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(Simon Avery) wrote in message ...
Mike Barnes wrote:
snip
MB| * notch the joists and strengthen them as necessary - I
MB| will be putting
MB| thick plywood and a limestone floor on top of them

Unless they're particularly thick joists, I'd not do this as the
limestone will be adding a lot of weight and you don't want to be
weakening them unduly.

MB| Comments?

snip

The Building Research Establishment do a "Defect Action Sheet" number
DAS 99 on the issue of notching and drilling joists in a way to
produce the minimum weakening.

A brief summary is:

Notches can be cut in the top of a joist in an area that stretches
from 7% of the span from one support to 25% of the span from the same
support. The notch must be no deeper than the lesser of 12.5% of the
depth of the joist or 250mm. There is a matching "notch zone" at the
other end of the joist.

Holes can be drilled in a zone that starts a distance of 25% of the
span from one support and ends 40% from the same end. The zone extends
from the centre line of the joist up 12.5% of the joist height and
down 12.5% of the joist height. There is a matching "hole zone" at
the other end of the joist. Holes must be spaced such that the
distance between centres is at least 3 times the diameter of each hole
(I guess if the holes are different sizes you take the larger
diameter).

They don't say how close notches and holes can get at the 25% point,
but I would say "as far away as you can keep them".

This is my interpretation of the document, not an official "you can do
it this way". If you want to be sure you could try contacting the
Building Research Establishment in Watford or your local planning
office (shudder!)