Whether forming the foundation of a flat or pitched roof, the installation of a set of wall plates remains unchanged. The prerequisite for the installation of the plates is for the internal skin of the masonry that will support the plates to established be at the correct height. Once installed, the top of the wall plates sit at the same height as the underside of the joists. If we ignore for a minute the thickness of the ceiling and floor coverings, for all intents and purposes, the height from the finished floor to the top of the wall plates will be the height of the finished room. This height is generally specified on the drawings, or must be matched to an existing ceiling height. Accuracy in these dimensions can be achieved through the creation of a layout rod by the carpenters and given to the bricklayers, denoting the level from the floor/datum to the top of the plate/wall.  

Once the masonry is established to the correct height, the wall plates are ready to be cut. To do so, simply lay whole lengths of straight 4”x2” treated timber on the top of the internal skin and mark them to length. Where the length of the wall is longer than the length of timber, a half lap should be cut and the plates should be joined. This same joint is cut at the corners of the internal skin, where applicable.

To cut this half lap, ensure the end of the length of timber is cut square to begin with. Use an offcut of timber as a physical reference, or measure the width of the plates and mark this measurement in from the end. Use a square to follow this line across the top face of the plate and down the two sides. Using a tape measure or steel rule, mark a line on the side of the plate at half the thickness of the material. Using a combi square set to depth, or using the fingers as a pencil guide, take this halfway line around the three faces of the plate, up to the shoulder line on both sides. Make a mark to denote the correct area of waste removal in order to avoid confusion, as well as to denote which side of the line is to be cut. Using a handsaw or circular saw set to depth, cut down the inside of the shoulder line to just above the halfway line just marked. Placing the timber on an appropriate surface, the waste can be cut or split away from the length.  

By visually identifying the direction of the grain, the material could be split away with a sharp chisel instead of being cut with a saw. If the grain is running straight with the length then the material will split away to our halfway line. Using a sharp chisel, any excess waste left behind can be quickly paired away. If the direction of the grain is not favourable, or if a defect such as a knot is present then the material waste needs to be cut away. Positioning oneself above the plate for this cut is the easiest method, with the end to be cut being elevated if possible. Using a circular saw is faster here to make the majority of the cuts, though a handsaw is still required here as most portable circular saws will not cut the typical 97mm from the end of the plate to the shoulder line. Using a handsaw, begin cutting along the inside of the waste line at an approximate 45 degree angle, making sure to keep the saw straight in the cut. Going slowly to start with can help to align the blade of the saw along the cutting lines down the end and the side of the plate. Keep cutting at this angle until the saw blade reaches the shoulder line and opposite corner of the plate. Flip the plate over and make the same cut again, taking note that we’ll be cutting on the other side of the line now. Cut to the shoulder line and opposite corner again, and then drop the saw to a 90-degree angle to remove the remaining waste. At this point, the waste should fall out. By repeating this process on another appropriate length of timber, the two pieces should fit together perfectly.

If the remaining pieces are too thick, the tops of the plates won't align correctly, and material should be paired off the inside face of the tenons. If the tenons are too long, there will be a gap somewhere between the shoulder and end of the plates. This can be rectified by running a saw down the tight shoulder and end of the plate.  

With all of the joints cut and the plates cut to length, they can be handed over to the bricklayers who will bed them onto the inside skin. An experienced bricklayer will install these plates perfectly level, provided that the timbers are nice and straight. They will typically weigh them down with concrete blocks or bricks overnight whilst the mortar sets to prevent the plates from curling or bowing. When the mortar has set the next day we can check that they are installed correctly and fix them in place. Ideally the plates are level, though we should double check to ensure they are. By hooking our tape over the far side of the plates we can check them for parallel. If we’re pitching a roof on these plates, we want to ensure they are perfectly parallel to within a few millimetres, whilst also maintaining them square to the end of the building. If a pole plate is being installed later to support the roof assembly then ensuring that these plates are parallel is less critical, but still desirable. At this point the plates will slide on top of the mortar bed if they need to be moved. Once happy that the plates are in the correct place, they can be temporarily fastened with nails into the mortar bed, joining the half laps together with skewed nails where applicable. Having located the plates in the correct place we can now carry out our layout of either joists or rafters, in order to be able to attach the plate straps in the correct place.  

Without these restraint straps, the plates are simply sat on top of the masonry with nothing physically holding them down. As the roof structure is assembled, the surface area of the total build increases, most of which is sat on the plates. During storms or windy days, negative pressure can exert a lot of force on roof structures, and without the restraint straps to hold the plates down, the roof may be lifted off.

If the plates are not paralleled before the installation of the roof, the ridge will fall or rise from one end to the other, with the total span shrinking or increasing down the length of the plates. Within a few millimetres of tolerance this is not an issue. However, if we simply align the plates with the masonry instead of paralleling the plates, they could taper wildly if the building is not square or parallel. It goes without saying that this can only be rectified by a small amount by paralleling the plates, as the plates have to have a full bearing on the masonry, and so if the building is widely out of square a larger problem is at hand, in this scenario, a pole plate may be required on top of the joists.  

If the plates are not level, are not straight along their lengths, or are at slightly different heights on either side of the building, then this will also negatively affect the end result of the roof. For the most part, deviations from true like this will result in the roof having humps or hollows along its flat surfaces, as well as causing the ridge to be out of level, or even being off centre to the span of the plates.