Civil Engineering Reference
In-Depth Information
with the birdsmouth checkout firmly against the outer wall (see Figure 8.27). Scribe a line
down the side of the ridge board at plumb cut of the rafter.
Take a measurement from one rafter to the other at the external wall, including the
rafters. The length of the ridge beam is to be the same as that of the rafters at the external
wall. If it is within 10 mm it is of little concern, more than this needs to be investigated
further. If the scribed mark is in the correct position, cut the ridge board to length and fix
the end rafter to the end of the ridge board. The first two rafters that were temporarily
fixed to the ridge are now to be permanently fixed in position. One by one, remove the
temporary nail from the rafter, and while holding the weight of the ridge and rafter,
reposition the rafter so that it is adjacent to the end of the ridge and level with the top of
the ridge or on the chalk line. Once in the correct position it can be fixed permanently to
the ridge board.
The end rafters now securely brace the roof, so you can complete the installation of
the common rafters. Refer to the details for installation of common rafters previously
detailed.
Joining the ridge beam
When the ridge board is joined, the first section of ridge to be fitted will have a 45-degree
angle cut with the point of the cut nearest the floor. Fit jack rafters 2 and 3 as detailed
above, and then a pair of common rafters at the other end of that section of ridge. Now,
fit jack rafter 1, which will stabilise that section of the roof. You are now to fit jack rafters
5 and 6 and lift the ridge into position between these rafters at one end while joining the
angle cut end of the ridge to the first section of ridge fitted. Nail through top and bottom
of the ridges to secure them together. Cut the ridge to length and fit jack rafter 4 as
detailed previously.
Look along the top of the ridge to determine any bows sideways in the ridge board
that need to be straightened. It may make it simpler if you fix a string line to the top of
the ridge as this will allow you to refer to the straight line as the balance of the rafters is
fitted.
In a perfect situation, the rafters will all be fixed to the top edge of a straight ridge
board, and all the birdsmouth checkouts will be sitting firmly against the outer edge of
the external walls all the way around the roof. As yet, after about 30 years in the trade, I
am yet to see this happen. The most important thing is to keep the ridge board straight
and the top of the rafters in line vertically, as this is the section of the roof where
discrepancies will be most visible.
The birdsmouth checkout may have a gap between it and the external wall or you
may even have to extend the birdsmouth so that the rafter can sit correctly at the ridge.
Obviously there is a limit as to the tolerance in this, however there are many factors that
need to be taken into consideration when establishing just how much variance there can
be. As a general rule, a 10 mm gap between the back of the birdsmouth and the external
face of the wall, or a need to extend the birdsmouth by 10 mm, is probably within the
bounds of acceptable. If you are at all concerned it is better to contact your building
consultant rather than to worry about whether you are doing the right thing.
