Joinery Roofs
(m), and the adjacent column, headed ‘LENGTH OF RAFTER’, provides the corresponding basic rafter length (BRL),1 which, in the case of this example, is 4.8826 (m).
35 DEGREES
RUN OF R
=
210 mm RISE IN 300 mm RUN
LENGTH LENGTH OF OF RAFTER H&V
ROOF MEMBER AND BEVEL
1
1.2206
1.5779
R
2
2.4413
3.1559
3
3.6619
4.7339
4
4.8826
6.3118
5
6.1032
7.8898
6
7.3239
9.4678
7
8.5445
8
FIGURES USED ON BLADE TONGUE
MARK CUT ON
PLUMB
300
210.0
R
SEAT
300
210.0
BLADE
H&V
PLUMB
424
210.0
TONGUE
H&V
SEAT
424
210.0
BLADE
H&V
EDGE
300
269.0
BLADE
HIP
BACK’G
300
134.0
TONGUE
11.046
PURLIN PLUMB
300
172.0
TONGUE
9.7652
12.624
PURLIN
EDGE
300
245.0
TONGUE
9
10.986
14.202
JACKS
EDGE
300
245.0
BLADE
10
12.206
15.779
MOVE DECIMAL POINT 1,2, OR 3 PLACES TO RIGHT AND UNITS BECOME TENS, HUNDREDS OR THOUSANDS. 0.4 AND UNDER IGNORE. 0.5 AND OVER ADD 1 TO UNITS.
Staying with a pitch of 35°, we’re going to set out, by means of geometry and using a scale of 1:25 on a sheet of A3 paper, a hipped roof with a span of 3m (Figure 5).
TONGUE
DIFFERENCE IN JACKS 300 350 400 450 500 550 600
mm mm mm mm mm mm mm
CENTRES CENTRES CENTRES CENTRES CENTRES CENTRES CENTRES
366 427 488 549 610 671 732
mm mm mm mm mm mm mm
Figure 3: The ‘35 Degree’ page of Roofing Ready Reckoner Source: Stobart Davies Ltd (www.stobartdavies.com)
Figure 4 illustrates this information pictorially with the BRL rounded to 4883mm. If the example roof had a hipped end, we would also refer to the third column, headed ‘LENGTH OF H & V’ (H: hip rafter, V: valley rafter), to find the hip rafter length of 6.3118 (m).
Figure 5: Hipped roof with a pitch of 35° and a span of 3m. Illustration: Mark Milner
When complete, we are going to cut out and fold the roof to form a three-dimensional model and compare its scale measurements with actual measurements given in the RRR. In doing so, the RRR tables – hitherto page after page of meaningless numbers in the mind of an inexperienced but budding roofer – are brought to life.
Stage 1 With the A3 paper in portrait orientation and working from a vertical line drawn down the centre of the sheet, two further vertical lines are drawn 1.5m (at 1:25 scale using a scale rule) each side of the centre line (Figure 6).
gth len er aft 3 mm r sic 88 Ba 4
Figure 6: A span of 3m equals a run of 1.5m. Photo: Mark Milner
Span 3 m Run 1.5 m
80
Scale 1:25
The RRR is undoubtedly a useful tool for roofing carpenters. The difficulty, from a teaching point of view, is presenting the tables in a way that allows students and apprentices to see the information in their mind’s eye; a difficulty made greater when the run is not listed in the tables. www.trada.co.uk
60 50
Geometry
60
Figure 4: The basic rafter length of the example roof. Illustration: Mark Milner
85
Run 4000 mm
85
Pitch 35°
Figure 7: Our setting out so far. Illustration: Mark Milner
Stage 2 Horizontal lines, suitably spaced to fit both an elevation and a plan of the roof on the A3 sheet, are drawn next. Suggested measurements are given in Figure 7. >>
Timber 2020 Industry Yearbook
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