Timber for boarding out loft.

[RichA]

I am due to board out my loft eaves with chipboard; unsure what type of timber is most suitable (and economic) to raise up the boards.

Current joists are a mix of 147mm (side eaves) & 174mm (rear eaves).

The boarding is to allow access and for lightweight storage.

Thanks

 

[ktuludays]

Why not use t&g loft boards, fairly cheap and due to their size easy to get into a loft

 

[RichA]

Thanks, that is what I intend to use; But need timber joists to raise them above the 270mm of insulation (I have edited my OP to make that clearer).

How much compressing of insulation is deemed ok?

 

[SFK]

So you need to make it 174-147 = 27mm taller. As it is the loft you could screw these 25mm batons on the front joists and ignore the 2mm raise at back. Wickes also have the toung and grove floor sheets.

https://www.wickes.co.uk/Wickes-Treated-Sawn-Timber---25mm-x-38mm-x-1-8m/p/9000036483y

 

[oldbutnotdead]

Ahh,, you want about 125mm of lift. Compressing loft insulation does degrade its performance quite a lot. First thing, have a measure up and see what linear metreage you need at what height (so if there are 20 joists at 4m each that need lifting by 100mm you need 80m of that depth).

Then have a look round the DIY sheds- there's usually an offer on with CLS timber, see what the per metre price is, remembering you'd probably need to double it up

Then tour the timber merchants, see how much 4 x 2 or 5 x 2 is going to cost you (again per metre)

Another method you can consider (for light loading) is either 150 x 25 fence boards (screwed to the sides of the joists) or ripped 25mm plywood (same dimensions)- the ply will be stronger but more expensive.

And finally there are those plastic loft legs- I've never used them, they look really flimsy and seem to cost far too much.

When you've worked the price out, consider what you're planning to put up there & ask yourself if it wouldn't be cheaper and better to get rid of it all- if it is going in the loft it means you're not using it much!

And final final- is your roof a cut roof or is it trussed? If trussed (skinny timbers with nail plates usually) then there is very little spare capacity in it for storing stuff anyway

 

[RichA]

Thanks. I was anticipating that I might raise the 147mm by 97mm (to 244mm) and the 174mm by 70mm (also to 244mm).

This would mean minimal squashing and presumably 18mm chipboard adds some insulation value...

What is the best way to attach to joists? A screw on the diagonal? (Some are cross/perpendicular, others are aligned/parallel)

 

[oldbutnotdead]

The R value of chipboard is negligible- slightly better than plasterboard but not much so you can forget that as a defence.
Fixing- either screw straight through or use joist hangars in reverse. Depending on how much traffic or rubbish you anticipate piling up there you might want to think about nogging between the upstands (to prevent the whole lot racking)- more of a potential issue on the 97mm and if you use decent sized screws through the chipboard you can probably get away without.

 

[cdbe]

I've never used the legs, but would consider them to avoid the cold bridging of the timber - normally (when not boarded) the subsequent layers of insulation are laid perpendicular to the joists and negate this - but I don't know what they cost. If you use new "joists" then try and lay them perpendicular to existing to minimise cold bridging.

 

[RichA]

Is 600mm centres ok for 18mm chipboard? It is round the eaves so no large/heavy objects (other than myself, occasionally).

 

[oldbutnotdead]

Yeah it'll do as long as you don't pile too much carp up there

 

[Mottie]

My ceiling joists are 4”x2” and I had 100mm of insulation. Last year I laid 6”x2” timbers on them, squeezed in 170mm of extra insulation and used the T&G chipboard loft boards. I only boarded the central area of the loft and just laid the insulation on the outer edges over the existing joists/insulation.