What is a roof truss?

Roof trusses are self-supporting timber frames that are used to form the structure of a roof. Traditionally, within the scope of historical post and beam construction, timber frame trusses were assembled and installed to create the roof structure of buildings. Large beams were hewn from logs and assembled into frames through intricate joinery methods in order to facilitate the creation of a roof. Depending on the desired outcome for the roof structure, many different styles of truss frames were used, each with its own strengths and weaknesses. These styles, some of which are illustrated below, range in scope from ease of assembly, to living space efficiency, all the way to decorative. 

Structurally, the development of these frames over the course of history allowed for larger buildings to be assembled. As mechanical principles were explored further, larger and more complex truss frames were developed, capable of spanning extreme distances whilst also supporting tremendous loads. Historical examples of this can still be seen in old churches and listed buildings, as well as post and beam framed barns. Due to the larger timbers used, and the ensuing stability offered, these frames were spaced further apart than modern trusses and rafters. Horizontal purlins provide lateral stability to the frames and support smaller timbers that are spaced more closely together to support the roof covering. As time moved forward, advances in technology allowed for smaller dimensional timber to be created with relative ease in comparison to hewing logs for post and beam construction. Whilst post and beam construction still continues to this day, the wider availability of timber, and importantly nails and other mechanical fixings, allowed for the modern process of hand cut roofs. These structures were quicker to assemble and required less timber to construct. During the period of rapid technological development from the 1700s onwards, the use of nails and dimensional timber grew in the context of cut roofs. Over the next few hundred years, cut roofs became the standard, and formed the basis of what we understand now in modern cut roofs.

Shortly after the Second World War, further advances in manufacturing technologies saw the widespread reintroduction of roof trusses, in a cost effective and less labour-intensive form. In contrast to the labour, materials, and skills required to cut a roof from scratch, roof trusses could be made to size in a workshop and shipped to a destination as a complete set to form a roof assembly. This eliminates much of the onsite cutting and fitting, instead allowing for a very fast paced construction process. Through the same mechanical principles that traditional truss frames followed, modern trusses are engineered to specific tolerances depending on the criteria. Through these mechanical principles, thinner timber (relative to that used for cut roofs) can be assembled in specific patterns to create incredibly strong truss frames. As with the historical truss frames, modern trusses can be manufactured to facilitate different finishes and utility within a building.  

Shapes and styles of truss

There are many different types of modern roof trusses. Here we shall take a look at the most common of these styles and their historical counterparts, as well as an insight into their development.  

King post

Queen post

Attic truss

Fink truss

It should be noted that this is just a drop in the ocean of truss development. Trusses can be engineered to accommodate any shape of roof.  

How are truss roofs assembled?

Whilst a lot of the assembly in the total process of creating the roof has already taken place off site, a fair amount of work is still required to pitch the trusses before the roof covering can be installed. Though there are no calculations or cutting required before we start pitching the trusses, the actual installation process is more or less the same as for a cut roof, following many of the same steps and techniques. Here we shall look at pitching a simple gable end truss roof. As mentioned previously, there are many different styles of truss roofs - too many to cover each in detail. That being said, the process remains mostly unchanged through all of the different styles.  

As with any roof, the first step is to parallel the wall plates and begin the layout. Seeing as the pitched portion of the trusses are not bearing directly on the wall plate, it's not imperative that they are parallel, in that this does not directly impact the plane of the roof. That being said, it's good practice to begin pitching any roof on a set of parallel plates. It will also enable us to accurately measure the overhang of the trusses along the wall plates if they are parallel to each other. With the plates paralleled the layout can begin. The exact style of this roof sees full height masonry gables at each end of the roof, a very common style for regular gable end truss roofs. To begin the layout, we start at one end of the roof and mark out the trusses. The first truss will sit 50mm away from the inside skin of the masonry gable to allow for ventilation - standard practice. Depending on the specification of the roof, the trusses may be spaced at standard 400mm or 600mm centres, with the latter being more typical. As such, we can work our way down from the first truss location at the appropriate centres, marking the locations on the plate. The very last rafter at the other end of the roof will also sit 50 mm away from the inside skin of the masonry gable, regardless of the on-centre layout of the trusses. Once one plate is marked out, we can transfer the layout onto a batten, and onto the other wall plates, ensuring to start the layout from the same end. Once the roof is assembled, 4x1 timber is installed with the roof to brace the trusses. We can use some of this 4x1 as a rod to transfer our layout marks at this point.

With the wall plates marked out, we can install the truss clips that will secure the trusses to the wall plate in order to aid in our installation. Truss clips are an ancillary piece of hardware that ties the truss in place on the wall plates, similar in concept and appearance to a timber joist hanger. Due to the thinner timber that is used in the creation of trusses, nailing or screwing the trusses to the plate provides less stability to the truss, and can also damage the trusses. As such, truss clips are required for fastening. These clips are installed either on the inside or outside face of the wall plates, and receive around the truss, providing secure fastening holes through the side of the truss. The clips are nailed to the plate with twist nails.  

Once the clips are installed, the trusses can be “stacked” on the wall plates, with the appropriate number of trusses to aid in the installation process. Starting at one end, each truss is laid on its side near the appropriate layout mark, overlapping the truss under it. Note that this method only works if there is appropriate space to do so. Otherwise, the trusses must be individually brought in and secured. Starting with the last truss on the stacked pile, we can pull it upright into position on its appropriate layout marks, receiving into the truss clips that we previously installed. Using a level to plumb the truss up, we can brace it in position off of the wall plate or scaffold. In order to achieve a flat roof surface, and a consistent soffit overhang within the roof, all of the trusses must be installed perfectly in line with one another. We can achieve this by measuring off of the wall plates to the end of the truss and centering it on the wall plates. A double check should be made to ensure that the overhang of the trusses is the same over the outside skin of the masonry, and not just off of the inside skin wall plates, as this is ultimately what will be on show once the roof is completed. With the truss centred on the building, we can secure it in place with nails or screws through the truss clip.  

Once the truss is fastened in place, we can double check that it is plumb in its final location. From here we can begin to stand each of the remaining trusses up in sequence, securing them evenly on the wall plates in their truss clips. The first few trusses are temporarily braced approximately plumb off of the first plumb truss. Once a few trusses are fastened in position, we can attach our layout batten over the top of the trusses towards the apex of the roof, making sure that our layout marks are aligned with the trusses. We can take the temporary braces off of the other rafters and move them into place, securing them in place with this lateral layout batten. If our first truss was accurately plumbed out and our layout was accurate, the sequential trusses should be plumb when they are aligned with the layout batten. As the trusses are installed, additional diagonal bracing should be installed to maintain stability within the roof assembly.  

Once all of the trusses are pitched and secured, permanent bracing can be installed inside of the roof space to stabilise the trusses in position so that the temporary braces can be removed. Diagonal wind braces are installed at 45-degree angles across the underside of the rake of the trusses. These diagonal braces span from the wall plate at the bottom to the top of the inside space of the trusses. Where a brace reaches the apex before all of the trusses are picked up, another set of bracing is installed a few rafters back from the end of the first brace, beginning a new diagonal wind brace. The diagonal braces on the other side of the roof are reversed for additional stability within the roof. Several longitudinal braces are then installed along the inside of the trusses, parallel with the direction of the wall plates. These timbers are located within the roof space, often close to any intersections within the truss assembly. Where the roof is longer than a longitudinal brace, an extension piece begins a few trusses back from the end of the mainbrace. These braces are commonplace in all truss roofs, though additional bracing may be required for specific roof assemblies in accordance with the specification.  

All the braces here are typically 4”x1” timber and are secured with mechanical fastenings.