Page 134 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Finding out where notches can be made in a joist, step by step

1 Measure the span of the joist from wall to wall.
2 Multiply the span measurement by 0.07. This will give a measurement equal
to 7 per cent of the span.
3 Measure from the wall the 7 per cent measurement and mark it on the joist.
No notches must be made within this mark.
4 Now, multiply the span measurement again by 0.25. This measurement is
equal to 25 per cent of the joist’s span.
5 Measure from the end of the joist again, find the 25 per cent distance and
mark it on the joist.
6 All notches must be within the 7 per cent and 25 per cent marks.
To put this into practice we must look more closely at the calculation:

Length of span of the joist = 4 m
7% of the span = 4 × 0.07 = 0.28 = 280 mm
25% of the span = 4 × 0.25 = 1 = 1 m

Therefore, notches in the joist must start 280 mm from the end of the joist and
must finish 1 m from the end of the joist. All notches required must be made
within a distance of 720 mm. This can be done from both ends of the joist, so
two sets of notches can be made.
The depth of the notch must not exceed 12.5 per cent (or / ) of the depth of
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the joist. So, if the above joist measured 250 mm in depth, then the depth of
any notches must not exceed:

Depth of the joist = 250 mm
12.5% of the depth = 250 × 0.125 = 31.25 mm

IMPROVE YOUR MATHS
1 The span of a joist measures 4.5 m long and 200 mm in depth. Using the
calculations shown above as a guide, calculate:
a the area where notches can be made
b the maximum depth of those notches.
2 The span of a joist measures 3 m long and 250 mm in depth. Using the
calculations shown above as a guide, calculate:
a the area where notches can be made
b the maximum depth of those notches.
3 The span of a joist measures 3.6 m long and 300 mm in depth. Using the
calculations shown above as a guide, calculate:
a the area where notches can be made
b the maximum depth of those notches.

Holes drilled or cut into joists follow a similar procedure. A hole must not begin
within 25 per cent of measurement of the span measured from the end of the
joist and must stop at a point equal to 40 per cent of the span, again measured
from the end. The size of the hole must not exceed a measurement equal to
25 per cent of the depth of the joist when measured from the centre line. This is
illustrated in Figure 2.42.
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