Blocking
Timber is a living material that reacts and moves in different temperature and moisture conditions. Through the process of acclimatisation, it's safe to assume that all timber materials will shrink or expand to some degree after their installation. In fact, all materials, construction related or not, will change a small amount in response to the environmental conditions around them. Over time as standard lengths of timber dry out in a warm environment, the natural tensions present in the structure of the wood can lead to movement in the form of cupping, bowing, twisting, and warping. This movement is rather undesirable in installed components, and can lead to damage or a poor quality finish. As such, it’s imperative that we limit the movement of drying timber through the pre-emptive installation of blocking.
What is blocking?
Within a timber joist system, solid blocking and herringbone strutting are two different components that are installed between the joists - acting to brace them laterally and prevent twisting. 1 row of blocking or strutting is typically installed through the central span of a joist system, though more rows may be installed depending on the length of the span and the structural criteria. When installed correctly, the blocking/strutting prevents the joists from moving laterally, as well as twisting along their length. If the blocking is installed tight between each joist, and keyed in at both ends, additional bracing against downwards deflection is provided to the total system.
Solid blocking refers to standard timber for use as blocking. The timber is cut to length between the joists, and registers against the side face of each joist. Ideally, the width of the material used for solid blocking is less than the depth of the joists, to allow the blocking to sit above and below the two edges of the joists. This prevents the blocking from protruding past the depth of the joist in the event that uneven shrinkage occurs. This method of blocking is the most common in modern construction, and is relatively fast and cost effective.
Herringbone strutting is a more traditional method of blocking, providing the same function of modern solid blocking. Whilst more aesthetic and less material intensive, the additional time required for the installation of herringbone strutting renders it a more expensive method of blocking. 1 set of strutting between 2 joists consists of two thin timber battens that form an “x” shape in their through section, with each strut going from the top of one joist to the bottom of the next. The advantages of herringbone strutting over solid timber blocking is that when installed correctly, the strutting will move over time with the movement and shrinkage of the joists, remaining tight as everything adjusts. In contrast, solid blocking, whilst faster and cheaper to install, may become loose and lose efficiency over time as the joists inevitably begin to shrink and twist.
Modern metal strutting systems have been invented that serve the same purpose as herringbone strutting, and take a fraction of the time to install. These modern systems are best suited for use with engineered “I joists”, due to the poor face registration of solid blocking on these components. When installed correctly, herringbone strutting is also more suitable than solid blocking for transferring load between joists, as shown in the diagram. The spaces that are present between the struts offer easy access for other trades to run utilities through them - another advantage of solid blocking.
What are the criteria for the installation of blocking?
For blocking to effectively fulfil its intended purpose, a variety of different criteria must be met. To begin, the correct number of rows of blocking needs to be installed to suit the span of the joist system. Short spans at around 2 metres and under don't necessarily require blocking, though inspectors may ask for them, and installing a row can’t hurt. For uninterrupted spans of up to 4.2 metres, 1 row of blocking is required though the centre of the span. For spans over 4.2 metres, 2 rows of blocking must be installed at third points along the span.
When installing joist systems, often the offcuts of timber created are long enough for use as solid blocking. Ideally, solid blocking should be be thinner in depth than the joist, so that uneven shrinking of the joists and blocking does not cause damage to the finished flooring or ceiling systems. If the same width of timber is used for the blocking as the joists, and the joists shrink faster than the blocking, the blocking can produce a hump in either the flooring system above or the ceiling system below. As such, it is preferable to either purchase a few lengths of timber specifically for use as blocking, at 1” less in width than the width of the joists, or to rip down the offcuts of the joists so they are an inch shorter in depth. Doing so allows for the blocking to be installed so that it is half an inch short of both the top and bottom of the joists, eliminating the possibility of damage caused by shrinkage.
Each row of blocking should be installed to a line to ensure an aesthetic finish as well as maximising the effectiveness of the blocking over time. Each piece of solid blocking should be cut with 2 square ends, with a length the same as the distance between the joists where they sit on the plate. This will ensure that each piece of blocking does not bow the joists out, keeping the entire row of joists nice and straight. When fixing each piece of solid blocking, care should be taken to ensure that each joist is in full contact with the blocking and is not twisted. Doing so will ensure a full row of tight blocking. The entire row of blocking should be installed tight and keyed in at both ends to ensure that the row of blocking is serving its purpose to its maximum capacity. The row should be installed in a perfectly straight line or staggered in an alternating pattern either side of a straight line for ease of installation.
Herringbone strutting must fulfil all of these standard criteria as well, though there are other additional criteria to consider. The wide, flat, square surfaces at each end of a piece of solid blocking help to align the joists and pull them out of twist and in line with one another. Strutting does not benefit from this feature, and so the joists must be pulled upright and out of twist through the use of layout rods before the installation of strutting. To maximise the effectiveness of the strutting though the relatively small surface area where the strutting meets the side of the joists, the layout and cutting of each piece must be precise. The strutting must not protrude above or below the joists, so as not to disrupt any floor or ceiling coverings. An inset of around 5-10mm in from the top and bottom edges of the joists is suitable for the top and bottom edge of each piece of strutting. The strutting must be cut and assembled from appropriate sized timber, with 1½” or 2” square batten being the most common choices. The strutting must be predrilled in its end to prevent splitting when fixing. Any defects such as splits within the strutting will reduce the overall effectiveness of the system. 3” nails or an appropriately sized wood screw should be used to fix the herringbone strutting in place. The pair of struts should not be fixed together through their centre point, as this restricts potential allowable movement within the joists and could lead to splits within the strutting. A slight gap should be left between the two struts to avoid any unwanted squeaks from occurring.
How is solid blocking installed?
For a standard single row of solid blocking at the halfway point of the span of a run of joists, start by marking the centre of the span on the underside of the joist at each end of the room. Solid blocking can be installed from above if desired, but it is less efficient, being more awkward, as well as unnecessarily risky. Installing the blocking from below is often the standard procedure, with benches or trestles and a plank being set up directly under the desired path of the row of blocking to allow for ease of working. A comfortable position to install the blocking from should allow us to stand with minimal clearance under the joists, without having to duck or lean back. In this position, we’re able to reach over the top of the joists to support ourselves, as well as apply force to the joist if necessary when nailing up the blocking. With the benches set up and the centre point of the span marked at both ends, a chalk line can be snapped between the two. This straight line through the centre of the span gives us a perfect reference point to install the blocking to. From here, a few more optional markings can be made to ensure that each piece of blocking is installed perfectly, though they are not necessary. Walking down the row of joists, a square can be used to strike a line up the side of each joist, straight up from the central chalk line.
When installing blocking, setting up a chop saw and a saw station near the area of operation can help to speed up the process, offering us accurate and quick cutting facilities. If we’re using the offcuts of joists for blocking, we’ll first have to rip them down to an inch thinner than the width of the joist. Using a circular saw, and setting up a quick stop to push the blocks into, we can rip each piece down the required width.
With the layout complete, we’re ready to start cutting and installing the blocking. When the joists were installed, a layout rod should have been pinned on top of the joists through the centre of the span to eliminate any bow in the length of the timber, and keep the centre of the joists on centre. For marking or measuring the blocking, the measurement should be taken between the fixed tops of the joists, as measuring between the bottoms could give a longer or shorter measurement than desired due to any twist or cup present in the joists.
With the offcuts ripped down to the right width, use the circular saw or chop saw station to cut one straight end on each piece. It’s important to highlight that from this point the overall task can be completed faster with two operators, one cutting and one fitting, though it's not essential. Starting at one end of the run, ignoring the very last gap between the joist and the wall, take a piece of blocking and butt the square edge against the top edge of one joist, letting the blocking come to rest on the top of the next joist. Using a pencil, strike a line against the joist where the blocking rests upon it, ensuring that the square edge is firmly and tightly located. Alternatively, a measurement can be taken between the two top edges of this gap.
Take the measurement or the marked block to the chop saw and make the appropriate cut. If marked/measured and cut correctly, the block should be a snug fit, possibly requiring a hammer to lightly tap it into position. If both joists are straight with no twist or bow then the block should be easily tapped into place. If one of the joists is twisting away at the bottom then care should be taken to support the blocking in its final place until a nail is inserted, so that it doesn't fall out. For a pair of joists with one leaning in at the bottom, a couple of tricks can be employed to get the blocking into place. Firstly, take the block and place the very top edge of it against the bottom edge of the joist that is leaning in. Using a hammer, hit the other end of the block whilst pushing upwards on it. The bowed joist should be quickly sprung out with the tap of a hammer enough to allow the block to slide into the gap. From here, use the hammer to tap it upwards into place, though it’ll be harder than usual because of the spring tension it's under. Alternatively, provided the block is a decent length, it can be started in the gap at an angle, registering on the layout lines of one of the joists. Use the hammer to tap it round sideways until it comes straight again. This method will push the joists outwards, and could possibly disrupt the batten above. Using a sacrificial striking block can help to reduce visible hammer marks on the timber whilst still allowing for very forceful blows to be made to the blocking to get it into place.
Whilst blocking can be installed in a perfectly straight line, doing so means we can only ever nail through the face of one joist, with the other end blocked by the last piece of blocking. As such, it's much more typical to see rows of blocking installed staggered in an alternating pattern either side of a straight line. For the first piece of blocking, choose a side and register the block against the layout lines. With the block in place, use either a nail gun with 90mm nails or a hammer and 4” nails to fix the block securely, nailing through the back side of each joist and into the blocking. Where this isn't possible, nail either sideways through the block diagonally into the joist, or upwards diagonally through the bottom. If either of the joists are bowed outwards, use a 4” nail or even a coach screw to pull it back into square, ensuring there is no gap down the intersection. For very large joists, it's not uncommon for a cup to be present in the length. In this instance, discretion must be used to decide whether the joist is upright, as pulling the cup out of a deep joist isn’t easily achievable, and we must make do.
With the first piece of blocking installed, continue down the line repeating the marking/cutting process, staggering the blocking either side of the line as you go. For efficiencies sake, measure or mark out 3 or 4 pieces at a time, cutting and installing them in batches to speed up the process. Cutting all of the blocking at once is not advisable, as the joists are subject to incremental movement as the blocking is installed. If the blocks are a few millimetres long or short each time, this compound effect will make the blocks at the far end either too long or too short, with time then required to rectify this issue. If working with another operator, one can be cutting the next block whilst the other is installing the previous one. Continue this procedure until all the blocking is installed between the main spans of the joists, leaving the two end gaps until last. At this point, we can cut and fit some folding wedges to lock the blocking into place. Using a hand saw or circular saw, cut 2 pairs of sliding wedges. Fit one set in the gap at one end, tapping it just over tight and then putting a nail through the joist into wedges to stop them moving. Take the other pair of wedges on the other end and install them until everything comes tight. Nail these, and the row of blocking is installed.
When installing blocking in steels, the procedure is much the same. The same measuring operations should be carried out, but there are a few variations to the method. Firstly, the blocking material needs to be ripped down to a width that will fit in the web of the steel. Second, no line needs to be struck, as the front edge of the lip of the steel will suffice to line the blocking up. Also, the nails will have to be slightly angled through the face of one joist, and then the face of the blocking into the joist at the other end of the blocking, due to the inability to stagger the blocking.
One final thing, when a row of blocking in a row of joists comes into a parallel steel, getting the final piece in there can be awkward because of the lip of the steel. As such, cut the block to the desired size, then cut the corner off at a 45 degrees angle, to about the halfway point of the thickness of the blocking. This will allow the blocking to be tapped round inside the steel and fixed into the joist. Make note that obviously no fixing can be made through the steel into this blocking. Alternatively, the steel can be blocked out flush to the front edge with timber to allow a regular piece of blocking to be installed.
How is herringbone strutting installed?
The installation of traditional timber herringbone is straightforward, though adherence to precision is required. With all of the joists in place and fixed on their correct layout marks the installation of the strutting can begin. As with the solid blocking, the desired number of rows of strutting must be determined based on the span and a central layout mark for these rows must be made with a chalk line across the joists.
With these lines made, the procedure for marking out the strutting can begin. To begin, accurately measure the depth of the joists, and subtract 10-20mm from this measurement. This will give us the total height that each strutting piece will occupy, with a 5-10mm inset from each edge of the joist to prevent disruptions with the floor or ceiling covering. On the end joist of the system, split this measurement over the centre line of the row that we just made, marking half the distance of the measurement in both directions. Carry out this same procedure at the other end of the joist system using the same measurement split over the centre line. By connecting these two sets of marks with a chalk line, we now have an effective registration point for marking our strutting across the whole of the joist system.
The installation of the strutting can be achieved from either above or below the joists, provided there is a safe passageway across the top of the joists. Marking out the strutting with the use of these registration marks is very easy at this point. Take a length of the strut material and lay it across the top of the joists. Line the outside faces of the timber up with the two outside registration marks we have already made. Using a pencil, mark a line on the underside of the strut against each inside face of the joists. Cut these lines with a saw leaving the line in. A mitre saw can be used to great effect here, as all of the angles of the strutting should be the same for every OC joist.
With this piece cut, we can hold it in place to check that it fits. If our layout was successful, both angles should be tight against the perfectly upright joists, with a small tolerance above and below the strutting for expansion. If it is correct we can predrill a hole for a fixing to prevent the piece from splitting. In terms of fixing the strutting in place we can either fix it through the top of the piece diagonally into the joists, or through the backside of the joist into the end of strutting. Traditionally, the former is more common. When fixing in this way, a pre-drilled hole should be made through the end of the strut with enough clearance for the fixing to meet minimal resistance within the hole. Additionally, or alternatively, a cut can be made down the length of the strut from the end of the piece until it meets the back of the hole. This cut will help to prevent splitting in the end of the piece. It also gives the strut room to move in relation to the fixing to allow for self adjustments of the struts to be made over time as the building settles. The hole should be drilled at an angle and location so that the fixing exits the strut centrally and fairly square to the angled cut on the face of the strut.
When fixing with either nails or screws, the fixing head should be inserted so it just enters the surface of the strut, without driving it too far - which could lead to damage occurring in the strut. With the first strut cut and fitted to one side of the central lines, another identical strut can be cut and installed within the same space alongside the first strut, on the other side of the central line, to form an x shape. All of the struts can be cut in this fashion, using the lines we marked early as registration points. Whilst measuring and cutting each strut individually would result in a far more accurate end result, provided the joists are correctly laid out OC, then for all intents and purposes the struts can all be cut at the same size. The beauty of their implementation is that even if the spacings between two joists creep ever so slightly off of the OC layout, the struts can be tilted more to still fit, with a minimal gap occurring between the face of the joist and the plumb cut of the strut. If the gap is too large or the joists begin to come off of centre for some structural or design reason, then the struts must be individually marked as previously explained. This being said, both struts that form a pair between two joists are identical.
With the explanation of how to install timber herringbone strutting completed, here is a brief explanation of how galvanised steel struts are implemented and installed. There are many specific styles and manufacturers of these products, though the overall premise and application is very similar to conventional timber strutting. These shaped strut pieces are attached with nails or screws to the joists and perform the same strutting and load distributing action as regular timber strutting. For the most part, these struts are sold in conjunction with engineered joists that are installed to a specifically designed joist layout, with the manufactured strutting maintaining the correct layout for the joists