Strengthening Loft floor

11 Jun 2012
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United Kingdom
I am looking into having my loft converted for storage. I want the floor boarded, roof plasterboarded and loft ladder installed.

The loft currently is full of a lot of boxes possibly 100kgs worth.

I have had someone come round to quote for the work and they
have advised me

''that they are not able to strengthen the original ceiling joists sufficiently by cross brassing them with 4' * 2' joists because the ceiling joists are 3' * 2' on a span of 3.5m and might deflect too much when weight is placed on them''.

They propose to fit a couple of steel beams so that we can lay a floor that won't rest on the ceiling, and in the process re support the roof on those steel beams giving you an open loft space. they would also need to strengthen the purlins supporting the roof since they are very thin, specially the one at the front of the house.

This is a 1870s cottage with an A frame but 2 trusts on either side.

The quote is twice as much as their website quoted and I just want to make sure I am not being taken for a ride. My question is could the floor be strengthened without the need for the rsj? is the point regarding the joists correct ?

any help greatly appreciated
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If the loft is just being used for light storage, it is usally possible to beef up the existing ceiling joists.
Glue and screw new 4x2s along the tops of the existing joists (NOT at right angles), Use 4" 12s screws or similar at 12" centres, (bore 2" deep by 1" dia holes first) and use plenty of PVA glue.
The new joists don't have to go right to the ends - stop them a foot or so short. Finish with plywood or OSB on top.
The cheapest way to strengthen a loft floor is cross bracing, not gluing and screwing new 4*2 on top of existing joists, that just add dead weight to existing joists, and if they are 3*2 that will make things worse. Tony1851 would be better off adding 6*2 from wall plate to supporting wall on a 3.5m to 4m span, with noggins in between.
Now ceiling joists are not designed to take a great load of weight, the best and safest way to do so is to have a floating floor, ie rsj with floor built from rsj rather than resting on ceilings. Ceiling joists are often 4*2, the thinner, the longer the weaker they are! It's more hassle and costs (two 9"*6" rsj were over £1k for my loft!) but weight of the roof can also be taken off the RSJs, strengthening your roof substantially in the process and giving it a new lease of life.
If your purlings are in poor conditions, you might want to sister a new 7*2 to existing ones with more strats from supporting walls, much cheaper than rsj ect but less space in the middle of loft since you'll have more strats. You will still have to strengthen you ceiling joists somehow if you want to avoid damaging you ceilings below!
In any case check with structural engineer or local building control. Good luck!
The cheapest way to strengthen a loft floor is cross bracing, not gluing and screwing new 4*2 on top of existing joists, that just add dead weight to existing joists, and if they are 3*2 that will make things worse. Tony1851 would be better off adding 6*2 from wall plate to supporting wall on a 3.5m to 4m span, with noggins in between.

Jostratham.If you read the OP properly, you would see that he just wanted to use the loft for storage. Your suggestion of new floor joists would be OK if he was doing a proper loft conversion, but would be expensive. He didn't necessarily want to go down the route of putting in a completely new floor supported off steels etc.
My suggestion of gluing and screwing additional 4x2s to the tops of the existing 3x2 is perfectly sound; it doesn't 'just add dead weight' to the existing ceiling joists, but effectively deepens them and so increases the 'S' and 'I' values. It won't turn them into 7x2s, but they won't be far off this in structural terms and it would be a cheap and effective way of strengthening the existing joists for storage.
If he wants to put steels in partly with a view to propping the existing purlins, then that's another matter, but we don't knoww the state of his roof; we are only going off what a builder has suggested and that advice may be in the builder's own interest.
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Hi again, do the 4*2 pieces have to be in one length or can it be cut in say 2 pieces then glued/screwed end to end? I am just wondering how I am going to get the joists into the loft. Thanks
if you really want it for storage, why do you want it plasterboarded?
Hi there I don't want it plasterboarded I just want to strengthen the loft floor so it can take more load. Someone suggested glueing and screwing 4*2 to existing 3*2s but I'm just wondering if the new 4*2s have to be single pieces as I'm trying to figure out how to get the new wood into the loft. Thanks
If you screw and glue new joists on top of the existing, the new ones dont have to go the full length uninterrupted. But, if there are to be any joint(s) in the new pieces, they should be as far away as possible from mid-span.
For example; if your ceiling joists have a span of, say, 12 ft, and you can only get, say, 8ft pieces in the loft, fix the new so that they are central over the span (ie leaving 2ft of ceiling joist each end) You can then piece-up with shorter pieces at the ends.
The reason for this is that the bending stress is greatest at mid-span, so that is where you don't want a joint. The stress tails off rapidly when you get to the ends of the joists.
Thanks tony 1851
Just out of interest, if the original joists are say 12ft and I put 8 ft joists on top, centrally as you describe...
If I don't intend to use the last 2ft of the edge of the loft (say because there is not enough height to access), is it ok to just put 8ft joists centrally on 12ft joists then only board and load the joists which have been deepened in the 8ft central portion?
Do I have to reinforce the last 2ft of the joist at each end if I don't intend to use that part of the joist?
I have read this thread with interest as I want to raise my loft floor to add additional 100mm insulation and replace the chipboard.

In the process I'd like to strengthen the floor rather than weaken it. I have similar 3x2 main span (about 7 metres) joists with a presumably somewhat supporting partition (lathe and plaster) wall mid-way underneath. Tony's idea of gluing and screwing additional 2x4s on top seems like good advice, though there are 3 cross joists/beams(?) on top dividing the main span into 4 sections, and so cannot get a good length of new supporting joist on top of the mid section of the old ones. I am inclined to cut 2x4s to fit into the 4 sections and secure them to the cross joists with screws through the latter into the ends of the 2x4s. Is this a reasonable way of doing it? Actually, I was thinking to reinforce only every other joist (14" spacing between each joist) which would be sufficient to support the chipboard. Would this be sufficient?

Incidentally, the cross joists are also embedded in the brick wall on end and disappear into woodwork around the eaves the other, and the middle one is as deep as the main-span joists and also appears to run along the partition wall. Would this seem to suggest a fairly strong floor anyway? It does seem to sag in the middle, however, and in both dimensions.

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