Of the Many Shapes of the Roof the gable will be Found Most Suitable for the Beginner. Marking Off the Wood for Cutting is Simplified by the Use of a Steel Square as Explained by Sparrow Legs Renovations.
THERE are various shapes of roof such as lean-to, gable, hip, hip and valley, gable and valley and Mansard.
For the average do-it-yourself man just starting on rafter cutting perhaps the gable roof is the most suitable. First, one should be familiar with the names of the different parts of a roof. Span is the overall width of building taken from outer edge of wall plates.
Run is the horizontal distance from wall plate to a point directly under ridge. This as a rule is one half of span.
Rise is the height of roof at centre of ridge (see Fig. 1).
The steepness of a roof is known as its pitch and pitches most commonly used are one-half, one-third, one-quarter and one-sixth. A roof with half pitch is half as high as the building is wide. If it is third pitch it is one-third as high as it is wide and so on.
Fig 1 - Diagram showing the various parts of the roof.
The Steel Square
A steel square enables you to mark off the various cuts and also to ascertain the length the rafter needs to be. The long arm of the square is called the blade and the shorter arm is called the tongue. The blade is 24in. long and the tongue 16 or 18in. The outer edge on one side of both blade and tongue is marked off in inches and twelfths of inches.
In using the square think of the blade as being the run of your roof, the tongue as being the rise and the hypotenuse (that is a line drawn from a point on the blade to a point on the tongue) as being the slope.
The measurements used on the square will be equal to one-twelfth that of the actual roof, i.e., inches and twelfths on the square will represent feet and inches on the roof. For an example let us say your building has a width (span) of 12ft. and you wish it to have a roof of one-third pitch. The rise will therefore be 4ft. (one-third of span) and the run 6ft. (half of span).
Lay the square on your timber so that the 4in. mark on the tongue and the 6in. mark on the blade is on the edge of timber. The tongue will now give you the bevel required for the top end of rafter-that is to fit the ridge board. This is known as a “plumb cut.” The blade will give the
bevel to fit on the plate (“seat cut”).
Length of Rafter
The length of the rafter can be found by any of the following methods:-
1. Measure hypotenuse from 4in. mark on tongue to 6in. mark on blade and multiply by 12.
2. Slide the square on the timber twelve times keeping square on marks 6 on blade and 4 on the tongue.
3. Read off rafter length from rafter scale found on the side of the square where the twelfths occur.
If method No. 2 is used it is made more easy to slide the square if two strips of wood are screwed together on the hypotenuse. Either method will give rafter length from the centre of the roof to the outer edge of the wall plate. Where a ridge board is used, deduct half thickness of the ridge board from the rafter length and add the allowance for overhang as a separate measurement.
Fig. 2 - A Steel rule showing the rafter scale which simplifies the task of marking off the wood.
Rafter Scale
Fig. 2 shows part of a rafter scale. The first column gives figures to be used in placing the square on the timber. In each a case in this scale 12 is the setting for the blade and the next figure is for the tongue.
Column 2 gives the pitch and the remaining columns give the rafter length. To use the scale for a roof of 12ft. span and one-third pitch select line 3 for one-third pitch and carry along to the column under the figure 6 (6ft. being the run of the roof).
Here you will read off 7-2-6 which means 7ft. 2 6/12in.
This will be the rafter length required and it is only necessary to deduct half thickness of the ridge board and add whatever is required for overhang.
