How to raise level of loft boards?

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So I want to add insulation and then board over it - the joists are already 170mm deep so I only need another 100mm insulation. There’s not much headspace and loft legs are too long. I was thinking of running some timber around 125mm x 45mm across the existing joists which are 170mm x 45mm, so I have 25mm air gap.

First will this add too much weight to the ceiling? What spacing should the timber be, and what type of timber should I look for? Can I fix it by driving pairs of screws in at 45 degrees where it crosses the joists? I was thinking of this stuff -https://www.travisperkins.co.uk/treated-timber/c24-kiln-dried-regularised-sawn-treated-timber-47mm-x-125mm-x-4-8m/p/285417

Or is there a better way to do this?
 
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Sounds fine, spacing depends on dimension of whatever you are using to overboard with.
You could raise the height less by using rigid insulation board (better insulation performance that rockwool )and board directly over that which would mean No extra timber required.
 
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Sounds fine, spacing depends on dimension of whatever you are using to overboard with.
You could raise the height less by using rigid insulation board (better insulation performance that rockwool )and board directly over that which would mean No extra timber required.
You mean just rest the boards on top of the insulation board? Doesn’t laying impermeable ins board on top of fibreglass roll prevent air flow ? I read somewhere there needs to be an air gap above fibreglass or rockwool.
 
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I'm guessing this is a cold roof space you want to board over so you can store things up there.

Run your timber perpendicular to the existing joists. This limits the cold spots. As you're not storing gold ingots and baby elephants, so use something like 63 x 38. Where the timber crosses the joist, put a block of 63 x 38 as a stand off so you will have raised the floor 126. This approach will be a third of the weight of 125 x 45, and remember, you have to lift all this timber through your loft hatch. Run the new mineral wool perpendicular to the existing layer ie over the existing joists. Now fit t&g loft panels running perpendicular to your new timber.
 
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I'm guessing this is a cold roof space you want to board over so you can store things up there.

Run your timber perpendicular to the existing joists. This limits the cold spots. As you're not storing gold ingots and baby elephants, so use something like 63 x 38. Where the timber crosses the joist, put a block of 63 x 38 as a stand off so you will have raised the floor 126. This approach will be a third of the weight of 125 x 45, and remember, you have to lift all this timber through your loft hatch. Run the new mineral wool perpendicular to the existing layer ie over the existing joists. Now fit t&g loft panels running perpendicular to your new timber.
Thanks . Too late though, I’ve done it with 90x45. I’m hoping because its a small area (1.3m x 2.4m ) the lack of air gap won’t be an issue. I suppose I could run something on top of the new joists before the boards but I’m pretty sick of being in the loft already
 

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