Pitched Roof Anatomy

There is a wide array of different roof styles with unique assemblies and procedures that are used in modern construction. Each style has its benefits and intended purposes, with the different criteria of the finished roof in relation to the building determining the style to be used. Whilst the styles themselves vary from building to building, the construction of all roofs revolve primarily around a small selection of common components. In the following section we’ll look at the basic terminology and components associated with modern roof construction.  

Terminology

To begin our explanation, we will look at the shell of the building before the roof is installed. By looking at the diagram, we can see that the timber wall plates are sitting on a bed of muck on top of the inside skin of masonry, in accordance with the procedure we’ve already looked at. Once the wall plates are parallel and secured in place,  the distance between the outside edges of the plates is known as the span of the roof. This term is also applicable to pole plates, which we’ll look at shortly. For a regular pitched roof, the centre of the roof assembly falls directly in line with the halfway point of the span, with the wall plate at each side of the building equidistant from this centre point. The distance between the outside of the plate to the halfway point of the span is known as the half-span, or run.  

Looking down the length of the building now, we can observe another critical measurement. The distance from the top of the wall plate to the geometric apex of the roof structure is known as the rise. The specific measurement of any given rise, in combination with the run, will determine the pitch of the roof. The pitch is the angle between the level horizontal plane of the joists, and the upwards slope of the rafters. As the measurements of the span and the rise change between roof structures, so too does the pitch.

In the UK, the pitch of a roof is most often described in degrees at the lower intersection of the joists and rafters. In the US, the pitch is typically described by a ratio of the relationship between the rise and the run in inches. For example, a 6/12 pitch sees the rafters travel at an upwards angle of 6 vertical inches (rise) for every 12 horizontal inches (run). This specific angle in degrees is 26.57. When the rafter travels upwards at the same rate as it does horizontally, the pitch is 45 degrees, also known as a 12/12 pitch. The diagrams display a selection of different common roof pitches, as well as a chart converting the two different methods of measurement.

When assembling a roof structure, these measurements and angles can be used to calculate the exact lengths of all the components of the roof, through processes we’ll look at later. 

With the basic terminology discussed, it's time to put names to the common roofing components. In these diagrams, we can see the two most common types of residential roof - the gable end roof and the hip and valley roof.  

By far the most numerous structural components within roof structures is the common rafter. These structural timbers form the bulk of the frame in most roof types. Broadly speaking, these rafters sit on top of the wall plate, with a special birdsmouth cut made to allow them to receive tightly over the wall plate. The horizontal portion of this notch is known as the seat cut. Another angled cut, the plumb cut, is made at the top of these rafters in relation to the pitch of the roof to allow them to fall neatly against the ridge. The specific angle of the plumb and seat cut vary between roof pitches, though they are always perfectly plumb and level respectively, in relation to the pitch. The angle of the seat cut in degrees is the pitch of the roof, when registered off of the long edge of the timber. The angle of the plumb cut in degrees is 90 degrees, less the pitch of the roof. The internal angles in a right angle triangle always add up to 180 degrees. When we look at the through section of a common pitched roof, we can see that the structure takes the shape of two right angled triangles, back to back. With a known 90 degree, and the known pitch in degrees, we can always determine the angle of the seat cut by adding up to 180. Something to consider - the angle of the plumb cut is the pitch of the roof when you cut it on a chop saw. The default position (0 degrees) of the chop saw, is actually 90 degrees to the long reference edge of the timber. This reverses the layout, meaning pretty much all plumb cuts for pitches up to around 55 degrees - the max of the chop saw- can be cut on it. Try finding a rafter with a pitch higher than that.  

The ridge is a timber component that runs down the centre of the roof at the half span of the plates, in order to support the tops of the common rafters, additionally tying the top of the roof structure together. The most basic form of a ridge is a timber ridge board, though variations such as beams or steels can often be implemented for structural purposes. 

As previously discussed, the roof joists are installed alongside the common rafters to prevent the wall plates from spreading apart under the weight of the roof. In larger roof structures, or in loft conversions where the underside of the roof structure forms part of the ceiling area, collars are installed. These collars create a flat surface between the tops of the rafters for the ceiling to receive plasterboard but also helps to prevent the rafters from spreading - similar to joists.  

Beyond the wall plate and associated birdsmouth notch, the rafter tails typically project beyond the footprint of the building to provide additional weathering qualities to the building. This overhand is known as the eaves. In modern construction, the rafter tails are cut with a plumb and seat cut, to receive fascia and soffit respectively, though in the past it was not uncommon for the eaves to remain exposed. These modern components seal the roof structure to prevent wind, insects, and animals from entering via the eaves.  

At each end of a gable end roof, particularly those that feature a soffit overhang, a gable ladder is installed. This component is assembled off of the roof and installed onto the last rafter in order to provide fixing points for the soffit and fascia. The fascia capping that runs up the pitch of the roof on these ladders are known as barge boards.  

On some roofs, the rafters return around the corner of the wall plates to create a hip, or hip end. A long structural timber, typically bigger dimensionally than the common rafters, spans all the way from the corner of the plates to the ridge to form the corner of the change in direction. This timber is known as a hip rafter. The top of the hip rafter meets the ridge intersection at a calculated point within the geometry of the roof. In order to continue the O/C layout of rafters along the roof at a hip end, jack rafters are implemented, featuring a special compound angle on their plumb cut to allow them to receive into the side of the hip rafter.

Sometimes, the direction of the roof changes around an internal corner of a building. This is known as a valley. Similarly to a hip, a long timber known as a valley rafter spans from the inside corner of the wall plates to the ridge intersection. This allows for a continuous frame around the corner. Alternatively, a lay board can be installed over an existing roof frame to allow for another roof structure to receive into it, creating a variation of a valley. In both instances, cripple jack rafters, or valley jack rafters are installed, that span from the ridge to the valley timber. These also feature complex angles to allow them to fit with the geometry of the roof. 

In modern construction, all roof timbers are scaled up in size to meet the structural requirements calculated by the engineer in relation to the load of the roof. For example, 6” timbers and wider are often used. This is in contrast to the much smaller timbers used in older roof structures. 4”x2” rafters are commonly found in old houses and bungalows. Traditionally, to prevent these smaller timbers from sagging under the weight of the roof tiles, purlins were installed around the inside of the roof to brace the centre point of the rafters. These purlins are supported by purlin struts - timber braces that distribute the load of the centre of the roof back onto the internal masonry walls of the structure. This configuration is not so common in modern construction.