Whoever rewired my bungalow ran the cables across the attic joists at
all angles to keep the runs as short as possible. This makes putting
any kind of flooring down a bit difficult. Isn't the correct way to
drill holes through the joists at half depth and run the cables
through? The joists are only 70mm deep, so would drilling holes all
over the place weaken them?


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Isn't the correct way to drill holes through the joists at half
depth and run the cables through? The joists are only 70mm deep,
so would drilling holes all over the place weaken them?

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.

In an earlier contribution to this discussion,
Rob Bradley wrote:

Whoever rewired my bungalow ran the cables across the attic joists at
all angles to keep the runs as short as possible. This makes putting
any kind of flooring down a bit difficult. Isn't the correct way to
drill holes through the joists at half depth and run the cables
through? The joists are only 70mm deep, so would drilling holes all
over the place weaken them?

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.
--
Cheers,
Set Square
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Christian McArdle wrote in message
et...
Isn't the correct way to drill holes through the joists at half
depth and run the cables through? The joists are only 70mm deep,
so would drilling holes all over the place weaken them?

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.


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


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The span is 4 metres max. Are there tables of prescribed dimensions
and spacings etc somewhere I can access?

Building regulations are available on www.odpm.gov.uk.

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.

47x72mm = 1.27m

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.

Rob Bradley wrote:

Whoever rewired my bungalow ran the cables across the attic joists at
all angles to keep the runs as short as possible. This makes putting
any kind of flooring down a bit difficult. Isn't the correct way to
drill holes through the joists at half depth and run the cables
through? The joists are only 70mm deep, so would drilling holes all
over the place weaken them?


Sigh. I know. *******s 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.



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