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Constructing a Flat Roof
Flat roofs are constructed basically with joists spanning the gap between two walls, or wall
plate to wall as is the case with a lot of extensions, covered by sheet timber or other material. In the case of timber
a further, waterproof cover is required. The weight of these last two coverings, together with the span, form the basis
of the calculation for the size of joists required. A further part of this equation is discovered in Approved document A
of the Building Regulations (Structure) where the possible snow loadings, depending on area of the country and height
above the ordinance datum, are taken into consideration.
|
Maximum Clear span of Flat
Roof Joists in Metres For SC3 Timber
|
|
Size
of roof
joist
|
Spacing between
joists
|
|
400mm
|
450mm
|
600mm
|
| 50 x72 |
1.23 |
1.21 |
1.16 |
|
50 x 97
|
1.81 |
1.78 |
1.70 |
|
50 x 122
|
2.42 |
2.37 |
2.25 |
|
50 x 147
|
3.04 |
2.97 |
2.80 |
|
50 x 170
|
3.61 |
3.53 |
3.30 |
|
50 x 195
|
4.23 |
4.13 |
3.78 |
|
50 x 220
|
4.85 |
4.67 |
4.25 |
It is therefore important, before building your structure that you have the
desired building regulation approval.
In the case of a felt or fibreglass covered roof, we very strongly advise the
installation of this by a professional. You will get a guarantee and that is worth its weight in gold. EPDM synthetic
rubber membrane is easy to
install, and if fitted as a single membrane as there are no joints to worry about, even when done DIY it will out last
any of the above methods.
The
following tables give details of the allowable spans and spacing for some of the more common timber sizes used in flat
roof construction. All figures are based on normal loading conditions found
in domestic construction. In any other situation you should refer to the
Approved Document Part A of the Building Regulations or ask your Building Control Surveyor for advice.
Before the joists are fitted it is usually necessary to fit 
a
wall plates, this is usually 1" x 4" rough cut timber which is fixed to the top of the wall, usually
with a combination of cement and fixings to make sure it is level. Wall plates allow easy, secure fixings when fitting
the joists. It is usually necessary to have the wall plates level with each other as well as along the length of the wall, but this is
not always the case a small discrepancy can be allowed especially if the fall produced is in the right direction The
joists are fitted level, usually 400mm spacing but can be 450mm, or 600mm depending on the size of the joists that have
been chosen. Noggins should be installed for every 2 metres of span this will stop the joists from twisting
as well as keeping the gap between joists constant.
A firing should be fitted to each joist. Firings are lengths of timber cut
down the length from corner to corner forming very long triangles so that they go from full thickness to nothing and
lifting the one side of the roof creating a fall. This fall being directly proportional to the thickness of the firing
in relation to its length. Firings can be obtained at all good timber merchants.
The decking boards can now be fitted along with the facia boards. It should
be noted that all boards should lie half way on a joist were it is to meet another board if it does not it should be cut
to do so.
The following stages depend on the method you choose to use as a final
covering on the roof. In most instances it would be recomended that EPDM be used due to the ease of
installation
and the life of the product.
(Note – the following insulation thickness would be
suitable, 250mm fibreglass, 120mm cellotex or 140mm styrofoam)
Please do not ask
us to specify your timbers as mistakes in dimensions etc can be very dangerous and we will not apply definitive
regulations to any project we do not have first hand knowledge of.
Source Delston
.
,