On Wednesday, October 17, 2012 3:28:07 PM UTC+1, John Rumm wrote:
On 17/10/2012 11:49, GMM wrote:
Sorry chaps, a bit of a long post but I have tried to get all the essential information in (!) Undoubtedly
there will be something I've missed out, even so....
I have to build a floor in a loft. At first this will provide a platform for some roof timber repairs to be
carried out but ultimately it will be used for storage, not living space, so there are no formal BR
requirements but, of course, I�d like it to be robust enough that it doesn�t all wind up in the bedroom below.
At present, there is a lath and plaster ceiling, with rather wimpy-looking ceiling joists, so not the most
stable platform to work on. It�s in pretty good condition (as a ceiling, not a floor) so I�d like to keep it
that way, which means installing joists clear of the ceiling timbers. This will, of course, create a void
for a good depth of insulation.
So question 1: To work above the ceiling, I�m thinking of supporting it with boards on acro jacks,
possibly moving these according to where I�m working as I go and putting some boards on the ceiling
joists to spread the load (mostly me!). Does this sound like a sensible thing to do � ie any better
suggestions?
Nothing wrong with it, but you may be able to get around the problem in
other ways, if you can get a few joists in without needing to load the
existing ceiling too much. Once that is done you can get a temporary
floor to work off and do the rest.
Indeed: Once I get the wall plates on, I'll bring phase 1 of the joists in through the (at that point open) roof, fix the roof timbers, then stash the rest of the floor materials before the roof is sealed again. Then I'll work from that 'platform'. What I'd like to do is avoid wrecking the ceiling at an early stage (!)
One side of the space is an internal wall and the other side is a chimney breast with single-brick party
walls in the recesses each side (these look pretty weak). Each end is the outside wall of the house
running up about 60cm from the ceiling to the eaves of the roof.
So the existing ceiling is not at the top of the wall as such then, but
is suspended a bit below it?
That's right: A bit unusual (and I haven't measured it) but about 2' of vertical wall above the ceiling. Just for a change, something that might make a job easier (famous last words!)
Since the shorter span (about 4m) is
from the internal wall to the chimney breast/party wall, the joists for the new floor will run this way (ie
parallel to the outside walls).
The plan is to use ledger boards (wall plates if you prefer) and joist hangers to mount 50 x 200mm
(-ish) joists at 400mm centres. I know that socketing into the brick is favoured by some but that isn�t
going to happen for many good reasons.
Socketing is rarely done these days it seems... hangers are the norm.
To avoid going near the single-brick party wall, I�ll have to
use trimmers across the (approx.) 1.8m recesses each side of the chimney breast, mounting joist
hangers on these, so this is where the greatest loads will be: With 4 or 5 joists on each trimmer, their
mountings will be carrying approaching 1/3 of the floor loading. The short ledger boards that these trimmers will mount on will, of course, run at 90 degrees to the others (ie along the returns of the
chimney breast and along the outside walls).
You can get strong shoes that rawl bolt to masonry, and are go for 10kN
and more... so one of those at each end of the joist will carry a
significant floor load. I needed to do one like this on my loft at my
previous place.
The bottom left of drawing:
http://internode.co.uk/temp/beam-layout.gif
Shows beam F terminating on one of these shoes. It in effect takes one
end of the entire floor load.
Do you have any details of what you used for that please? They could come in handy here...
So question 2: What�s the panel�s view on the best way to mount the ledger boards, particularly those
that carry the greatest loads? I�m thinking of resin studs between each pair of joist positions (in the
past I would have used expanding bolts but this is Victorian brick), but would a single large stud (M16)
be better or worse than a couple of smaller ones (M12) and is there any benefit in supplementing
these, with anything further (eg a pattern of multi-montis)? For the short ledger boards, I�m thinking
four studs in a rectangle around each joist hanger.
Using more mountings spreads the load, but over-perforating the boards would weaken them (though
I suspect it would take a lot for this to be a big issue).
I can't see much advantage going for M16 over M12 since they are both
going to be significantly stronger than the timber (especially as the
loading is across the grain).
The type of hanger used also makes a difference. With a suitably rigid
one that will not attempt to "unwind" and flip the wall plate over, most
of the load is simply pushing the wall plate hard against the top of the
masonry - there should be very little lateral load.
It seems there's definitely a case for a good snug fit on the joists, which would ensure the forces all resolve in the right directions.
Another option would be the masonry hangers that are designed to be
built into a leaf of brickwork. These can be retrofitted by raking out
some mortar and then mortaring back in. They don't require any bolting
as such.
I had thought of these but sort of dismissed them as it's pretty unlikely that the mortar courses are level enough across the space to end up with a level floor. That may not be a very good excuse nor that I didn't fancy spending too much time up there chiselling out mortar (!) but it just looks like they might not be such a robust fix as the timber mounted ones. I could easily be wrong there.
Lastly, to get decent access into the loft space (currently a 2x2� hatch in a cupboard), I need to create a
new doorway. The only way I can approach this is from the inside of the loft (due to obstructions I
can�t practically re-position until the opening is formed). Normally, cutting a new opening would be
best done using strongboys to support the triangle of brisk above until the lintel is installed but I�d
hesitate to jack against the top of the ceiling and getting them through the hatch wouldn�t be easy.
So question 3: Instead of using strongboys, would a board (4x1 or so) fixed to each brick (multimonti
into the brick centre) above the lintel do an adequate job of stopping everything moving while I get a
lintel in? (A bit non-standard I know, but the best option I can think of in the circumstances!)
That ought to do it. Is this a single leaf or double?
Just a single, so not too much to fall down !
Many thanks for your input John.
Cheers