Roof Joists

What are roof joists?

Roof joists are a structural timber component that form a part of a standard pitched or flat roof. They are identical in practical application to regular floor or ceiling joists, in that they span from plate to plate, taking the ceiling, and possibly creating a basis for a floor system in the loft. The only difference between these roof joists and regular joists is that they are a critical part of a roof system. It's also important to verbally distinguish from regular floor joists, as they may be a different size within the building, and we want to avoid confusion by being specific in our terminology. The joists sit directly beside the rafters that are also pitched on the same wall plate. The critical purpose of the roof joists aside from supporting the ceiling is to prevent the wall plates from spreading apart due to the outwards force of the roof. The joists are nailed to the rafters where they sit beside one another for additional strength, though in larger roof assemblies, the two components may be required to be bolted together.    

What are the criteria of a roof joist system?

Roof joists must sit hard on the plate and must be of a size suitable for the span of the plates - this is specified by the engineer on the drawings. The plates must be levelled before the joist installation to ensure that the joists are level from plate to plate. The joists must be set out at specific intervals, typically 400mm centres, and positioned in relation to each room to accommodate the ceiling covering. A full bearing is required on the plates, though the end of joists will have to be cut/shaped to prevent them from protruding past the rafters. As per standard practice, the joists should be rounded upwards. The joists should be nailed appropriately to the wall plates in order to securely fasten them in place.  

How are roof joists installed?

The installation of roof joists is relatively straightforward, though a careful layout is required in order to move directly onto a successful roof. The process is as follows. Bear in mind that the following steps are slightly different if pole plates are to be implemented into the roof assembly. 

The process begins by checking the wall plates for level and paralleling them across the building. For a successful roof structure to be achieved, the span of the plates must be equal down their length. Once the joists are fixed in place to the plates, we restrict ourselves from any movement in the future, and so time must be taken to ensure that they are correctly located before they are fixed.  

Once the position of the wall plates is correct, they can be tacked in place through the use of small 3-inch nails directly into the masonry below. This will prevent them from moving around whilst the layout and fixing of the joists is carried out. After the layout has been carried out, wall plate straps can be installed clear of any components, securing the wall plates properly to the masonry below.   

The layout of the roof joists takes place in much the same way as regular floor joists, with a 50mm ventilation gap at each end of the roof, and spaced at common 400mm centres. Double or triple beams may come into play here depending on the requirements for the roof; to accommodate roof components or to form access hatches into the roof space from the floor below. These critical components take priority in the layout of the roof, with the remaining joists installed to accommodate the correct centres for the installation of the plasterboard on the ceiling surface below. The diagram shows a standard gable end roof joist layout. All of the timbers fall at 400mm centres to allow for the effective installation of the plasterboard, with the doubles that form the loft hatch also falling on this layout. When transferring the marking from one plate to the other through the use of a layout rod, it's important to ensure that the markings are laid out from the same end of the building, to ensure continuity in the layout. With the plates properly marked out for the joists, we can see where the rafters will fall in relation alongside the joists, how many of each we will need to cut, and where we can place the wall straps so that they will not conflict with any other components.  

The next step is to cut the joists. Depending on the exact detail called for on the drawings of the intersection between the joist ends and the rafter tails, a specific length and angle may be required. Whereas regular floor joists are cut to the span of the outside of the internal skin, excess material on the ends of the roof joists may be required in order to bolt them to the rafters. That being said, we don’t want them so long that they project past the top of the rafters and into the roof. They can always be cut off afterwards if they are long, but this is often untidy, and entirely avoidable. The easiest way would be to physically mark the profile of the cuts against a pair of pattern rafters, by holding them all in place on the roof. This gives us the specific angle of the cuts on both ends of the joists, and all we need to do is reduce the overall length of these marks by a couple of inches to ensure the ends of the joints remain under the line of the rafters. This will provide adequate material on the ends of the joist for fixing the joist to the rafters.

Once all of the joists are cut, they can be rolled out onto the plates and fixed into place. A block can be cut from a length of batten to the length of the overhang of the joists past the plate. This can be registered off of the plate to install all of the joists centrally across their span. For a smaller roof assembly, the joists may be installed alongside the rafters and filled in as required. For larger roofs, the joists must be installed and braced, in order to create a stable platform off of which the rafters can be pitched. For a gable end roof, particularly those with gable ladders, it's important for the last joist and last pair of rafters at each end of the roof to be temporarily fixed. This is to allow for later movement if required when installing the gable ladders - allowing for the whole assembly to move to create an even soffit width and barge overhang.    

By placing the joists on the layout marks, they can be fixed securely in place with 4-inch nails skewed through the sides. Regular checks at intervals down the structure should be made to ensure that the plates have not shifted and are still parallel to each other.  

In accordance with the other practices already studied, doubles or access hatches may be installed in this assembly to allow for access into the loft after the fact. For loft conversions, these joists are floor joists, though they are also part of the roof assembly. With the joists in place, the rest of the roof can be assembled, now that there is no risk of the plates spreading apart. 

Might have to leave the last few joist out when installing hip end. Wait until the hip is formed with all of the jack rafters in place - as these are the critical components, then find where the joists will fit afterwards, filling in to meet the criteria of the ceiling joists.