Boarding garage ceiling

I've got a detached double garage, recent 1990s construction with 100x40 joists running front to back, spaced at around 56cm.
I'd like to board the ceiling on top of the joists to provide some storage space, nothing too heavy, currently it's used for empty boxes etc. Would the joists support this weight (plus a person) or do I need to add more joists? I've never worried about this in the loft, I'm not sure why I'm worrying about the garage.
I'm also going to somehow board underneath the joists (plasterboard whatever - not decided yet) with insulation in the gap. Then the pitched roof will get insulated, but I'm getting ahead of myself.
There's also a total of 4 planks running diagonally across the top of the joists, assuming they're there to stop the joists spreading I can do away with these once the flooring is screwed down?
If it means the difference between putting extra joists in and not I'd be willing to spend more money on a more expensive flooring solution.
Thanks
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Mike Buckley
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On Sun, 26 Sep 2010 17:07:09 +0100, Mike Buckley wrote:

That's 2x4's on 16" spacings in old money, isn't it? Across the width of a double garage, that doesn't sound like it'll support much at all (there are various deflection calculators online - but I don't know of one that'll tell you max weight before your floor outright collapses :-)

In your loft, surely you have quite a few walls underneath, so the unsupported spans aren't that great? (Are the 'floor' ones for the loft really only 2x4s, too? I've never seen a house like that - ours are 2x8s - but I've never lived in anything modern)

I suspect ply might be better - less weight and less prone to moisture.

Yes, that sounds sensible.
cheers
Jules
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About 6 metres, give or take.
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A long span for 1.5x4 at 16" spacing. About half that span would be fair enough. I'd either use as is but load lightly, or better, toughen it up. http://wiki.diyfaq.org.uk/index.php?title=Strengthen_a_Wood_Floor
Green chip would be the 2nd cheapest flooring option, with half inch scrap chip the cheapest.
There are also alternative options, if you wanted to play. For example if you underboarded one section with green chip and filled it level with a tough papercrete mix with added plaster, that would get you in one go more strength, insulation, a floor and a ceiling. When its set, move the geen chip and repeat. Put polythene on top of the chip before fixing it, and screws or nails into the joists to maximise grip. With such approaches its best to load test it once cured before using it - note it doesnt stop you overboarding it if it doesnt meet point load spec.
NT
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Green chip sounds good, was thinking chipboard unless I can find some scrap "something else".

That's interesting, although I'd probably end up overboarding. Seems odd that adding more weight between the joists makes them stronger.
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The papercrete adds structural strength as well as weight. It effectively turns your 1.5x4s into 16x4s, albeit not made of wood.
You'd only need to overboard if you got it wrong and it failed to do the job. Scrap paper's free, its only cement you pay for.
NT
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On 26 Sep,

Are they TDA (prefabricated) W trusses, or a conventional roof? If th latter what support is there in between? purlins???
Pictures?
If TDA trusses (so the ceiling is effectively supported at 2metre spacing) you shouldn't have a problem unless you stuff a lot of weight up there.
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Pre-fab trusses as far as I can tell, W trusses with metal connector plates. We're part of a massive estate built in the early 90s, no way I can see David Wilson using anything other than pre-fab.
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On 28 Sep,

No problem, With the amount of weight in my loft, with similarly sized trusses supported just at their ends, I haven't had a problem in 30 odd years, bar slight rotting behind the fascias.
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On 26/09/2010 17:07, Mike Buckley wrote:

What are you ultimately trying to achieve with the end result though - comnventional garage below with just storage space above? If so, why all the insulation?
By putting insulation below the joists are you hoping to keep the upstairs warm, or the downstairs? (Or both?)
My garage sounds quite similar except that my joists are deeper so I have no worries about weight. All sorts of junk and boxes live up there; there's no insulation whatsoever which causes me no issues.
David
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The boarding above the joists is for storage, the boarding below is to make it more like a room, white ceiling plus some decent lighting. This will let me work on my bikes and also make it into more of a room and less of a garage. Might as well insulate in the gap while I'm doing it.
There'll be a decent floor, plus some door insulation going in as well. I've not found anybody online that's managed to make a single skinned garage into an all year room, so I'm not even going to try, but I'd like something that I can store stuff in that I can keep frost free easily.
Sort of a 9 months of the year games room-lite. With tools, and motorbikes.
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People have converted single skin garages into compliant housing, it is doable. Bitumen paint it, add battens, insulate between them and plasterboard with ali foil as a VB. Ply might be better than PB if you wanted more of a workshop.
NT
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