Civil Engineering Reference
In-Depth Information
The pitch of the roof will be expressed on your approved plans as degree. To prepare the
roofing boat we will need to know the amount of rise in the roof for every horizontal
metre. Let us use a 30-degree pitch as an example. At 30 degrees you will be able to walk
on the roof when it is dry, if you are wearing Dunlop volley sneakers; at 33 degrees you
will need a roof ladder. Please note: if you are on any roof you must have safety rails
around the roof or wear a safety harness to comply with safety regulations.
A roof with a 30-degree pitch
A roof with a 30-degree pitch will have approximately 578 mm of rise for every metre of
run, or to put it another way, 57.8 mm vertical lift (see 'B' on Figure 8.13) for every
100 mm of run (see 'A' on Figure 8.13) on the horizontal plane.
Making a roofing boat for a roof with a 30-degree pitch
The timber to make the roofing boat will be approximately 400 mm long, the depth of
the rafters and approximately 20 mm thick. An offcut of sheet flooring is ideal. Line 'C'
represents the top edge of the roofing boat. Line 'B' represents the plumb cut at the front
of the roofing boat (see Figure 8.13).
C = hypotenuse
C
A = 100
B = 57.8
B1
D
Figure 8.13
Transfer the calculated details to the roofing boat. B1= vertical birdsmouth line. D= Top plate horizontal
birdsmouth
Once you have marked line 'B' on the roofing boat for the plumb cut, transpose this
line to the other end of the boat, keeping it at least 30 mm from the end of the timber
(see Line B1 in Figure 8.13).
The birdsmouth notch is to be not greater than one-third of the depth of the rafter.
For example, rafters 150 mm deep would have birdsmouth notches of not greater than
50 mm. Establish the depth of the birdsmouth for your rafters and from this ascertain the
depth of timber to be above the birdsmouth. From the top of the roofing boat, mark down
line B to the depth equal to the timber to be left in the rafters above the birdsmouth. Mark
the point that this depth meets line B1. From this point draw a line toward the front of the
roofing boat at right angles to line B1 (see Figure 8.13 for line 'D'). This line represents the
section of the rafter that sits on top of the external wall timber.
You now have all the relevant lines on the roofing boat, so carefully cut the plumb cut
line and the birdsmouth notch. Measure the distance from the front of the roofing boat,
