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How much load can this garage loft handle?

The garage is 24' wide. The floor of the loft is 2x10s 24" OC. Those are 2x6s to the left of the plates (the 2x10s run all the way from those left plates to the right wall). All the vertical lumber is 2x4s. 2x6s under the garage roof. The garage walls are 16" OC.

Bottom of loft View in loft Some dimensions 2' on centers 2x10 on one end 2x6 on the other end

I'm hoping to move some power tools up there and grow that space into woodworking shop. But I'm not sure if the space will support the added weight (or if it'll be a good idea... I'd much rather have a shop on the concrete pad and without having to climb stairs... but that's another topic).

I understand any advice given on this site is not legally binding. I'm just looking for an informed guess.

Matt Thomas
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    It's impossible to tell or even guess based on some photos you took. Do yourself a favor and hire a qualified structural engineer to analyze your current building and determine what is needed to make your plan work. – jwh20 Nov 17 '21 at 13:12
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    Probably not as much as you would like. Those trusses mainly made to support weight from the top, not from the bottom. Would need to add support posts and beams from the floor with proper footings to hold weight for a work shop deal. – crip659 Nov 17 '21 at 13:14
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    Something was pre-planned with those 2x10s under the 'walking' area of the trusses, but I'm not sure what, exactly, given the butt joint to the 2x6. So I'd agree that you need an engineer on-site to assess. – Aloysius Defenestrate Nov 17 '21 at 14:36
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    Now that I see them, they're obviously built to create a room. Therefore, they're designed for typical habitation loads. I would send photos and dimensions to an engineer or truss plant for verification. – isherwood Nov 17 '21 at 21:54
  • Man, those metal plates would worry me. You can figure out the load a single board can take doing the calcs yourself, but you would want a structural engineer to tell you about those metal plates tying those boards together - that just doesn't look right to me. Another option would be to sister a bunch of boards across there, perhaps even for the 24' length, though that adds some non-zero weight to your load on your walls. – daveola Nov 18 '21 at 02:06
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    @daveola Why do those metal plates worry you? They look like the correct truss plates used in a sensible design to me. Remember that the bottom chord of a truss is loaded mostly in tension - they're not designed like a floor. Likely that's a 40psf design but would be a bit bouncy as a floor, though functional. Sistering over the bottom chord splices would firm it up and would be a good idea if the attic is to be loaded and occupied. – J... Nov 18 '21 at 16:12
  • To be very clear, I am *not* a structural engineer. I just do a bunch of ad-hoc building, and I wouldn't personally feel like butting the ends of two beams together and holding them with metal plates would be strong enough, but I have never done the calcs or testing on it. It sure doesn't feel intuitively to be as strong as a continuous piece of wood, but I'm very prepared to be way wrong on that. :) Regardless, they certainly make it much harder to determine the strength of that span then if there was just a single wooden beam of fixed cross sectional dimension. – daveola Nov 19 '21 at 01:01
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    @daveola I'm pretty sure that looks perfectly fine. The area where the truss chords double as a floor are wider lumber to add stiffness where there are no vertical supports to the rest of the truss, but at the joint where you're looking the weight on the floor joist is being supported by the vertical and diagonal members through the truss plate, so there's actually not a lot of bending force on the joint at all. – J... Nov 22 '21 at 17:48

4 Answers4

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The Code requires trusses be designed to support the Live Load (snow loads, etc.) plus Dead Loads (lumber supporting the roof and ceiling) plus 10 psf Live Load in the ceiling. (10 psf is not much… and you can’t stack it high in one area just because you don’t have anything in another area.)

I don’t have an idea what your trusses will support, but I’ll bet it’s far more than 10 psf, because a 2x4 bottom chord will support 10 psf for just a 24’ span.

I’d go to your local lumber yard where they make trusses and ask them. They have software that can calculate the size of the chords and webs in about 2 seconds. You can do this in two ways: 1) The hard way: Take all the sizes of the chords and web members to them and let them input the members sizes into their program , or 2) Tell them you want 40 psf , or 50 psf , Live Load etc. on the bottom chord and let them calculate what size the chords and web members sizes. Then take their printout home and match it up with your trusses.

Lee Sam
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  • Would the lumberyard be able to wrap their heads around the 2x10-turning-into-a-2x6 bottom chord thing? Are those joining plates usually sufficient to connect things for human loads? – Aloysius Defenestrate Nov 17 '21 at 14:40
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    @Aloysius Defenestrate Looking at the second picture, it looks like the middle of the span was planned as a room (habitable space). Notice there are no web members extending through that space. – Lee Sam Nov 17 '21 at 14:47
  • Right. I've built a number of homes with such trusses either over the garage or over the second story as storage. The entire bottom chord (and indeed the entire truss) is overbuilt for extra load. The interior bottom chord spans further and carries more load, so it's oversized further. – isherwood Nov 17 '21 at 21:52
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From the second picture and your opening statement, For permitting purposes, I think the structure was categorized as the "garage with attic for other uses (non-sleeping)", and designed accordingly.

For the open space between the vertical truss hangers/posts, the code specifies a minimum of 40 psf design live-load on top of the material dead load. It could be higher, but can't be safely assumed or calculated without resorting to the original truss supplier or a structural engineer.

enter image description here

r13
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  • OP's Picture #2 suggests more of a dutch-barn style of roof, to maximise headroom and minimise losses to the smaller triangular spaces. – Criggie Nov 18 '21 at 02:02
  • @Criggie Yes it is more of a dutch style. As you stated, picture #2 shows half of the profile. Mirror it and you have the other half. – Matt Thomas Nov 18 '21 at 15:39
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Your span of 24 ft is slightly smaller than my house's 27 ft span, but your photos indicate the truss dimensions are similar to mine.

My attic trusses are rated for 40ft psf live and 10ft psf dead load, within the open space. I have a 2x6 bottom chord with an additional 2x4 on top of the ~10ft wide opening.

Here is a screenshot from my truss calculations.

attic truss calculations

Daniel
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I would suggest you cut a single pole to measure the concrete-to-under-span height.

Compare the height difference between empty, and with an assistant standing exactly over that beam, or a decent weight of about 100 kg focussed in that one spot.

Then estimate the load you might put into the wood shop and scale up the measured deflection. This will give you an idea of how the trusses will react to the full weight.

Personally, that looks like a storage space, and not a workshop. You may have problems with:

  • Getting longer lengths of timber in there,
  • Extracting finished items safely
  • Installing larger machine tools like a drill press
  • Excess heat, and fresh air ventilation.

I would suggest you build the workspace you want downstairs, and plan on adding a new garage or carport for vehicle protection outside.

Criggie
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    Oh, I thought you were going to suggest parking the cars upstairs. :D They are, after all, a lower priority than a good table saw & planer! – FreeMan Nov 18 '21 at 00:20
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    @FreeMan vehicles are important - you can use them to go look at more machines, and to buy/transport smaller tools and consumables. But larger workshop machinery doesn't like being rolled around so in an ideal world they get parked and levelled, then left. I'd suggest using the loft for summer/winter clothes, christmas trees, emergency food storage, etc, and not everyday items. – Criggie Nov 18 '21 at 02:00
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    @FreeMan Exactly my thoughts!! – Matt Thomas Nov 18 '21 at 15:38
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    @Criggie If you look closely at picture #2 you'll see I have a small cable winch for lifting awkward objects like tires and wheels up through the ~4'x4' hole caused by the stair width. Don't worry, you're protected from falling through that hole by the _very_ sturdy window screen propped up there (yikes!) But I agree it would be hard to get a jointer up there, or a finished table down! – Matt Thomas Nov 18 '21 at 15:58