Replacing a joist on a flat roof

[Snowbound]

Re- boarding my ceiling and I've discovered one of the joist has irrepairable dry rot!! Joists are spaced at 400mm. Can I remove the rotten joist and replace without having to support the ceiling or will I have to get an acro??

Many Thanks.

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[Deluks]

Tricky one, as I assume the roof itself will be fixed to the top of the joist. Does the roof leak? The rot will be there for a reason, moisture and or lack of ventilation will cause it in a roof void.

Is there any insulation in the roofspace? What type? airgap?

When replacing the ceiling be sure to use foil backed plasterboard.

Ok just noticed photos, you'll need to remove all timber about 500mm past the rotten area, and fix a new one to the side of the old one with a couple of m12 bolts. Joist hangers fixed to the wall are far better than building joist ends into the brickwork. The roofing deck could also be infested so the whole lot will need treating.

As well as foil backed plasterboard, you'd be wise to get some insulation up there as well. Full fill would normally be used but given this problem I'd insulate partially and remove some of the bricks right at the top to let some air in. Remove at least one half brick between each set of joists, and at the other end as well, this will ensure a through flow of air between each set of joists and under the plywood deck. Cover gaps with some vents or stainless mesh.

When removing the rotten member cut upwards at an angle so the rotten end can be easily removed. You need not replace the short section of firring strip, just pack out every 400mm between the joists top/deck with some offcuts.

 

[Snowbound]

There's no evidence of any leak. Standard fibre loft insulation with about a 70mm air gap. Would there be any issues if I used standard plasterboard?

 

[Deluks]

Don't use standard plasterboard, that's what causes problems like this in the first place by letting moisture from the room into the roof void.
If you have bought a load already then you can buy polythene vapour barrier which should be stapled to the joists before fitting the plasterboard, tape all holes/overlaps and leave no gaps at all.

Have you checked gutter? Could be leaking inwards and soaking fascias/joist end?

Replacing the insulation with a celotex/kingspan/xtratherm type will keep a lot more heat in. You could also just keep the existing and fix board insulation under the joists, will cost 20-30mm of ceiling height, but would be worthwhile.

You can get plasterboard with the insulation already fixed to it, but if you already have standard plasterboard then just buy the insulation seperate and screw through it with extra long screws when fixing the PB.

 

[Chukka63]

you wont need an acrow for what you want to do.

 

[Deluks]

you wont need an acrow for what you want to do.

Nice one, all that typing and I didn't even answer the original question (which TBH is the least of his probs)

 

[Chukka63]

no but you saved me the trouble

 

[Snowbound]

Thanks guys! very much appreciated!!