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craigsalisbury

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Hi chaps,
As it will start to cool down soon, i want to get the joists covered and insulated for winter.

I was thinking about 100mm slab insulation between the joists but then do i use plasterboard or OSB? MDF? Ply? I'm wondering it the weight of the sturdier materials could impact the joists as we use it for storing stuff, would I need to double up on the joists also? i think they are currently 6x2's with 600 centres.

as pictured

IMG_20200909_083326.jpg
 

DBT85

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Plasterboard or osb will be fine. Not sure if you need to do anything regarding vapour barriers or anything.

Mike will know.
 

MikeG.

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Interesting shaped room!

All of those sheet products are heavy, so the choice boils down to price (plasterboard) or robust-ness (OSB, MDF, or ply). You need a vapour barrier, so that would mean foil backed plasterboard, OSB or ply, or messing about with polythene. For me, in a workshop where walls and ceilings get the occasional ding from clumsy material handling, I go for painted OSB. Ply would do roughly the same job for more money, but arguably look slightly more attractive.
 

craigsalisbury

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Interesting shaped room!

All of those sheet products are heavy, so the choice boils down to price (plasterboard) or robust-ness (OSB, MDF, or ply). You need a vapour barrier, so that would mean foil backed plasterboard, OSB or ply, or messing about with polythene. For me, in a workshop where walls and ceilings get the occasional ding from clumsy material handling, I go for painted OSB. Ply would do roughly the same job for more money, but arguably look slightly more attractive.
do they do a foil backed ply? 6 or 9mm be ok? ill of course paint it white.

The garage is an odd shape, the door end width is 4.1m and the other end width is 2.8m. the depth is 5.2m, im not complaining though, its a decent size for a single garage
 

Myfordman

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I used 100mm fibreglass matting between the joists and held in place with polythene sheet stapled to the ceiling joists to support it, holding it place and providing a vapour barrier. Then I clad the area in 6mm mdf as being relatively light, cheap etc painted with white emulsion to reflect light.
 

frank horton

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as Myfordman suggests
but I'd be sistering up the joists as well.....
they look a little long and wide spacing to be very secure.....
I'd be using cup square headed bolts and these
shopping-2.png

double sided fixings, very sharp so watch out.....most are pre punched for 12mm bolts....
they'll take out any droop and stiffen the floor/ceiling no end.....think they are 30p each from screwfix / toolstation....
I use em a lot.....put wooden chicken sheds together that have never moved in 15 years.....
u can use em on ply but they need a little help to dig in with something heavy.....
 

AJB Temple

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If you buy from the sheds (worth getting a trade account from Wickes as it is dead easy and gives 10% discount on everything) there is not a lot in it between OSB and shuttering ply price wise in practice. Much easier to roll or paint a decent white finish on ply. Will improve your lighting a lot (reflective).
 

craigsalisbury

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as Myfordman suggests
but I'd be sistering up the joists as well.....
they look a little long and wide spacing to be very secure.....
I'd be using cup square headed bolts and theseView attachment 91854
double sided fixings, very sharp so watch out.....most are pre punched for 12mm bolts....
they'll take out any droop and stiffen the floor/ceiling no end.....think they are 30p each from screwfix / toolstation....
I use em a lot.....put wooden chicken sheds together that have never moved in 15 years.....
u can use em on ply but they need a little help to dig in with something heavy.....
ill have to pickup one of those jack things to make sure the joists arent sagging before fixing side by side.
 

MikeG.

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do they do a foil backed ply? 6 or 9mm be ok? ill of course paint it white.
Ah no.......you're confusing things. My commas were placed carefully! :)

OSB and ply each act as a vapour barrier in themselves (they have so much glue in their construction that they are almost vapour-impermeable). Plasterboard doesn't, and MDF doesn't. If you choose plasterboard, it needs to be foil backed. If you choose MDF, it needs polythene behind it, which you can also do for plasterboard instead of using foil-backed.
 

MikeG.

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No need for sistering. Your span is only about 3.5m in the middle of the room, and if there is any doubt about the load bearing of those truss bottom chords then one simple beam across the underside of the middle would remove all doubt.
 

craigsalisbury

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so it looks like im going with 100mm slab insulation and 6mm ply (painted white). let the winter hit!

Edited after @MikeJhn 's (shed god) reply
 

MikeG.

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If you buy from the sheds (worth getting a trade account from Wickes as it is dead easy and gives 10% discount on everything) there is not a lot in it between OSB and shuttering ply price wise in practice. Much easier to roll or paint a decent white finish on ply. Will improve your lighting a lot (reflective).
True, but OSB tends to be flatter than shuttering ply, which can be twisted and cupped horribly. You can be fine if you choose your own sheets.
 

Myfordman

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The joists do look fairly minimal but I assume they were adequately specified during the build. Adding the dead load of 6mm mdf is not likely to bring the structute down.
MDF will be cheaper than py for a given thickness.
 

MikeG.

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so it looks like im going with 100mm slab insulation and 6mm ply (painted white). let the winter hit!

Edited after @MikeJhn 's (shed god) reply
6mm ply is too thin. It will sag across that gap (600mm). You need to be using 10-ishmm miinimum. Beware, however, that shuttering ply tends to be thicker stuff. If you buy thin ply it might not be shuttering ply, but instead be the awful far-Eastern ply which is soft and weak.
 

MikeG.

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The joists do look fairly minimal but I assume they were adequately specified during the build.....
Exactly. It's worth remembering that the bottom chord is actually suspended from the rafters, and particularly the ridge, by the intermediate "W" timbers. Unless there is some extravagant load up there I wouldn't be worried......after all, these things support water tanks in most houses.
 

frank horton

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Vamos, Crete, GREECE.......
Craig,
if u have a saggy bottom......ooops sorry......hahaha....joist....
save ur money from buing an ACCROW jack....use a heavy joist type lump of timber and a sledge hammer or the like to prop up the joist.....if the floor hight changes just use decent wedges for the extra lift....to protect ur floor if it's painted or fancy use a piece of plywood.....
DO NOT USE A CAR JACK with wheels.....
thats unless u can scrounge an Accrow.....
dead easy to set by eye or u can use a string line.....well worth the effort....
then u can load up the roof even more......hahaha....
 

craigsalisbury

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ok scrap 6mm ply in favour of 11mm OSB, after a coat or 2 of white paint i imagine that will be ok and its a bit cheaper than ply. I wonder if the long edge is the same way as the joists it may add a bit of strength?

I love all this diversity of opinion, but my god it makes it harder a numpty like me to make a decision, so i reckon going with this ^^
 
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MikeG.

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No. I doesn't matter which way you hang it, it's still a dead load on the trusses.

-

Let's stop this conversation about the trusses dead by actually measuring. Measure the floor to u/s of chord at the wall, and measure it again in the middle. (Unless you have a laser. And we have to assume the floor is flat). If there is a sag of more than say 12mm in the middle, THEN let's have a conversation about strengthening them.

Better, pin a string at each end of the chord say 10mm up from the bottom edge, pull it tight, and measure the distance from the string to the bottom edge in the middle.
 

craigsalisbury

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right then :) doesnt really seem to be much if any sag, the only thing which makes no difference is a slope in the joists i.e. not level

IMG_20200909_111803.jpg
IMG_20200909_111839.jpg


Next to laser
IMG_20200909_112532.jpg


Middle of garage
IMG_20200909_112520.jpg


End of garage

IMG_20200909_112508.jpg


Hope this helps
 
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