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I have a detached, single skin garage that I'm gonna turn into a workshop/hobby space and I'm going to put some insulation in there to make things a little more comfortable. The first thing I'm going to be doing is the ceiling. The roof space is a trussed pitched roof with the joists of the trusses running the length of the garage (about 5.4m or 17.7ft).

Grey is the breezeblock piers, red is brick, brown is the truss joists. Measurements are approx.

The reason why I opted for a ceiling is because my options were to insulate between the rafters and then beneath the rafters, but there's a diagonal support on the trusses that I know I can't move so I'd have to insulate around it? Which sounds like a faff and honestly impossible to create a complete barrier with the 2nd layer of insulation. Further to that, I'd still have to insulate the walls in the roof space. So I decided on adding a ceiling with loft roll between the truss joists.

Every topic in this area seems to have a fair share of people saying that I can fix a ceiling to the trusses or people saying if I touch the trusses I'll bring the whole garage down. So I am playing things safe and not touching the trusses.

So my plan is to get some timber (50x75mm or 2x3 I think?) fixed to the walls across the width of the garage (about 2.8m or 7.8ft) just below the trusses (I won't fix them to the trusses either) and from there, add noggings, loft insulation and finally a ceiling.

Darker brown represents the new joists below the trusses

I have a few questions about this.

  1. Does this approach sound OK? I can probably get some wood long enough to run the length of the garage and have the new timber for the ceiling fixed to that for extra support.

  2. I'm still a bit concerned about weight. What is a reasonable material to use for the ceiling itself? I was thinking 9mm (~3/8") plasterboard as its a little more affordable now but looking around it seems a single board is about 20kg (or 44lbs). Are there lighter options?

photo of roof space

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  • A bit confusing for me in metric because I'm an American...but I was a little surprised by the weight. Then I compared it US drywall and 3/8" 4'x8' drywall weighs 45 lbs - which is basically the same as 9mm (arbitrary unknown but presumably similar sheet size) plasterboard 20 kg. Unfortunately, I don't have an answer to the question though. Sep 8, 2023 at 14:50
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    Maybe a real picture of the trusses so we are all using the same words to describe things. Most homes with truss roofs attach the ceiling to the trusses with no problem. Might have a problem with adding a floor above for living space.
    – crip659
    Sep 8, 2023 at 14:55
  • I've added Imperial measurements (to the best of my ability) as well as a photo of the roof space as requested.
    – Tensketch
    Sep 8, 2023 at 15:06
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    @isherwood Sorry if I caused a problem. I truly was confused and despite knowing most of the key numbers (2.54cm = 1", 2.2 kg ~= 1 lb.) it still took me a minute to think it through and then to double-check it with calculators and then to actually find out how much drywall weighs. I was using this as an excuse for myself and did not mean to say all (or even most) Americans are as bad as I am. Sep 8, 2023 at 15:46
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    It was a good effort, @Tensketch, thanks, but 1/3 inch isn't a measurement we Imperial Americans are familiar with, either. :D I changed that to 3/8" because that's close enough and we're quite comfortable (oddly) with eights... (Also, no need to include an "edit" tag - if someone wants to see the edits made, they can just click the "edited..." text to show the whole edit history.)
    – FreeMan
    Sep 8, 2023 at 16:03

1 Answer 1

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You've been misled (though I can't imagine by whom--I've never seen such a warning from anyone I consider knowledgeable). Those trusses (and all similar trusses numbering millions) are absolutely designed to carry a ceiling and a reasonable amount of insulation. You do not need extra lumber, and you don't need to worry about damaging anything if using common screws.

Have at it. Install a vapor barrier on the truss bottoms, then your drywall (or steel if you want to get classy--I found a deal on a leftover lot for mine). Blow or lay insulation above. Done and done.

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    What I have seen quite a bit - and I think legitimately so - is concerns about using existing trusses as a storage area - i.e., on top, not a ceiling hanging from below. Sep 8, 2023 at 15:56
  • It was a thread about a similar topic (may have been DIYNot) and someone said that trussed roof structures are engineered to hold the weight of the tiles with extra allowance for snow. Anything else pushing them down will risk bringing the whole thing down. Made it sound like a horror story tbh.
    – Tensketch
    Sep 8, 2023 at 19:45
  • We've also seen a lot of rafter ties (not trusses) on 4-foot/1.2m spacing. In any case, the OP has a new build, so the specs of the truss lower chord load capacity should be a phone call away, or just looking at a pile of paper or computer image files.
    – Ecnerwal
    Sep 8, 2023 at 22:24
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    @Ecnerwal is right, I am in a new build. Got up my ladder and found a label on the trusses for Howarth Engineered Timber Solutions. It's out of office hours, so I've dropped them an email enquiry. Hopefully I get a response next week.
    – Tensketch
    Sep 9, 2023 at 10:32
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    They responded to me yesterday and confirmed exactly what you said. They've been engineered to support a ceiling and loft roll! I'm relieved!
    – Tensketch
    Sep 12, 2023 at 9:45

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