How do you use timber span tables?
Span tables give the largest clear span for a solid timber joist or rafter of a given size, strength class and spacing, under a given load. For houses in England and Wales, the tables building control usually expects are TRADA's, which Approved Document A refers to.
Where the tables come from
The usual source is TRADA's "Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings". They used to be printed in Approved Document A. Since 2004 the Approved Document has pointed to them instead, as one way of meeting the loading requirement for certain members in floors and roofs.
They apply to houses up to three storeys above ground. They don't cover trussed rafter roofs, which the truss maker designs, or anything outside their stated limits. The tables are TRADA's copyright, now in their 4th edition and covering C16 and C24 softwood. Buy a copy from TRADA's span tables page rather than relying on a figure from a forum.
What you need to read a table
Pick the right table for the member (floor joist, ceiling joist, rafter, purlin, flat roof joist and so on), then read across with four things:
- the strength class of the timber, usually C16 or C24 (C24 is stronger, so it spans further at the same size), stamped on graded timber
- the size of the timber, width by depth, in millimetres
- the spacing, or centres, usually 400, 450 or 600 mm
- the load the member carries, such as the dead load of the floor or roof build-up and, for roofs, the snow and the pitch
The figure you get is the maximum clear span between supports. If your span is longer, go up a size, use a stronger class or close up the centres.
Things that catch people out
Clear span is the distance between the faces of the supports, not the overall length of the joist, and the bearing at each end has to be adequate too. Heavy point loads, such as a bath, a water tank, a wood-burner's hearth or a wall built off the floor, are outside the ordinary tables: LABC Warranty advises doubling joists under a full bath, for example. Openings need trimmer and trimming joists sized for the load they collect.
The edition matters. LABC Warranty says the edition current when the work is built is the one that applies, and the 2014 edition changed the imposed loads and deflection limits from the 2004 one.
When to get an engineer
Anything outside the tables needs a structural engineer: steel beams, long or heavily loaded spans, point loads, engineered joists (which come with their own maker's tables), alterations to an existing roof, and anything building control queries. Building control checks the sizes as part of the application, so put the member sizes, class and centres on the drawings.