Boarding out loft after insulation

[matt80]

My loft is currently about 70% boarded out and I plan on getting the insulation updated as it is currently quite thin. I'd like to pull back the existing boarding so that the new insulation can be installed underneath.

However, I do need some storage space up there so I would like to put the boards back - but since the new insulation will be really thick am I right in thinking I'll need to build up the joists and create some sort of suspended floor to get the clearance I need?

Are there any gotchas to doing this - is it safe? What is the best material to use and best way of fixing it to the joists? Any advice would be much appreciated.

 

[JohnD]

You can add cross-battens.

For example, suppose your old timbers are 4" deep and run North/South.

You can put additional timbers, say 2"x2" on top of them running East/West.

That enables you to put 100mm insulation between the old timbers, and 50mm between the new ones, and put your boards on top. It also avoids you having boards resting on cables or pipes

Since the insulation will be unroled in different directions it reduces the risk of having thin spots or gaps.

Space the new timbers to be a close fit with the insulation you are going to use; but also so that there is one at the edge of your boards, under the joins.

This will not make your loft "floor" any stronger as a whole, so you can't use it as habitable space or storage for heavy objects, but it will spread your weight while you are walking about up there, and if all screwed tightly together, will make it more rigid.

 

[noseall]

love the north, south, east and west analogy John.

 

[matt80]

Thanks - that all makes good sense. I've not done an awful lot of jobs with timber before, what would you say is the best way of attaching the east-west timbers to the north-south timbers?

 

[JohnD]

long screws

or skew-nailed

but use screws not nails as hammering wil crack the plaster and make it fall off (can you say "skew-screwed"?)

Also, I'd drill pilot holes if I were you

 

[matt80]

just one last question: what does skew-nailed mean?

 

[JohnD]

the nails are not parallel to each other so they resist pulling out. You have to clamp or hold the timbers tightly in position before drilling your pilot and/or nailing, because the act of hammering or screwing in does not draw the timbers together in the same way that a straight screw would, and, in fact, tends to push them sideways.

It is not considered an elegant way of holding wood together, and you would be shocked to see it in furniture. It is adequate for rough carpentry that will not be moved about.

It has the advantage that you do not need to drive your screw or nail through the entire depth of the top piece.

It is used a lot in stud partitions where there is no real stress on the joint after construction.

 

[matt80]

Thanks John.

I've been up into the loft today and had a better look around. My existing timbers are around 2" width and 3" deep. To have the 10" insulation installed, that means I'm going to need pretty thick pieces of timber - will the extra weight of this cause any load problems on the existing joists? I guess they must be able to take quite a bit of weight as people have obviously been walking around up there for a number of years, but I want to be sure!

Matt

 

[JohnD]

yes, the weight will cause a problem.

two points:

although 250mm is recommended now, it gives surprisingly little benefit over 150mm. this is because you get diminishing returns on each additional thickness. 250mm is also quite impractical for walking about in

It is possible, after counterbattening, to lay rigid foam insulation slabs under the boarding. You will need long screws to fasten the boards down to the timbers, and it is essential that you do not step on the slabs as they may break.

 

[matt80]

I'm actually taking advantage of a local council grant to get the work done, so my choice of insulation is fixed. Back to the drawing board? I only need a relatively small area of storage, 2m x 2m would suffice...

 

[JohnD]

I suppose you could make a wooden platform raised 200mm above the original timbers. you could even cut back the fibreglass and put a couple of layers of rigid foam there instead over this small section.

If it is above a wall the roof timbers will be supported better rather than being in the middle of an unsupported span

 

[matt80]

Another thought! There are actually two much larger pieces of timber that run the entire length of the loft (i want to call them purlins, but i don't know if this is the right term or not).

Could these be used to create my raised floor level? I would guess they're 2 to 3 metres apart.

 

[JohnD]

yes but at that size gap you will have to put timbers across them like joists. With luck someone will be along in a minute with a table of sizes/loads/lengths

A picture might help

 

[Symptoms]

Easy solution - take-up boards, get the firm in to insulate, re-fix your 2m x 2m storage area after removing (lowering) small quantity of insulation. Your're not going to loose any meaningful insulating properties 'cos the timber decking will inherently have some.