Yet another loft boarding question

[SimonH2]

About to start on boarding out a mates loft. It's a 70's or perhaps 80's bungalow with trussed roof. I've read many threads, on boarding attics but not seen answers to all my questions.

To keep some thickness to the insulation, I was looking at 6x1 joists run crossways on - screwed to the existing joists (around 3x2 ish). Two of the joists will be touching truss diagonals, so by the time the flooring is on it will be impossible for any sideways movement (6x1 toppling over).

So questions :
1) Is this timber from Jewsons suitable ? Or should I be looking at something else. We have a local branch, and (looking online) none of the others seem to offer wide sections without also going fairly thick and heavy.

2) Would 18mm ply be sufficient ? Bear in mind it's for storage, and the loft hatch at a little under 600x600 does limit the size of things that can be put up there !
I'm thinking ply because a) I don't care too much for chipboard, and b) the only T&G floor boards I could get through the hatch (too little height above) would be the tiddly little ones that cost a fortune from the likes of B&Q. I reckon I'll be OK with 4x2 pieces (with some 16" longer/shorter to stagger the joins)

3) From what I've read, 400 centres seem to be advised. Could I get away with 600 or would that mean much thicker ply negating any weight saving from reduced number of joists ?

4) There's an internal wall which will be about 3/4 the way along (of a 4.2m total length). I was wondering about bolting the end joist to the wall (with packing pieces to fill the gap) to support the long end so less of the wight goes on the trusses. Is this a good idea, and what sort of fixings would you suggest ?

EDIT: I've worked out that we can just get the 4.2m lengths in one piece and manoeuvre them into position.


Assuming this goes OK and doesn't completely put me off, after that will be the girlfriends loft. That's a 60's house with purlins, rafters etc. I assume much the same applies ? Since there's no truss diagonals to brace against, I assume I'd best put some noggins in - how may given that the individual joists will be laterally supported top and bottom (screwed to existing joists, ply screws down on top) ?

 

[SimonH2]

Guessing my original post was TL;DR

I also think I worded Q4 badly. What I meant was, there's the top of an internal wall at about 3/4 of the overall span, so I was looking at packing up off that, and bolting the end original joist (at the other end of my new joists) to the wall. That way, the new joists are taking some of the weight themselves by spanning load bearing supports (about 3m apart) - so less load on the trusses.

 

[jonwish]

From a personal point of view as someone who is not a professional trade but used to work in building maintenance for many years and has renovated a few houses I would say the 22mm timbers are too thin (need at least double that) and 600mm spacing is too much (especially if your not using tongue and groove).

Just my opinion

 

[SimonH2]

From a personal point of view as someone who is not a professional trade but used to work in building maintenance for many years and has renovated a few houses I would say the 22mm timbers are too thin (need at least double that)

I assume that's based on them being self-supporting across a span ? For that, the tendency to twist and "fall over" in the middle of the span is important, as well as how much they sag.

My reasoning is that these won't have that problem. Every 600mm they'll be screwed to the joist below, and they'll be sheeted on top. Thus they won't be able to twist.
Also, as they are mostly supported by the existing joists they are sat across, the actual strength (in terms of bending and sagging in the middle) isn't really an issue.
So these timbers only really need to lift the boards up to leave room for insulation, and support the boards across 600mm of gap.

and 600mm spacing is too much (especially if your not using tongue and groove).

That seems to be the general consensus


Any thoughts on how best to bolt the end trusses to the wall ? Whatever hole I drill in the blockwork has to go through the joist first - so expanding bolts (rawlbolts) would need a rather large hole in the timber. Do you think chemical anchors and a bit of studding would work ? Ie, drill 8mm hole, insert resin phial (or squirt from gun), knock in a bit of 8mm studding and twist to mix.