Installing a loft floor

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, Id like it to be robust enough that it doesnt 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. Its in pretty good condition (as a ceiling, not a floor) so Id 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, Im thinking of supporting it with boards on acro jacks, possibly moving these according to where Im 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?
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. 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 isnt going to happen for many good reasons. To avoid going near the single-brick party wall, Ill 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).
So question 2: Whats the panels view on the best way to mount the ledger boards, particularly those that carry the greatest loads? Im 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, Im 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).
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 cant 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 Id hesitate to jack against the top of the ceiling and getting them through the hatch wouldnt 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!)
Cheers
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On 17/10/2012 11:49, GMM wrote:

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.

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

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

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:
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.

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.

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

John.

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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.
--
Tim Lamb

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On Wednesday, October 17, 2012 4:05:44 PM UTC+1, Tim Lamb wrote:

Absolutely - If there was too much danger of damaging the ceiling, I thin it 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 accommodate a loft conversion.
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On Wednesday, October 17, 2012 3:28:07 PM UTC+1, John Rumm wrote:

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 (!)

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!)

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

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 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.

Just a single, so not too much to fall down !
Many thanks for your input John.
Cheers
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On 17/10/2012 20:42, GMM wrote:

Its was a masonry fix shoe. In fact there is a picture there of it:
http://internode.co.uk/temp/shoe.jpg
I 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.
--
Cheers,

John.

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On Wednesday, October 17, 2012 11:49:06 AM UTC+1, GMM wrote:


2x8 joists is unusually large for loft storage. Almost all old joists are quite able to handle storage. So I wonder if you've fully and correctly evaluated the situation, ie what the current joist dimensions are (all 3).
NT
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On Wednesday, October 17, 2012 8:56:57 PM UTC+1, (unknown) wrote:

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!
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On Wednesday, October 17, 2012 9:07:08 PM UTC+1, GMM wrote:

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 roof, and not be at the remotest risk. A 10' 2x4 can deflect at least 6" safely, 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 than 2x8s. And I don't expect anyone lives in your loft anyway. :)
NT
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On 18/10/2012 13:11, snipped-for-privacy@care2.com wrote:

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.
--
Cheers,

John.

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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.
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On Thursday, October 18, 2012 1:49:59 PM UTC+1, John Rumm wrote:

Whether that's true depends on how you define acceptable. Limiting deflection to 3mm over 10' has absolutely nothing to do with safety or what I would call acceptable deflection limits.

Well, not in any sense of acceptable that I would buy into. Lots of old houses have ceiling deflection levels numerous times as big as BR allows, and I don't see any problems or complaints resulting.

Timber that light isn't being discussed, and the OP's new joists won't be supporting a ceiling.
If 3.1mm deflection would upset the op, go with the tables. I personally wouldnt have any problem with half an inch on a heavily loaded section
NT
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On 18/10/2012 23:56, snipped-for-privacy@care2.com wrote:

(the limit is somewhat higher than 3mm in 10')
However, its not just about safety, but also producing an adequate quality of building, where the floors don't bounce up and down, and all the joints crack in the plaster because there is too much movement.
There are other requirements that are also taken into account with the sizes commonly used, such as minimizing twisting of joists (although strapping will still be required over 2.5m), and providing lateral restraint of walls where required.

I have yet to be in a house where the floor deflects 6" while walking across a room. Personally I would take that as an indication one should get out fast!
However it is true that what was acceptable in the past would no longer meet current standards. That is partly a reflection on more stringent requirements for air tightness of properties, and also changes in materials. Lime mortar, and soft bricks will accept more movement than modern materials for example.

I was responding to your comments about the 2x4 which can deflect 6" safely rather than the OPs storage floor. A 6" deflection may not cause it to fail, but it would be excessive by any measure, and would not be suitable even for loft storage.
The OP can use lighter timbers than those that would be required to meet the standard of a floor if he wants, although if there was a suggestion that at some point in the future it was upgraded to a habitable room it would be sensible to build it to the required standards now obviously.

3.1mm would be fine even for a floor (the limit on that length would be around 8.4mm IIRC).
--
Cheers,

John.

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On Friday, October 19, 2012 1:26:36 AM UTC+1, John Rumm wrote:

No, its not. Millions of old houses have timber a fraction the size of the current requirements and don't suffer any of those problems.

obviously that's not relevant

I've never known a Victorian ceiling/loft floor be draughty

Slight movement of loft floor does not move the brickwork. Modern PB is much more tolerant of movement than lath & plaster.

Obviously a floor structure that deflects 6" in use has never been proposed. My point was that deflection levels encountered in real life floors are a fraction of failure limits

Standards change, so its not obvious, its just an option

NT
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On 19/10/2012 13:38, snipped-for-privacy@care2.com wrote:

True, so long as the fraction you have in mind is say 7/8ths...
Many places in the past would have used say 7x2 where these days 8x2 would be deemed adequate.
There are places which use significantly shallower timbers, but then tend to have more frequent cross members and hence shorter effective spans.
Many of our downstairs floors here are only on 4x2" joists - but these span dwarf walls often at 4' spacings - so they are quite rigid.

Modern buildings are in general very much less draughty than Victorian places. There have been many changes in building practices to achieve this. One example being joist support. It is no longer common to set joist ends into openings in walls since (among other things) it also creates a potential air gap. The switch to joist hangers also frees you from the limitation of needing to use joist sizes that match a brick height multiple in height.

I did not say it did - it was an example of building material that is more tolerant of movement. Perhaps horsehair reinforced plaster would have been a better example.

Its different - not necessarily always better.

No one (including the building regs) is attempting to suggest otherwise.

Indeed, but they rarely reduce, so its fairly obvious (to me at least).
Also building regs are not retrospective. So if a floor was designed as a floor, and was compliant with the standards of the time, you would be able to use as the basis of your room in the roof, it even if the standards applying had changed since it was built.

--
Cheers,

John.

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On Friday, October 19, 2012 3:53:36 PM UTC+1, John Rumm wrote:

I'm talking about 3, 4 & 5" timber where new builds would use double to triple the size

But one place draughts don't come in is through Victorian ceilings

There's no way a BCO will accept a loft conversion in a 1924 house on its original 3" loft floor joists.
NT
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On 20/10/2012 01:49, snipped-for-privacy@care2.com wrote:

I just noticed that you superficially said *loft* floors there, and my comments that followed were more generally about "floors" in general - my fault for not reading carefully.

There is a general drive to cut down air currents through void spaces in direct contact with heated surfaces. If you have a penetration through a wall, then air will flow through it. Even if that never discharges directly into the living space, it lowers the thermal performance of the building.

I doubt a loft with 3" joists would not have been deemed acceptable as a proper floor for a habitable space - even in 1924.
However, my point was, that if you upgrade something now to the current standards of a floor in a habitable room, then there would be no need to upgrade it further if one later made the space habitable - even if the standards for a floor have changed by then.
--
Cheers,

John.

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On Saturday, October 20, 2012 4:42:00 PM UTC+1, John Rumm wrote:

3x3 was the smallest standard habitable flooring joist size in Victorian houses. It was much used for short spans, such as across corridors & landings.
IIRC the 1924 BR didn't specify joist sizes, so 3x3 would still be compliant for habitation then. It could be used in loft floors above corridors, where the span was short.
I challenge you to find any BCO that would accept that in a loft conversion today.
NT
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On 20/10/2012 20:33, snipped-for-privacy@care2.com wrote:

And it still might be acceptable now (for short lengths)

A loft floor is not a floor in the accepted sense though - its not expected to carry significant load.

A BCO would be happy with a loft using 3x2 - its a good deal better than many a lofts built with modern trusses. However that is a very different thing from a loft floor which going to be used for a habitable room. If you are converting the loft, then the same spec as would apply to any other floor in the building will kick in.
Out of interest I had an experiment with superbeam to see what you can get away with on a 3x2 (well 72x47mm) and a typical floor load (uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about the limit - so you could probably still do a landing with it and comply with modern building regs. (having said that, its generally simpler to use one depth all over to save having to buy lots of timber sizes)
--
Cheers,

John.

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On Saturday, October 20, 2012 10:33:20 PM UTC+1, John Rumm wrote:

For clarity, lets take it a step further. Say the loft got 2x2s in 4' spans in 1924, hopelessly unsuitable for habitable rooms, but still compliant for them in 1924. So it was built in compliance with BR standards for habitation at the time, and you can indeed walk on them, just about. But no BCO in their right mind would accept a conversion to habitable now on 2x2s.
NT
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