Raising Roof Joists for Loft Flooring

[Dave Page]

I am planning on insulating my loft to the recommended 270mm, and would also like to lay floorboards above the insulation so I can keep using the loft for storage. The gap between the ceiling and the top of the existing joists is 70mm, so there's 200mm to go.

I'm intending on adding two strips of 40 x 100 on top of the existing joists, screwing them down, and laying the insulation between them and boards on top. It seems like a fairly simple job.

There's one bit of information I'm missing; what sort of timber would be appropriate for this job?

Dave

[Lobster]

When topping up old insulation (Rockwool) it's normal and more effective to lay the second layer perpendicular to the first. So you'd be better off fitting the new joists perpendicular to the originals I'd have thought. However, your original ceiling joists are not very meaty, so depending on the span, you may have a problem there, if you are going to pile lots of new joists, floorboards and storage items up there.

Just bog standard timber joists from the timber yard - haven't a clue what species it is!

David

[Dave Page]

I was intending to replace the existing insulation - it's only about an inch thick, dead old (over 10 years), and patchily installed. Plus insulation seems to come (from Wickes at least, who are selling rolls ad near-half-price until this weekend) in 170mm and 100mm depths.

The joists are 14" / 355mm apart, so there are plenty of them (this is particularly annoying since insulation roll seems to come in 400mm widths). The thought of laying new joists crosswise is interesting, not least because it would save me a few screws.

As it was, I'm not planning on putting too much weight up there, and was considering only raising every other joist to save on timber. On the other hand, the loft hatch is only 750mm wide, so if I want the boards to be supported by three joists, I'll need to space the joists at roughly 350mm anyway :)

That sounds pretty useful. Wickes have some 40 x 100mm pre-cut 2.4m lengths, I'll ask someone at the store whether it's suitable for my purposes if it works out cheaper than a timber merchant.

Dave

[The Natural Philosopher]

I think you mean 'orthogonal'

Perpendicular is walls... ;-)

[Ian Stirling]

Umm, no. Perpendicular can also mean simply "a straight line at right angles to another line".

[The Natural Philosopher]

...that is laying horizontal.

Orthogonal is better. Less implications of verticality.,

[Dave Page]

I understood what Lobster meant, and feel that "perpendicular" was quite appropriate. After all, laying insulation foam vertically wouldn't make sense.

Dave

[Lobster]

I wouldn't bother lifting the old stuff - 'orrible job, and it won't do any harm being added to by what you're going to lay on top. I've used the Wickes stuff; it's still intended to be laid perpendicularly (or orthogonally - FFS!) even if you're starting from scratch (I think their instruction sheet says so).

That's just the job, their 'kiln-dried', unfinished stuff is what you want, with the purple sticker! Look down its length to check it's straight before you put it on your trolley; especially if they haven't got much stock in, a lot of it can be banana-shaped. Personally, I find Wickes cheaper than timber yards as they won't give me decent trade prices.

Still not too happy about your weight issue though, but I could be wrong

- hopefully someone with more knowledge than me will comment?

David

[John Cartmell]

'Perpendicular' without the sense of vertical makes no sense at all. What would it be hanging (-pend-) from?

[Lobster]

Ah, but would you have understood if I'd said "orthogonal"? :-)

David

[Dave Page]

Probably, yes. Or even "crossways".

Dave

[Bob Mannix]

Yes it does. It only implies vertical in some of its meanings (architecture, physical landscape) and not in others (maths and the conjunction of two lines or planes).

A quote from Round the World in Eighty Days:

"It was supplied with two rows of seats, perpendicular to the direction of the train on either side of an aisle which conducted to the front and rear platforms."

Orthogonal could just as easily be vertical or horizontal as well.

Of course one could have used "at right angles to" :o)

[John Cartmell]

AFAIK it should only be used in maths & drawing where it's used to represent a physical verticality. Where you are describing a physical (3 or more dimensions) situation 'perpendicular' cannot mean horizontal.

The translator made an error.

But not 'perpendicular'.

[John Cartmell]

As a board game designer - yes! I use it in practically every set of rules that I produce ...

.. but feel obliged to define it every time! ;-)

[Ian Stirling]

It would - but it tends to slide off :) I'm wondering ATM if any of the common '270mm' rolls of stuff on the market can be split into 3 or 4 parts - for insulating the garage.

[Dave Page]

Frankly, it's all I can do to stop it blowing away in a draft.

Fair enough. In that case, I'm not sure what the best thing to do is; if I lay the insulation perpendicular to the existing joists, I'm not going to be able to get 270mm down over the existing joints (or a little way either side) unless I lay *more* joisting over the top. Seems the best thing to do might be to lay down the 100mm rolls *between* the existing joists, lay another 200mm of joisting perpendicular to that (squishing down some of the insulation beneath), and rolling the 170mm insulation between the new joisting.

Well, if there's concern, I'll hold off for a little while until consensus forms. Can't wait too long though, I want to make the purchase before the Wickes special offer on insulation ends on Saturday, and I'm busy all day Friday.

Dave

[Bob Mannix]

Oh no, definitely not - two lines in maths/geometry perpendicular to each other are usually representing no physical reality at all but are always described as perpendicular

It cannot *mean* horizontal per se, I agree, but could be used to describe things which form lines at right angles in some arbitrary plane (which might be horizontal) by considering them merely as geometric concepts. If the information being conveyed included information on verticality then perpendicular would clearly mean vertical.

That one's for a rainy day, I fear, not having a French copy to hand :o)

[John Cartmell]

No. You have to define the horizontal plane first. Of course that may only be assumed but, if you have assumed a line across the page to be the horizon and the top of the page as up, then a line at right angles to the first line is perpendicular to it *because you have defined the first line as the horizon*. In lazy maths today we generally forget that we have implicitly made that initial assumption.

You can indeed make any arbitrary plane the horizontal plane. In the case in hand you are saying that you have assumed that a vertical plane through the house is the horizontal plane.

Can we please return to reality before the thought makes me sea-sick! ;-)

;-)

[Weatherlawyer]

First thing get the cheapest offer you can with the insulation. But for heaven's sake phone around all the other places too first. Builders merchants not only stock it they deiver.

Secondly, why are you intent on adding timber? If you must walk around up there for maintainance in the future all you need is a walkway built up. If that. You could just lay a plank or two over the wool.

The rolls of insulate come in bags of three, sometimes you have to cut them apart. But if you lay them crosswise over the beams you can lay a full three rolls at a time.

Remember there must be a draught in the roof at all times as the amount of moisture that vents through it is quite amazing.

Third: However much you put up there at first, it will have shrunk to a lot less after a few months settlement. Talk it over with the salesmen in the timberyards you get quotes from. They will be far more knowledgeable than staff at Wickes's or similar. Though I must admit a few B&Q staffers I have met seemed on the ball.

[Ian Stirling]

Normally.