another loft insulation question

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.
Any thoughts?
AJH
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andrew wrote:

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.
some points.
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 loose insulation.
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.
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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:
http://www.screwfix.com/prods/11067/Ironmongery/Brackets/Brackets/Corner-Braces-Zinc-Plated-39-x-39-x-16mm-Pack-of-10
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.
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Did what? There were two options.
MBQ
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*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.
Cheers Richard
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geraldthehamster wrote:

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.
AJH
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wibbled on Sunday 21 February 2010 20:57

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 1 fails.
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Bearing in mind that a 4x4 beam is rated less than an 8x2... ;-)
Cheers Richard
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wibbled on Monday 22 February 2010 15:57

Vertically stacked, not horizontally...
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Oh I see, sorry.
Cheers Richard
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The size of the joists will depend on the distance they span. There were tables for this in the old, 1992 edition of Part A of the building regs, which can still be found at:
http://www.planningportal.gov.uk/wales/professionals/buildingregs/technicalguidance/bcstructuralsafetyparta/bcapproveddocuments

Probably, depending on the volume added by the dormers:
http://www.planningportal.gov.uk/uploads/miniguides/extensions/extensions.html
The law on PD changed in 2008
Cheers Richard
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