The tables below can be used to help calculate the size of timbers necessary to give adequate
support to floors, ceilings and roofs. Using these tables, you can work
out whether your joists are strong enough to support themselves and the load
they carry, without the help of any walls underneath. Building Control may ask for additional engineer's calculations.
The weight of the timber
floor itself consists of the
timber joists, the plasterboard ceiling underneath it, the floorboards and the nails or screws used to fix both of the aforementioned coverings. All of
these are known as "the dead load" . The joists must be able to support this dead
load without sagging. This dead load is generally taken by architects and
planners to be no more than 0.50 Kilo Newtons per square metre. (kN/sq.m)
The weight we
place upon a floor by way of bathroom suites, beds, wardrobes etc, is known as
the "imposed load". It is again accepted that, for normal household requirements, the
imposed load will not exceed 1.5kN/sq.m.
The building
regulations tables A1 and A2, list the size of joist necessary to support this
weight, over a maximum span. Table A2 uses timbers known as "SC4"
which are high strength timbers containing very few, if any, knots. These
timbers if you have them in your property, will be stamped C24 by the timber
yard that supplied them. They are not common in modern day construction unless
specified and we will deal with the more generally used timbers, dealt with in
table A1. These timbers are known as SC3 and will have C16 stamped on them.
This table is
for a dead load of more than 0.25 but not more than 0.50 and allows for an
imposed loading of no more than 1.5 kN/sq.m.
You need to measure the complete span of your floor joists, together with the
distance between them (the 'centres') and the size of the timbers. This should be compared to
the table. If your old joists do not meet the requirements set by the table, they
are either supported somewhere from underneath or they were installed pre
regulations (November 1985).
RECOMMENDED MAX. SPANS for timber
floor joists
Spacing (distance apart) of
joists - centres
Size of
joist
mm
|
|
400mm
|
|
450mm
|
|
600mm
|
|
SPAN
m
|
SPAN
m
|
SPAN
m
|
|
38 x 97
|
1.72
|
1.56
|
1.21
|
38 x 122
|
2.37
|
2.22
|
1.76
|
38 x 140
|
2.72
|
2.59
|
2.17
|
38 x 147
|
2.85
|
2.71
|
2.33
|
38 x 170
|
3.28
|
3.1
|
2.69
|
38 x 184
|
3.53
|
3.33
|
2.9
|
38 x 195
|
3.72
|
3.52
|
3.06
|
38 x 220
|
4.16
|
3.93
|
3.42
|
38 x 235
|
4.43
|
4.18
|
3.64
|
|
|
|
47 x 97
|
|
1.92
|
1.82
|
1.46
|
47 x 122
|
2.55
|
2.45
|
2.09
|
47 x 147
|
3.06
|
2.95
|
2.61
|
47 x 170
|
3.53
|
3.4
|
2.99
|
47 x 195
|
4.04
|
3.89
|
3.39
|
47 x 220
|
4.55
|
4.35
|
3.79
|
|
|
|
|
50 x 97
|
1.98
|
1.87
|
1.54
|
50 x 122
|
2.6
|
2.5
|
2.19
|
50 x 147
|
|
3.13
|
|
3.01
|
|
2.69
|
|
50 x 170
|
3.61
|
3.47
|
3.08
|
50 x 195
|
4.13
|
3.97
|
3.5
|
50 x 220
|
4.64
|
4.47
|
3.91
|
|
|
|
|
63 x 97
|
2.19
|
2.08
|
1.82
|
63 x 122
|
2.81
|
2.7
|
2.45
|
63 x 147
|
3.37
|
3.24
|
2.95
|
63 x 170
|
3.89
|
3.74
|
3.4
|
63 x 195
|
4.44
|
4.28
|
3.9
|
63 x 220
|
4.91
|
4.77
|
4.37
|
|
|
|
|
75 x 122
|
2.97
|
2.86
|
2.6
|
75 x 147
|
3.56
|
3.43
|
3.13
|
75 x 170
|
4.11
|
3.96
|
3.61
|
75 x 195
|
4.68
|
4.52
|
4.13
|
75 x 220
|
5.11
|
4.97
|
4.64
|
This is an abridged representation of
Building regulations Table A1



ROOF SPANS


CONCRETE MIXES

NHBC Foundation Depth Calculator
Easy to use. Simply line up a tree height and tree
distance, then read off the foundation depth.

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