Dave Page wrote:
Lobster wrote:

When topping up old insulation (Rockwool) it's normal and more
effective to lay the second layer perpendicular to the first.

I was intending to replace the existing insulation - it's only about an
inch thick, dead old (over 10 years), and patchily installed. Plus
insulation seems to come (from Wickes at least, who are selling rolls ad
near-half-price until this weekend) in 170mm and 100mm depths.

I wouldn't bother lifting the old stuff - 'orrible job, and it won't do
any harm being added to by what you're going to lay on top. I've used
the Wickes stuff; it's still intended to be laid perpendicularly (or
orthogonally - FFS!) even if you're starting from scratch (I think their
instruction sheet says so).

There's one bit of information I'm missing; what sort of timber would
be appropriate for this job?

Just bog standard timber joists from the timber yard - haven't a clue
what species it is!

That sounds pretty useful. Wickes have some 40 x 100mm pre-cut 2.4m
lengths, I'll ask someone at the store whether it's suitable for my
purposes if it works out cheaper than a timber merchant.

That's just the job, their 'kiln-dried', unfinished stuff is what you
want, with the purple sticker! Look down its length to check it's
straight before you put it on your trolley; especially if they haven't
got much stock in, a lot of it can be banana-shaped. Personally, I find
Wickes cheaper than timber yards as they won't give me decent trade prices.

Still not too happy about your weight issue though, but I could be wrong
- hopefully someone with more knowledge than me will comment?

David