Spans of timber joists

The tables below can be used to help calculate the size of timbers necessary to give adequate support to floors, ceilings and roofs. Using these tables, you can work out whether your joists are strong enough to support themselves and the load they carry, without the help of any walls underneath. Building Control may ask for additional engineer's calculations.

The weight of the timber floor itself consists of the timber joists, the plasterboard ceiling underneath it, the floorboards and the nails or screws used to fix both of the aforementioned coverings. All of these are known as "the dead load" . The joists  must be able to support this dead load without sagging. This dead load is generally taken by architects and planners to be no more than 0.50 Kilo Newtons per square metre. (kN/sq.m)

The weight we place upon a floor by way of bathroom suites, beds, wardrobes etc, is known as the "imposed load". It is again accepted that, for normal household requirements, the imposed load will not exceed 1.5kN/sq.m.

The building regulations tables A1 and A2, list the size of joist necessary to support this weight, over a maximum span. Table A2 uses timbers known as "SC4" which are high strength timbers containing very few, if any, knots. These timbers if you have them in your property, will be stamped C24 by the timber yard that supplied them. They are not common in modern day construction unless specified and we will deal with the more generally used timbers, dealt with in table A1. These timbers are known as SC3 and will have C16 stamped on them.

This table is for a dead load of more than 0.25 but not more than 0.50 and allows for an imposed loading of no more than 1.5 kN/sq.m.

You need to measure the complete span of your floor joists, together with the distance between them (the 'centres') and the size of the timbers. This should be compared to the table. If your old joists do not meet the requirements set by the table, they are either supported somewhere from underneath or they were installed pre regulations (November 1985).

RECOMMENDED MAX. SPANS for timber floor joists                         

                                                  Spacing  (distance apart)  of joists - centres

ROOF SPANS

CONCRETE MIXES

   NHBC  Foundation Depth Calculator

Easy to use. Simply line up a tree height and tree distance, then read off the foundation depth.