Installing a loft floor

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

Reply to
GMM
Loading thread data ...

information in (!) Undoubtedly

some roof timber repairs to be

there are no formal BR

all wind up in the bedroom below.

ceiling joists, so not the most

a floor) so I?d like to keep it

will, of course, create a void

boards on acro jacks,

boards on the ceiling

to do ? ie any better

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.

breast with single-brick party

outside wall of the house

So the existing ceiling is not at the top of the wall as such then, but is suspended a bit below it?

new floor will run this way (ie

to mount 50 x 200mm

favoured by some but that isn?t

Socketing is rarely done these days it seems... hangers are the norm.

breast, mounting joist

joists on each trimmer, their

ledger boards that these trimmers will mount on will, of course, run at 90 degrees to the others (ie along the returns of the

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:

formatting link
beam F terminating on one of these shoes. It in effect takes one end of the entire floor load.

boards, particularly those

of joist positions (in the

a single large stud (M16)

benefit in supplementing

ledger boards, I?m thinking

weaken them (though

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.

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.

cupboard), I need to create a

(due to obstructions I

a new opening would be

lintel is installed but I?d

hatch wouldn?t be easy.

to each brick (multimonti

everything moving while I get a

the circumstances!)

That ought to do it. Is this a single leaf or double?

Reply to
John Rumm

Couple of thoughts... someone in here reinforced their existing joists rather than install new.

The other.. don't allow the new timbers anywhere near the lath and plaster. We once had a chalet bungalow where the previous owners had stiffened up the old loft floor to take the new bedrooms load. These joists spanned the outside walls but were not attached to the old ceiling ones. Inevitably the new timber moved and pinged nail head plaster off the downstairs ceiling.

Reply to
Tim Lamb

tial information in (!) Undoubtedly

rm for some roof timber repairs to be

e, so there are no formal BR

at it doesn=EF=BF=BDt all wind up in the bedroom below.

ing ceiling joists, so not the most

a ceiling, not a floor) so I=EF=BF=BDd like to keep it

This will, of course, create a void

orting it with boards on acro jacks,

d putting some boards on the ceiling

e thing to do =EF=BF=BD ie any better

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 th= e rest of the floor materials before the roof is sealed again. Then I'll w= ork from that 'platform'. What I'd like to do is avoid wrecking the ceilin= g at an early stage (!)

ey breast with single-brick party

the outside wall of the house

That's right: A bit unusual (and I haven't measured it) but about 2' of ve= rtical wall above the ceiling. Just for a change, something that might mak= e a job easier (famous last words!)

the new floor will run this way (ie

hangers to mount 50 x 200mm

s favoured by some but that isn=EF=BF=BDt

y breast, mounting joist

r 5 joists on each trimmer, their

hort ledger boards that these trimmers will mount on will, of course, run a= t 90 degrees to the others (ie along the returns of the

Do you have any details of what you used for that please? They could come = in handy here...

to mount the ledger boards, particularly those

ween each pair of joist positions (in the

t would a single large stud (M16)

benefit in supplementing

hort ledger boards, I=EF=BF=BDm thinking

would weaken them (though

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.

I had thought of these but sort of dismissed them as it's pretty unlikely t= hat 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 sp= ending too much time up there chiselling out mortar (!) but it just looks l= ike they might not be such a robust fix as the timber mounted ones. I coul= d easily be wrong there.

=BD hatch in a cupboard), I need to create a

e loft (due to obstructions I

rmally, cutting a new opening would be

the lintel is installed but I=EF=BF=BDd

h the hatch wouldn=EF=BF=BDt be easy.

fixed to each brick (multimonti

everything moving while I get a

of in the circumstances!)

Just a single, so not too much to fall down !

Many thanks for your input John.

Cheers

Reply to
GMM

Absolutely - If there was too much danger of damaging the ceiling, I thin i= t would make more sense to just take the lot down to start with, then put a= floor and new ceiling into the space, which was what the builder did next = door in the same place. They were dropping the ceiling height however to ac= commodate a loft conversion.

Reply to
GMM

for some roof timber repairs to be=20

so there are no formal BR=20

esn=92t all wind up in the bedroom below.

g ceiling joists, so not the most=20

g, not a floor) so I=92d like to keep it=20

2x8 joists is unusually large for loft storage. Almost all old joists are q= uite able to handle storage. So I wonder if you've fully and correctly eval= uated the situation, ie what the current joist dimensions are (all 3).

NT

Reply to
meow2222

rm for some roof timber repairs to be=20

e, so there are no formal BR=20

doesn=92t all wind up in the bedroom below.

ing ceiling joists, so not the most=20

ing, not a floor) so I=92d like to keep it=20

quite able to handle storage. So I wonder if you've fully and correctly ev= aluated the situation, ie what the current joist dimensions are (all 3).

Just going by the standard joist span tables for moderately loaded floors. = Of courser a lighter joist would be cheaper/easier/more desirable. On the= other hand, I'd rather not have the whole thing collapse!

Reply to
GMM

Its was a masonry fix shoe. In fact there is a picture there of it:

formatting link
just had a quick look at the design load, and it was 8.47 kN. From memory the shoe was rated at 12 or 15 kN.

Yup. You could by the sounds of it simply bolt a timber to the wall, and then nail hangers to that.

Its easy enough to add a timber packer under the beam where it goes into a shoe to tweak they height it sits in the shoe. For that matter you can even trim the bottom of the a small amount if needs be.

Reply to
John Rumm

Did the OP ever say exactly what his current joists were?

Reply to
Tim Watts

Not that I noticed:-)

The ones holding up our lath and plaster ceiling are around 3"x2". Raised tie/included attic so about 12' span.

>
Reply to
Tim Lamb

The simple answer is there aren't any - apart from the skinny (3x2 or so) c= eiling joists. Since these are 150 years old, I wouldn't walk on them with= out care, let alone base any kind of structure on them.

Reply to
GMM

ceiling joists. Since these are 150 years old, I wouldn't walk on them without care, let alone base any kind of structure on them.

You'll be walking on those when you do the work. Will you support them temporarily?

Reply to
GB

. Of courser a lighter joist would be cheaper/easier/more desirable. On t= he other hand, I'd rather not have the whole thing collapse!

This is a very common misunderstanding. BR requirements are based on sound = transmission requirements, producing sizes far in excess of those required = for safety. If the table says 2x8 you could use 2x4, fill the loft to the r= oof, and not be at the remotest risk. A 10' 2x4 can deflect at least 6" saf= ely, a full loft on 10' 2x4s provides a fraction of its safe load limit.

Re noise transmission, the gap between the 2 joist layers has more effect t= han 2x8s. And I don't expect anyone lives in your loft anyway. :)

NT

Reply to
meow2222

They are based on acceptable* levels of deflection rather than sound transmission (while there are building regs on sound transmission, they don't figure at all in structural calculations of joist sizes at all as far as I am aware)

(* Where acceptable is usually specified as a function of the joist length, or an absolute number of mm if over a certain length. The calculations will also check that the beam is not likely to fail in shear at the ends, or bending in the middle)

As you highlight, the purpose of the deflection limits is not because that is where a joist will fail catastrophically, but that is where any more movement would become unacceptable. L&P ceilings can move a fair amount, but don't expect one that is supported by a floor structure that deflects 5 inches every time someone walks over it to last long!

Loft floors (once strengthened) are actually very good for noise insulation since they typically have two separate sets of joists carrying the floor and ceiling loads.

Reply to
John Rumm

rs. Of courser a lighter joist would be cheaper/easier/more desirable. On= the other hand, I'd rather not have the whole thing collapse!

d transmission requirements, producing sizes far in excess of those require= d for safety. If the table says 2x8 you could use 2x4, fill the loft to the= roof, and not be at the remotest risk. A 10' 2x4 can deflect at least 6" s= afely, a full loft on 10' 2x4s provides a fraction of its safe load limit.

than 2x8s. And I don't expect anyone lives in your loft anyway. :)

Well, you're right: Nobody does live in my loft ...at least not that I'm d= are of...though you never know=20 who might sneak in there (according to the Daily Mail at least....(!) I'm = anticipating that this floor will=20 ultimately be loaded with all the junk you normally find in lofts - broken = toys, old masters and all that.

On the other hand, I'm not sure it would feel too good to walk on an unsupp= orted 14' span of 2 x 4=20 and if it deflects 6', then I would have to mount them higher than 2 x 8s s= o they didn't bang on the=20 top of the ceiling.

I have been working from the BR tables as a) they would seem to give the be= st outcome and b) I=20 couldn't find anything else that gave useful information.

I shall take another look and see if I can find any useful info on this. P= erhaps 2 x 6s would do the job=20 reasonably, although there is an attraction in 2 x 8s in that a future (cur= rently completely off the radar)=20 loft conversion would be possible without having to take it all down and st= art again.

Of course, there is an argument that smaller timbers in themselves reduce t= he total dead weight of the=20 floor a bit but I should think this is a pretty minor effect as is the impa= ct on overall cost. =20 Probably the most compelling argument (for me) could be the fact that I hav= e to lug them up 3 floors=20 to get them in, but that only needs to be done once.

Reply to
GMM

Put in the void as much insulation as possible. Also you might want to renew the lighing cables while you are it, and install more in cae of new light positions in the romms below. And install some 2.5mm cable ready to fit sockets up there in case. Even some pipe ready to replace a tank, etc. You don't want to rip it all up a few years down the line.

Reply to
Doctor Drivel

Not to mention the possibility of seasickness every time anyone bounces around up there !

I'm hoping this one will be good for heat insulation too, with a full load of insulation between the ceiling and the new floor.

Reply to
GMM

I couldn't agree more: Flooring over old cable would be a significant error and my first job will be to put in some decent (and redundant in case one blows) lighting to replace the one batten holder there at present, along with a socket or two, spurred from a handy point close by. The (for want of a better term) internal gable end wall is adjacent to the stairs and top floor rooms so the plan will be to put a layer of celotex over this once I have a floor I can stand on to do it.

Reply to
GMM

ceiling joists. Since these are 150 years old, I wouldn't walk on them without care, let alone base any kind of structure on them.

He said he was thinking of boards on Acro props. Knowing exactly which bits are supported from below when working above might be a bit tricky, but at the least they'll effectively have a much shorter span.

Reply to
Alan Braggins

ceiling joists. Since these are 150 years old, I wouldn't walk on them without care, let alone base any kind of structure on them.

Quite straightforward really: The loft is exactly the same as the room beneath, so props can be put directly under the points where work is taking place and moved as it progresses. Using boards above and below will spread the load. As you say, even off (but close to) a board this will effectively shorten the span of timber so reduce stresses. Certainly, in my view, preferable to buggering up the ceiling.

Reply to
GMM

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