In uk.d-i-y, Matt Beard wrote:
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!)

Thanks, excellent stuff. The bad news is that that formula allows only a
16 mm notch. Actually I'm pretty sure that there are holes and notches
outside those specifications under just about every floor I've ever
taken up :-( however I'm talking about substantial old houses where the
joists probably started out much stronger than they needed to be.

Thinking on, I wonder if some reinforcement might help. Two steel plates
say 600x100 with a 50 diameter hole in the middle, put them either side
of the joist, bolt them through, then drill out the wood. Obviously I'd
want someone qualified to do the sums but I'm still at the ideas stage.

A previous house had one joist on the ground floor not unlike that. I
could see it from the cellar. The end of the joist had obviously rotted
so someone had made a new piece about 600 mm long, butted it up to the
original, and bolted the two pieces together using steel cheeks.

--
Mike Barnes