Timber floor joists are still the most popular way of constructing suspended floors. If holes have to be drilled through the joists for cables or pipes, then providing this is done sensibly, you won't need any calculations doing. The generally accepted guidance for what is acceptable is:

- holes should be drilled at mid depth of the joists
- the diameter of the hole should not exceed 25% of the joist depth.
- holes should be located between 0.25 and 0.4 times the span from the support
- holes should be at least 3 diameters apart horizontally
- notches should no more than 12.5% of the joist depth
- notches should be located between 0.07 and 0.25 times the span from the support

In our opinion these guidelines can be a bit generous, particularly if there are several holes /notches in adjacent beams.

The chances are, if you need this type of calculation doing, it's because the Building Inspector is alarmed at what has actually been done by an over ambitious plumber! If this is the case, then having this design check done first is a better option than automatically replacing the floor.

Our structural calculations for checking holes or notches in timber floor joists includes an assessment of the remaining strength of the joists. Our structural design also includes preparing the calculations for any necessary strengthening work. Please note though that if the size of the hole is more than the size of the joist remaining it is unlikely that anything other than the complete replacement of the floor will work.

We need the following information to check holes in timber joists:

the span of the joists

details of any unusual loadings

details of any partition walls the joists have to support

details of the size of the holes or notches and their position along the length of the joist

Design information

All design work is carried out in accordance with BS5268 (The structural use of timber).

The calculations provided will be suitable for Building Regulation approval.