Raising loft joists

[kieranmc33]

I want to add another layer of insulation to the loft, but I also want to partially floor it for storage. I have blanketed the shallow sides of the loft but for the centre within the purlins I want to raise the floor by adding joists at right angles to the existing ones, fill this in with insulation and then floor over the top.

Assuming that's ok and the right way to do it, what's the best timber for the new joists? 4x2? (100mmx47mm). And what's the best way to fix them to the original joists? Just screw straight down through? Or is from the sides ok? Or is there a fixing bracket or plate I should be using??

Thanks

 

[PrenticeBoyofDerry]

Your about spot on with what you are proposing to do.
New timbers perpemdicular to your existing loft joist.
They want to be centred at 400mm, so they can take yor loft panels/boards with minimum of cuts.
The recommended loft insulation depth is 270mm, so if your exisiting joist are say 100mm, your additional ones should be 170mm to make the space up to 270mm.
Any sawn timber will do, when boarding your loft out, it is best to start at the loft hatch.
Also consideration should be made to the electrical cables, they will heat up if totally surrounded by insulation.
Lighting circuits on 5/6 amp fuse/breakers are normally ok, but any socket or shower circuits up there should be calculated and installed using the correct methods or the cable may need to be upgraded.
Screw from side/top or use fixing brackets, the choice is yours.
I personally go in from the sides (both sides).
Remember when insulating to leave gaps at the eaves of the roof to allow for ventilation, this will help prevent condensation.

 

[kieranmc33]

Thanks for that PrenticeBoy, very comprehensive. I've used the slack in the electrical cables to lift them and sit them on top of the new layer of insulation, so they're not surrounded. And I've not gone tight into the eaves, but I'll keep an eye out for condensation over time

 

[officialsimon]

Surely lying cables on top of insulation is the same fire risk as laying underneath on the ceiling below? If they are touching the rockwool and heat up they can still be the fire risk? Isn't the best way to conduit them?

 

[PrenticeBoyofDerry]

officialsimon,
If the cables are laid above thermal insulation, heat can dissipate through the uncovered side and is not considered a risk.
If the cables were run through conduit and that was also within thermal insulation, again a derating of the cable will need to be made.
It is a preferred method not to have the cables enclosed within insulation.
But this does not mean it can not be done, providing preventative measured are made against the risk of fire.
This would mean either down rating fuse, not normally practical.
upgrading of cable size or the correct installation methods used to prevent the upgrading of cables.
But as you quite rightly have mentioned, the cables can be routed below the insulation or clipped to joist, providing the cable is not surrounded by 50mm plus of insullation. Then the heat can dissipate
through the timber joists or plasterboard ceiling.

 

[geraint]

Your about spot on with what you are proposing to do.
New timbers perpemdicular to your existing loft joist.
They want to be centred at 400mm, so they can take yor loft panels/boards with minimum of cuts.
The recommended loft insulation depth is 270mm, so if your exisiting joist are say 100mm, your additional ones should be 170mm to make the space up to 270mm.
Any sawn timber will do, when boarding your loft out, it is best to start at the loft hatch.
Also consideration should be made to the electrical cables, they will heat up if totally surrounded by insulation.
Lighting circuits on 5/6 amp fuse/breakers are normally ok, but any socket or shower circuits up there should be calculated and installed using the correct methods or the cable may need to be upgraded.
Screw from side/top or use fixing brackets, the choice is yours.
I personally go in from the sides (both sides).
Remember when insulating to leave gaps at the eaves of the roof to allow for ventilation, this will help prevent condensation.

400 mm centres...

surely you want the flooring to join over a joist....