Sorry chaps, a bit of a long post but I have tried to get all the essential= information in (!) Undoubtedly=20 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 f= or some roof timber repairs to be=20 carried out but ultimately it will be used for storage, not living space, s= o there are no formal BR=20 requirements but, of course, I=92d like it to be robust enough that it does= n=92t 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=20 stable platform to work on. It=92s in pretty good condition (as a ceiling,= not a floor) so I=92d like to keep it=20 that way, which means installing joists clear of the ceiling timbers. This= will, of course, create a void=20 for a good depth of insulation.
So question 1: To work above the ceiling, I=92m thinking of supporting it = with boards on acro jacks,=20 possibly moving these according to where I=92m working as I go and putting = some boards on the ceiling=20 joists to spread the load (mostly me!). Does this sound like a sensible th= ing to do =96 ie any better=20 suggestions?
One side of the space is an internal wall and the other side is a chimney b= reast with single-brick party=20 walls in the recesses each side (these look pretty weak). Each end is the = outside wall of the house=20 running up about 60cm from the ceiling to the eaves of the roof. Since the= shorter span (about 4m) is=20 from the internal wall to the chimney breast/party wall, the joists for the= new floor will run this way (ie=20 parallel to the outside walls).
The plan is to use ledger boards (wall plates if you prefer) and joist hang= ers to mount 50 x 200mm=20 (-ish) joists at 400mm centres. I know that socketing into the brick is fa= voured by some but that isn=92t=20 going to happen for many good reasons. To avoid going near the single-bric= k party wall, I=92ll have to=20 use trimmers across the (approx.) 1.8m recesses each side of the chimney br= east, mounting joist=20 hangers on these, so this is where the greatest loads will be: With 4 or 5 = joists on each trimmer, their=20 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=20 chimney breast and along the outside walls).
So question 2: What=92s the panel=92s view on the best way to mount the le= dger boards, particularly those=20 that carry the greatest loads? I=92m thinking of resin studs between each = pair of joist positions (in the=20 past I would have used expanding bolts but this is Victorian brick), but wo= uld a single large stud (M16)=20 be better or worse than a couple of smaller ones (M12) and is there any ben= efit in supplementing=20 these, with anything further (eg a pattern of multi-montis)? For the short= ledger boards, I=92m thinking=20 four studs in a rectangle around each joist hanger. =20 Using more mountings spreads the load, but over-perforating the boards woul= d weaken them (though=20 I suspect it would take a lot for this to be a big issue).
Lastly, to get decent access into the loft space (currently a 2x2=92 hatch = in a cupboard), I need to create a=20 new doorway. The only way I can approach this is from the inside of the lo= ft (due to obstructions I=20 can=92t practically re-position until the opening is formed). Normally, cu= tting a new opening would be=20 best done using strongboys to support the triangle of brisk above until the= lintel is installed but I=92d=20 hesitate to jack against the top of the ceiling and getting them through th= e hatch wouldn=92t be easy. =20
So question 3: Instead of using strongboys, would a board (4x1 or so) fixe= d to each brick (multimonti=20 into the brick centre) above the lintel do an adequate job of stopping ever= ything moving while I get a=20 lintel in? (A bit non-standard I know, but the best option I can think of = in the circumstances!)
Cheers