Civil Engineering Reference
In-Depth Information
plumb cut of one of the rafters, as it will be difficult to hold it in position while holding
the ends of the rafters up. It is a lot easier if you have a second person to help with the
next process, however it is possible to perform alone.
Lift the two rafters up into their respective positions with the birdsmouth checkout
pulled firmly against the outside of timber at the external wall. Bring the two plumb cuts
of the rafters together to check that the rafters meet the ridge without too much gap
between the ridge and the rafters. If there is too much variation, first check that you have
cut the rafters to the calculated length. While holding the rafters in position, measure the
height of the underside of the rafter at its highest point to the underside of the ceiling
joist, and check this height against the calculations. Next check that the calculations for
the length of the timber are correct. If all of these measurements are correct it most likely
that there is a discrepancy in the construction in the roofing boat, which will mean that
the plumb cut will need to be adjusted. A 2-3 mm gap is not ideal, but particularly if it is
at the top of the rafter to the ridge it is not too much of a concern. Before you make any
adjustments to the rafters, check to see how they meet at the other end of the building.
Once any necessary adjustments are made to the rafters, check them in position
again. If you are now happy with the way they meet and the height at which they meet,
you can cut the balance of the common rafters. The common rafters are the rafters that
have a square plumb cut, whereas the rafters that are fixed to the hip, known as creepers,
will have a 45-degree angle cut on them so that they match the line of the hip. Spread the
common rafters on top of the ceiling in their approximate positions.
Installing the ridge beam
For a gable roof the ridge beam will be the same length as the side wall of the house; for a
hip roof, it is equal to the distance over the jack rafters, including the thickness of the jack
rafters. Allow an additional 100 mm of length for the ridge, as it will be cut to the correct
length just prior to fitting the final jack rafter.
Mark the ridge with the positions of the rafters
Before any rafters are fitted, it is best to mark the ridge to indicate the positions that the
rafters are to be joined to the ridge. The ridge is to be fixed parallel to the external wall
with the rafters running square from the external wall to the ridge. Consequently, the
marks on the ridge are to be opposite the marks on the external wall. When the rafters are
fixed to the ridge it is important that the top of the rafters are all at the same level. It is
normal for the top of the rafters to be fitted level with the top of the ridge, however, if the
ridge has a bow in it this could result in a hump up in the top of your ridge. Site along the
ridge to see if it is bowed; if it is, use your chalk line to mark a straight line along each
side of the ridge. In this instance, the rafters will be fitted with the top of the rafter level
with the chalk line rather than the top of the ridge.
Joining the ridge beam
On many occasions the ridge will be made up of more than one piece of timber. To join
the ridge timber, cut a 45-degree angle on the ends of the ridge timbers where they are to
join. If at all possible, make the join in the ridge beam between rafters rather than have a
rafter fixed over a joint. Before cutting the ridge beams, establish which piece of timber is
