Raising Roof Joists for Loft Flooring

Wickes deliver too, for a small fee. I'm not sure where all these "other places" are. I don't know any reputable builder's merchants, and wouldn't know how to tell if one *were* reputable. Wickes was recommended to me by the nice man who did my central heating, and did a very nice job of it too.

As mentioned in my original post:

That's why I'm intent on adding timber :)

Yep, I wasn't going to lay the insulation all the way to the eaves, or to the ceiling.

Bugger, this was the sort of thing I was worried about. Will I need to stick the timber up in the loft for a few months until it adjusts to the weather conditions up there before I can cut and fit it?

Dave

Reply to
Dave Page
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I do hope that was meant mathematically! ;-)

Reply to
John Cartmell

Crossways is good.

Plain Builders English.

Reply to
The Natural Philosopher

Orthogonal means 'at right angles to'

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Reply to
The Natural Philosopher

Lay the joists at right angles and use joist hangers where they meet the walls if any (gable end). Use 6" screws. Unifix supply these quite cheap. Having the at 90 degrees reduces thermal bridging enormously.

Reply to
Doctor Drivel

As he is removing the old insulation, it is best seal up the holes where cable and pipes penetrate with silicon, then lat a poly vapour barrier on the plasterboard/joist. Pin it with a staple gun to side of the joists and tape the joins with big overlaps. Then lay the insulation and lay the new joists. Then make sure the hatch is sealed and insulated too.

Reply to
Doctor Drivel

Lets review the options.

  1. Lay new joists on old at 90 degrees This will leave you with a very weak floor, as well as being a pain to insulate.

  1. Lay new joists at 90 degrees on joist hangers into the walls This is illegal in a lot of properties under the Party Wall Act, and might get you into a bundle of trouble.

  2. Add new 4x2 joists to the top of the old ones, inline, attaching with glue and screws. This is legal, and will bring a dramatic increase in strength to your floor/ceiling structure, enabling you to stack goods to the roof without worry of collapse, distortion, etc. When joining timbers, (since it isnt easy to get a full length one in there) the structure will be stronger if you glue joint them rather than just butt them, but eiher will help. Laying insulation is easy.

  1. Add new wood on every other joist This will cause damage to the ceiling, as the differing loads will try to mildly corrugate the non-flexible plaster ceiling. Expect cracking and damage.

Any 2x4 will do, as long as its straight. Spruce aka whitewood is cheapest.

NT

Reply to
meow2222

This is a well accepted method when the existing ceiling rafters are already almost deep enough.

The aim of gluing and screwing the two parts together is to prevent the composite beam from bending by one part sliding over the other. The glue is much more important than the screws because the large bond area distributes the shear forces more evenly. The main contribution of the screws is to help make a good glue bond, and afterwards they are almost incidental.

In loft conversions, this is usually supplemented by glueing and screwing the chipboard floor down, which adds yet more stiffness to the whole structure.

Reply to
Ian White

That means the load on the loft floor would only be supported by half of the joists, and the boards would be spanning a considerable gap - neither of these is a good thing.

The boards go across the joists, not along them.

Reply to
Rob Morley

I'd have thought 'normal' was the more usual term.

Reply to
Rob Morley

Is that just to stop drafts, or for any other reason? The ceiling appears to be chipboard rather than plasterboard here.

Dave

Reply to
Dave Page

Hmm, why would this lead to a weak floor? I figured that laying the new joists at 90 degrees to the old ones would spread load across the old joists, and laying the new floorboards across the

This method has the advantage that I can space my new joists at 400mm (the pre-cut width of the insulation roll) rather than having to deal with the existing 350mm joist spacing, and means that I can do the loft in fewer "strips" of insulation (three or four as opposed to a dozen).

Other people have commented that insulation roll is "supposed" to be laid *across* the joists. I have to assume that this precludes flooring the loft space, and that if I want to floor my loft space, I'll need to lay the roll *along* the joists.

Yeah, this was my original plan, and looks like being what I'll end up doing after all, possibly using the poly vapour barrier mentioned by Doctor Drivel in also.

OK, I appreciate now that this is a bad idea.

Dave

Reply to
Dave Page

Lay insulation between the joists and another layer 90 degrees to that.

I have seen a loft floor raised over 1 foot by using short pieces of 4X2. all the same length. These were on end on the existing joists vertically, and secured by small angle pieces. On top on the 4x2s long lengths of 4x2s were laid and 6" screws driven in. These were secured to the gable end wall by joist hangers, which also takes weight off the ceiling. The odd cross brace was inserted so the floor did not wobble, and the chipboard laid on top being well screwed down. The chipboard tightened up the whole floor. MDF was used around the loft hatch, which also added rigidity to the floor too. Easy to do if you have a chop saw and an Impact Driver. This gap was filled with spray-in Warmcell insulation, which also makes the ceiling air-tight too. Warmcell has the equiv performance of Rockwool that is 25% thicker.

Reply to
Doctor Drivel

About 42% of heat loss in UK homes is via air-leakages. Preventing water vapour from entering the cold loft prevents condensation on the timbers, hence the sealed loft hatch.

Reply to
Doctor Drivel

I can't do that without laying the new joists at 90 degrees to the old joists, which I'm told upthread would be a bad idea, or covering the joists.

Hmm, this sounds like a bigger project in terms of time and complexity than I can budget for at the moment. Nice idea though.

Dave

Reply to
Dave Page

you'd have exactly the same wood supporting as you have now, and 3" is marginal. Spreading a roomful of load out over a room makes little difference, it only helps with high point loads, not with total load, deflection or rigidity.

minor in comparison to leaving yourself with a borderline floor support structure.

thats how its done when theres no flooring. With flooring, the goods themselves will trap air, if packed in cardboard boxes, and so will insulate some more. You can roll insulation out on any unused floored areas if you wish.

yes

NT

Reply to
meow2222

It is not. It speads the load and vastly reduces cold bridging.

Not expensive to do. Yet the results are well pleasing in many aspects.

Reply to
Doctor Drivel

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