Flat Roof Joist Thickness

There are several variables to consider when determining the proper rafter size.

Flat roof joist thickness. Span calculator for wood joists and rafters also available for the android os. As the single ply roofing association spra points out this type of flat roofing is strong durable completely sealed and usually lasts at least 25 30 years. Double nailed at each joist. A minimum thickness of 18mm these will be able to span up to 600mm.

Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size. Weather you re building a shed roof or adding a dormer to your second story it is important to choose the proper size rafter. Buildingregs4plans flat roof guidance provides information on flat roof deck design and materials including timberboarding plywood oriented strandboard. There are two types of flat roof.

Standard asphalt shingles add very little thickness to the roof because a standard three tab shingle is about 3 16 inch thick. Also available for the android os. Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice. As opposed to old built up felt roofing with joins new single ply waterproof membranes are now most commonly used.

Cold roofs and warm roofs. Notches are not to exceed 0 125 of the depth of the joist and are to be located between 0 07 and 0 25 of the span from the support. Usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size automatically calculate sizes suitable for supporting the loads below. The joists will normally be placed at 400mm centres but no more than 600mm centres.

Timber to be covered these span tables do not apply to timber which is fully exposed to the elements. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. Wherever possible joists should span the shortest distance.

This sizing guide for roof rafters will help determine what you need for your small project.