What are ashlar walls?

Ashlar walls are a form of short structural wall that are installed within roof assemblies to provide stability to the roof structure, as well as forming the vertical edges of rooms within a loft conversion. Ashlar walls bear on the roof joists, and transfer some of the load of the rafters back onto the joists. Additional bird mouths are cut into the rafters in order to receive the top plate of the ashlar wall. These birds' mouths can be cut on the rafters off of the pattern to allow for close consistency in the location of the ashlar wall. The bottom plate of the wall is typically doubled up, with the top plate being occasionally doubled depending on the criteria. When the studs are installed directly under the rafters on the same 400mm centre layout, maximum stability in the structure is achieved. As the rafters follow the layout of the joists, this means the studs also sit almost directly above the joists. 

By installing an ashlar wall, the unsupported span of the rafter is reduced, preventing excessive deflection over time. These walls also help to define the boundaries of a room within a loft conversion or room within a roof structure. Without such walls, the floor space would extend all the way to the very tight points at the edges of the room. This is impractical to live in and keep clean. It would also be near impossible to finish traditionally with plaster, skirting, and flooring. By installing these walls, edges are defined in the room beginning at a decent head height. The height of an ashlar wall is typically around 1 metre, though this can be less or more depending on the criteria. The ashlar wall is also a good receptacle for insulation, allowing for additional warmth in the roof space. The void created between the rafters, and the back of the wall is often used for storage in loft conversions, with structural openings being created to receive small doors. These spaces are perfect for storing suitcases or Christmas decorations - the space being otherwise unused.  

As mentioned, the bird's mouths can be cut off of the pattern into new rafters to allow for the quick and easy installation of the ashlar wall. This being said, when carrying out loft conversion alongside existing rafters, the birdsmouth will have to be marked out and cut whilst the rafters are in situ. This can be a little tricky due to the space restrictions and typical cramped conditions - though there is a very decent method to accurately marking out the cuts. In an old existing roof, the rafters are likely to have sagged and deflected over time, especially in the centre of a run of rafters, and so the birdsmouth is going to be slightly different for every rafter. To accurately mark these out, the process is as follows.  

How are ashlar walls installed in situ?

We must first begin by installing our plates on the floor in the correct location on top of the joists. We can find the location of the wall on the drawings and install the bottom plates accordingly. Realistically, the wall should be running parallel with the wall plates and the ridge board, and so these can be used as a reference point to measure off of. Care must be taken here to ensure that the plates are straight and level, as this will form the basis of our accurate layout. With the two bottom plates installed in place, two long studs should be installed at each end of the bottom plate. At this point, these are just a layout tool and should be cut long enough to sail up off of the bottom plate and up the side of the existing rafters. They should be fixed in place, perfectly plumb off of the sole plates. By pulling a string line tight between the tops of the two studs, we now have a definite line to follow for the top edges of the studs, directly above the bottom plate. We now need to cut a stud that will represent the length of an actual stud, plus the thickness of the top plate. We will use this timber to mark out the locations of the birdsmouth so that they are all cut to a consistent height off of the bottom plate. As mentioned, the roof is most likely sagged more in the middle of the run that towards the edges, and so the stud should be set out from here. What works for a ⅓ birdsmouth in the sagged centre may decrease in size towards the less deflected ends. In some extreme cases, this ⅓ notch in the centre may not even touch the rafters towards the ends, and so an additional plate may be required to gain the height, or a deeper initial notch may be required, up to half the thickness of the rafter. With a size selected that fits on all rafters, simply go down the line of rafters and mark the birdsmouth off of the stud. Align the stud with the front edge of the bottom plate and touch the string line, then trace the top edge onto the side of the rafter. With all of the notches marked, remove the string line, and cut the notches with a saw. In these tight spaces, a jigsaw and oscillating multitool are one's best friend. With the notches cut, install a plate into the bird's mouths and fix it in place. If the marking out was accurate, all of the studs between the two plates should be exactly the same size. Cut and install the studs as usual, with a stud under each rafter.