What is a rake wall?

In some instances, within modern UK construction, the gable walls of a gable end roof may be constructed from timber instead of a traditional cavity masonry assembly. These components, known as rake walls, are most often associated with loft conversions and modern timber framed houses. This type of component is also very common in modern US house framing. The primary advantage of a timber rake wall over a masonry gable wall presents itself in the skills required for assembly. After the assembly of a gable end roof by the carpenters, preparations must be made for the bricklayers to come in and build up the gable after the fact. This requires additional setup, lead times, organisation, and materials. On the other hand, if the gable is erected from timber, this can be done during the process of erecting the roof, leading to maximum efficiency in the schedule. That being said, timber framed gables are only suitable for gable end walls that are being cladded on the exterior, as face brickwork and rendered gables must be assembled from masonry.  

Within a typical loft conversion scenario, where the through section of the roof is triangular, the floor joists and potential collars serve to prevent the wall plates from spreading. This means that the roof is self supporting, with the gable walls not serving a load bearing criteria. In this scenario, the triangular rake wall sits directly underneath the rungs of the gable ladder, to support the overhanging barge board, as a traditional masonry gable wall would. This style of wall can be assembled on the floor and lifted into place before the installation of the roof or assembled in situ after the roof is assembled. This is due to the fact that the rake wall is not supporting the main roof structure.  

As we look at more advanced loft conversions/timber frame roof assemblies, we can see that the criteria for the finished rake wall changes. In this through section, we can see that a large portion of the roof structure consists of two dormers, one each side of the roof. The external vertical timber walls of the dormers are structural in nature, designed to accommodate the wall plate and the rafters pitched off of it. In this vaulted ceiling arrangement, the lack of joists spanning between the wall plates makes the roof susceptible to spreading outwards. The collars serve only to flatten the apex of the ceiling, and are too high in the structure to adequately prevent the roof from spreading. As such, the roof assembly is supported by the ridge, which is in turn supported by load bearing struts.  

In a more lightweight application, the ridge is assembled as a double timber beam with a flitch plate. The beam is supported by the rake wall at each end of the roof structure, with a slight variation in the design of the rake wall to accommodate this. Internal structural stud walls that match the profile of the external rake walls are located strategically within the roof assembly to both define the individual rooms within the structure, as well as support the ridge beams. Due to the length of the ridge, the beam is assembled from multiple timbers and flitch plates that are staggered in their assembly, with the joins falling on top of the supporting walls. In this instance, the rake walls and internal supporting walls must be assembled and installed before the ridge can be installed.  

In more heavy-duty applications, with larger ridge spans and heavier, longer rafters, the ridge beam may be a full RSJ. In this particular arrangement, multiple sections of the ridge beams are joined together atop “goal post” style steel frames. These frames are similarly strategically located within the building to support the weight of the ridge beams, and the roof structure. The hollow in the goal post style frame allows for passageways to be framed in timber within them. The total steel frame must be assembled before the roof assembly can begin, as well as the outer timber dormer walls. The inside of both the internal and external frames can be filled in with timber after the fact to receive the respective sheathing.  

Whilst rake walls feature many of the same structural design features as standard load bearing stud partitions, the “rake” of the top plate, and the associated varied length studs can be a little tricky to set out and get right without the proper know-how. The following section covers the different methods of assembling and installing rake walls.  

How are rake walls assembled?

Due to the shape of the rake wall, accurately assembling the frame on the floor can be tricky, as the tolerances are very small. With a regular stud partition framed on the floor, every stud is identical in length, the top and bottom plates are parallel, as is every stud, with every cut, every interacting component, and the total being square/90 degrees. This makes the assembly very easy, as well as installing the finished frame.  

When it comes to the rake wall, we can see a stark contrast. The top plates are angled to match the pitch of the roof, with each stud growing incrementally in accordance with the on centre layout to accommodate these rising plates. Any discrepancies in the lengths of studs can affect the pitch of the top plate, creating a poor fit for the frame within the opening. If the desired method is to assemble the frame on the floor, here are some tips to aid in the process. Note that most of the total procedure follows that of a regular stud partition. 

If the room allows for it, the finished floor surface can be used to aid in the marking out of the rake wall. Lines can be marked out on the subfloor to match the profile of the rake wall to ensure that the correct shape is achieved, based on the span and the known pitch of the rafters. Even better than this, timber can be used to create physical stops for the rake wall to be formed against. Actual rafters can be cut in relation to the span of the roof, and screwed to the subfloor in relation to the wall plates as if they were pitched. The diagram shows this method for both triangular rake walls, and those installed in relation to dormer walls. A pair of rafters are cut for the span of the roof, and screwed to the subfloor. An offcut of the ridge board can be placed between the two plumb cuts to accurately display the depth of the ridge in the roof structure. With a faux wall plate placed in each birdsmouth notch, a line struck between the two denotes the bottom of the sole plate. If dormer walls are present, battens cut to the total height of the dormer walls can be placed into the birdsmouths to display the top of the joists. With the structure of the roof simulated on the subfloor, accurate lengths for plates and studs can be measured and assembled - creating a perfect frame for the pitch of the roof. If the rake wall is intended to support a wide ridge beam, an offcut of plywood cut to the shape of the through section of the beam can be placed between the rafter plumb cuts. The supporting studs can then be accurately assembled around the profile of the beam, so that it will slot into place during assembly.  Once assembled, the frame can be stood up ready for the roof or other components to be assembled.  

Alternatively, if the style of roof allows for it, the rake wall can be assembled in situ. When setting out the roof, we know precisely where the rake wall will sit, and so our layout, and the width of the gable ladder will accommodate this position. As such, once the roof structure is assembled, we can plumb up lines using a laser or level from the position of the rake wall, up to the ridge and down the rungs of the gable ladder. Plates can be measured between the existing components and screwed to the gable ladder and substrate in relation to these layout lines. The on-centre layout can then be marked on the sole plates, with each stud being accurately marked against the top plate in accordance with the procedures we studied earlier. This method is impossible to get wrong in relation to the previous method, due to the fact the wall is assembled within the existing frame of the roof- though it can be more laborious with all of the up and downing. 

In both scenarios, knowing how to calculate the “diminish” of the studs can be beneficial to the process. After we know the length of either the shortest or longest stud, we can add or subtract the diminish value to accurately mark and cut each subsequent stud. This is more efficient than measuring and marking each stud, and saves time and labour. To calculate the diminish, we can rearrange some of the formulas relating to finding the pitch of the roof that we’ve already looked at. Realistically we could use a rafter square on a rafter of the correct pitch to determine the rise per 400mm on centre layout to calculate the diminish of the studs. Alternatively, we can simply use the information and formula we already know to calculate the rise per O/C increment at any given pitch. The process is as follows. 

Calculate the length of the rafter per distance equivalent to the on-centre layout of the wall (400mm in this instance). We can use the formula for calculating the length of a rafter via trigonometry - H = run/cos(angle). Instead of calculating the full length of the rafter for the run of the roof, we can find the length of rafter per 400mm (or any centre) at any given pitch by typing “OC layout/cos(angle)” into a calculator. For a regular 400mm layout, this formula will look like - 400/cos(angle). 

This formula returns the value of the length of the hypotenuse of a right angled triangle. Using Pythagorean principles, we can rearrange  a² + b² = c² into c² - a² = b² to find the value of the rise per OC layout. As we look at the right-angle triangle, we know the value of a, as well as the value of c. C is the hypotenuse, a is the oc measurement, and b is the rise per distance at a specific angle 

To find b, we simply plug in the values we already have. Square the value of the hypotenuse, minus the value of a squared. This returns the unsquared value of b. Copy the value and square root it. This is the value of b. In other words, this is the length that each stud grows per on centre layout of the wall. There are other methods of calculating this, but this works pretty well, both in imperial and metric measuring systems.  

An example - 400mm oc studs for a 22.5-degree pitch roof 

400/cos(22.5) = 432.956880117 

432.956880117²  - 400² = 27451.660040646 

√27451.660040646 = 165.6854249493 

Rounded up, every stud at 400mm centres in a 22.5 degree pitched rake wall grows incrementally by 166mm.