eaves tight together with slight upward pressure, reducing water ingress. 

Up to you how you want to do it. You can calculate all of this on the ground and cut the soffit and fascia line on the ground as part of the pattern. This reduces the amount of work on the roof, but is going to lead to the least precise outcome. Any deviation in the shape of the rafters and placement of the wall plate is going to show once the fascia is installed. The most accurate, but also most awkward process is to cut both the soffit and fascia line to a string line after the rafters are installed. This allows for a very precise outcome, but cutting the soffit line after the rafters are installed can be pretty awkward. A comfortable compromise is to cut the seat cut on the pattern, and leave the plumb cuts until the rafters are installed. This way we can cut the fascia line to a string line, enabling a perfectly straight result, without struggling to cut the seat cut. The seat cuts cut from the pattern are unlikely to deviate more than 5mm in total provided that decent timbers were used and an accurate pattern was made. This is perfectly acceptable here, as no one is ever at an eye level height to look down the soffit line. The far more critical aspect, and the thing that can be seen from the ground, is the straightness of the fascia line.  

How are common rafters installed?

Once the pattern common rafter is marked out, more common rafters are required to pitch the initial frame of the roof. By cutting two more rafters off of the pattern, we can check the fit of the rafters on the roof. The pair can be pitched off of the plates with a small offcut of ridge timber between them to ensure that the plumb cut angle is correct, and that the birdsmouth is sitting tightly around the corner of the wall plates. Checking that the pattern is correct before cutting all of the rafters is very important. Having set the pattern out, we could cut all the rafters required - a costly mistake if the pattern is not right. Sometimes a small adjustment is required to the rafters to ensure that they fit correctly, especially in scenarios where we are tying into an existing roof. Regardless of how accurate our calculations are, a natural bow in the timber, as well as a misshapen existing roof can cause the pattern to be slightly incorrect. Once we’re happy that the pattern is correct, more rafters can be cut.  

Quick aside - if the rafters are too short, theyll have to be marked out again and recut - we can’t lengthen the short boards- though they’ll come in somewhere else so it’s not the end of the world. However, if the rafters are too long, dont just trim bits off until they fit. Youll probably get there eventually, but its not good practice. Instead, recalculate the length of the rafter in the desired method, and mark it out once again, paying careful attention to accuracy. This will be more accurate than guessing how much to cut off, and will also serve as a learning opportunity to see where we went wrong. It’s possible that the calculation that we made was incorrect, though more often than not, if the rafters are only slightly too long, then we were not accurate enough with our layout.  

The pattern is left at the cutting station to mark out the other common rafters until the very end. More rafters are cut as required to assemble the roof. The first step in actually pitching the roof is to establish the ridge board in combination with a few pairs of rafters to form the frame of the roof. Typically, a pair of rafters are pitched at each end of the length of the ridge board on the wall plates, in order to support the ridge board before the installation of the rest of the common rafters. This stage is far more easily achieved with 2-3 operatives, as there are too many components to keep track of for one person. Each rafter can be temporarily fixed with a coach screw through the back of the birdsmouth and into the wall plate to hold it in place. Each pair will rest against each other at the plumb cut intersection, though the angle will look slightly incorrect at this point due to the lack of ridge thickness between them. In order to prevent the pairs from tipping over before the roof is correctly braced, temporary braces can be installed on the rafters to a fixed point. Ordinarily, this will be a batten fixed to the midspan of the rafters, fixed either to a static point within the roof such as the fixed joists, or onto the static scaffolding. With the pairs in place and braced, the ridge board can be installed. The layout of the rafters can be marked on the ridge board off of the wall plates before the ridge is installed, to display where the top of the rafter should fall.   

The easiest method of installing the ridge board is to lift the ridge timber upwards from below the rafters, straight up between the plumb cuts. As the timber is lifted upwards, the rafters spread apart and the ridge timber slides between the plumb cuts. The downwards forces acting upon the top of the rafters pinch the ridge timber between them, preventing it from falling out whilst adjustments are made. Temporary coach screws can be used to hold the ridge securely in place. If the ridge needs to be moved downwards, one of the rafters can be lifted slightly and the ridge will come free. For a traditional roof, the desired position of the ridge board sees the top of the rafter plumb cuts kiss the top edge of the ridge timber. With the ridge timber in place, the spacings at the top of the rafters can be readjusted so that each rafter is plumbed up from the initial marking on the wall plates. Once the rafters fall on their layout and are plumbed up, the ridge can be braced off a fixed point to prevent any further movement.  

Once the ridge board is in place, the rest of the common rafters can be fitted against it. For long rafters, two operatives may be required to pitch the rafters safely, as they are long and heavy. For shorter rafters, once operative can pitch the rafters whilst standing at the plate.  

The rafter can be thrusted over the top of the ridge and pulled slowly backwards until the birdsmouth drops over the corner of the wall plate, and the plumb cut hits the side of the ridge board. As the rafter drops down, it may move backwards further than desired. By applying downwards force to the rafter tail beyond the wall plate, the top of the rafter can be easily manipulated through the use of leverage. As the plumb cut rises away from the ridge board, the rafter can be shunted forwards until the birdsmouth is tight around the wall plate. Steady pressure on the top of the rafter into the corner of the wallplate can be maintained until a fixing is installed through the rafter and into the plate to keep it tight. Once the rafter is fixed in place at the plate, the top can be moved around to accommodate the on centre layout of the roof. The top of the rafters are traditionally fixed into the ridge board with 3” nails - 1 through the top of the rafter into the ridgeboard perpendicular to the top edge of the rafter, and 1 through each cheek of the plumb cut into the ridge board to prevent the rafter from twisting over time. The bottom of the rafter is fixed to the wall plates with 4” nails, skewed through the rafter into the plate behind the birdsmouth. Fixings shouldnt be placed in the short grain before the birdsmouth, as the timber can split out due to the interrupted strength of the grain. Where rafters sit directly next to joists in a common roof assembly, 4” nail are also hammered through the side face of the rafter and into the tail of the joist to prevent the rafter and joist from twisting over time.  

Once a good portion of the common rafters are installed, diagonal bracing can be placed on the underside of the rafters so that the temporary braces can be removed out of the way. This diagonal bracing may also be temporary and removed at a later date depending on the specific requirements of the roof but can be installed to more effectively brace the roof against the increased forces acting against it. Diagonal bracing is installed as close to 45 degrees as possible across the underside plane of the common rafters, from the wall plate to the ridge board. Typically, wider 4”x1” timber is used, though common batten is acceptable. Fixings such as structural screws or tacked nails are installed through the bracing on each rafter to effectively resist the forces of the roof. As more weight is added to the roof, downwards forces of the roof are going to want to push the rafters and ridge outwards in both directions. The joists that bind the plates together resist the forces in that direction. The diagonal bracing is installed to resist the racking forces of the roof and to prevent it from falling over.