Ever wondered how far a timber joist will span before it starts to deflect noticeably? More than likely not. But if you are designing houses, extensions or loft conversions, it’s important stuff. You can look the info up on a Span Table (here’s the NHBC’s version).
So where do you find such a table? This information really ought to be in the public domain – i.e. available for free on some website – but it doesn’t appear to be. It’s not exactly a trade secret and I don’t think it has any valuable Intellectual Property Rights attached to it. But if you want to access the timber span tables, you have to pay.
What’s bizarre is that the span tables used to be embedded in the building regs, which are now in the public domain thanks to the internet. Get hold of a copy of the old Part A for England & Wales and there they are, in Appendix A, a 28-page blockbuster which covered every conceivable timber span you could ever think of, from humble floor joists to rafters and purlins.
But the new improved Part A, 2004 version, has seen fit to eliminate this useful information. Clause 2B1 states: Guidance on the sizing of certain members in floors and roofs is given in "Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings”, published by TRADA, available from Chiltern House, Stocking Lane, Hughenden Valley, High Wycombe, HP14 4ND, Bucks.
Very nice for TRADA who charge £17.50 for their span table booklet. You can find simplified versions of the span tables in the NHBC Handbook, which is also available at a price of £35, but obviously includes far more information than just timber span tables.
But it begs the question, why remove something useful from the public domain? What harm is it to have the span tables available for free?
Have just checked the ODPM building regs website and the 1992 edition of Part A is still available for free download, complete with the Span Table Appendix. So hurry and get yours before they hit the delete button.
If you can’t be arsed to wade through page after page of boring tables, here’s a really useful nugget you might care to remember. Halve the depth of the joist in inches and you get the span of the joist in metres. Thus a 9” joist will happily span 4.5m. And a 3.5m span will require at least 7” joists. Now you don’t even need a span table.