My daughter's new home, a 1930s semi with a decent sized square hipped roof
space has 2" of insulation, so I will up this to 180mm of fibreglass.
As I don't want to compromise use of the loft space and she may wish to fit
a dormer window I was wondering about both ceiling joist strength and
making space for the new insulation. First thought was to add 2 by 4s
perpendicular to the joists, fill gaps with insulation and lay boards over
that, with the knowledge that the insulation over the joists would be
compressed at these points.
Next I thought that adding 2 by 4s to the top of the current joists would
give better strength.
You are slightly better in terms of strength/stiffness and cold bridging
to go perpendicular on the second set. No need to compress..lay one
layer between first joists, add new one across and lay second layer
across first later, then plate.
plating in my case made a huge difference by reducing airflow through
rockwool. A well vented loft with a howling gale strips heat out of
make sure the edges are windproof too. I.e. put boards vertical at eave
level to stop wind getting in between the flooring and ceiling.
I have just done exactly this. Plus as the ceilings were sagging a bit
I propped them from below and also put some weight on the new timbers
before fixing them to the old joists with these:
So the whole lot is now kind of sprung. It's a good idea to make sure
that the new timbers are long enough to span from wall to wall before
pushing them down though.
*If* you can get where the ends of the existing joists sit. and if you
can lay a new timber the full length of the joists, then the strongest
option is to glue and screw 3 or 4x2 on top of your existing, then
your joists will be good enough for floor joists.
Another consideration is that if she later fitted a dormer window, her
local council Building Control would probably interpret this as
converting to a habitable space (it's a bit hard to do a dormer on the
quiet), in which case regulations would have to be complied with -
including the strength of the floor joists. The existing ceiling
joists would almost certainly not be up to scratch, in which case, if
she is really likely to want to do that in the future, it might be
worth considering whether it's feasible to add proper floor joists now
(which might be 8x2 or similar, depending on the span. The new floor
joists should be independent of the ceiling joists, running between
them (with the advantage that you can jump up and down on the floor
without deflecting the ceiling). That would give her a head start,
provided they were properly specced. When I did something similar to a
16 foot square room in the first floor of my bungalow, it cost me
around 250 quid for the timber.
This is what I thought when I asked as I'm told a floor joist needs to be
8x2. I was thinking of just screwing the new 2x4 on top of the existing one
to achieve this but will check with BCO to see if this is acceptable.
Getting the 8x2 into the loft is likely to be difficult but the GBP250 you
paid seems reasonable and the size is similar.
If we do go for the dormer it will be within PD rights so its only the
building regs I'll need to meet.
If he doesn't go for the gluing and screwing, you could also suggest side
laminated 4x2 + 4x2 - 2 bits of 11mm ply or structural OSB glued and screwed
both sides - like an inverse filtch beam.
The effect is the same (stops one beam sliding over the other therefore
increases stiffness) and adds a little extra stiffness itself. It looks very
convincing when I did it so it could be an easier sell to the BCO of option
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