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BUILDING A FLAT
ROOF
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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 and lean-to's, 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.
It is therefore important, before building your structure that you have the desired building regulation approval. 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.
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. It is usually easiest to put your insulation in at this stage if the cold roof system is being used. 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. All boards should be of exterior quality usually Plywood or OSB. If the warm roof method of insulation is being employed then it should be installed at this stage The installation of an EPDM covering onto a new roof surface is easily done and gives excellent results and would make it easier should you decide to use one of the other methods of installation such as mechanical or fully bonded.
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.
Although these pages are checked regularly and every care is taken with the information within. Delston can not accept any responsibility or liability for information or results displayed within due to third party interference. All information was deemed correct at time of publication. Author Myrddin
a addurl associate_sites c conversion diy e epdm order fitalytrims g greenroofinstall greenroofs help home index instructions j joists laddersafety m mechanical o query r t
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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 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.