Cable runs

No, it is acceptable if the space is not likely to be used. Indeed, it prevents the need to drill the joists, thus weakening them. As you are now converting the area into a storage area, you'll have to solve the problem.

This can be done by drilling the holes in the prescribed manner (you should only drill holes in certain defined parts of the joist of certain prescribed sizes and quantities). You can also add battens to the joists, so the flooring sits on those instead.

If your joists are only 75mm, then they are very weak. If the span is quite large also, then the storage capacity of the area in terms of weight will be pretty low, especially after the weight of flooring/battens is considered. Don't start putting large boxes of books up there.

Christian.

If you drill the joists, you'll have a *major* job - because you'll have to disconnect one end of each cable to thread it through the holes, and the cables probably go down the walls in top-hat section trunking under the plaster!

It's far easier to use battens on top of the joists to space the flooring above the cables.

As Christian says, watch the loading. Unless you have a lot of intermediate supporting walls, 70mm joists aren't cable of supporting much besides themselves and the plasterboard ceiling.

considered.

Thanks, Christian, I was a bit worried about holey joists. They are nearer 70mm (2¾ inches) than 75mm. In fact I'd already laid some boards on battens so maybe that's the way to go. The span is 4 metres max. Are there tables of prescribed dimensions and spacings etc somewhere I can access? Just off to move those large boxes of books!

Rob

Building regulations are available on

The old approved document Part A has tables for common wooden beam sizing, including joists. (New part A offloads these into separate documents from trade bodies that you have to pay for, so grab a copy now).

With 4m span, for table A3 for ceiling joists with < 0.25 dead load using SC3 grade wood and 400mm (close) joist spacing.

A 47x170mm is good for 3.96m.

Therefore, your joists are already very substantially below currently allowed specifications. Note that the ceiling joist tables show similar figures as for normal floors. Presumably they woke up to the fact that lofts are used for storage, whether they liked it or not!

Christian.

Sigh. I know. Bastards aren't they?

at 75mm, just lay yer insulation down, then cross batten with 2x1 or so lay another layer of insulatin cross ways and lay teh floor down on that. Cables will sit betwen the battens one way or another.