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karamazov
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# Posted: 26 Jan 2024 04:24pm
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Hi all,
I've been planning out a workshop that I'm going to build behind my garage for a while. I've settled on an overall size and design, but I am looking for some advice on what to do for the foundation.
Design for reference: https://imgur.com/311zfCk
It's a 14x32 with a gambrel roof and a loft. Currently I have it set up on 3 3-ply 2x8 beams for the skids and it's sitting on blocks (3x5 pattern). I'm planning to set up a compacted gravel pad slightly larger than the structure itself. To deal with wind I'm planning to use mobile home tie downs around the perimeter (I am not in a high wind or hurricane zone but just want to be safe).
Where I am the soil is pretty sandy for the first foot or two, and our frostline is something like 6".
Does anyone have experience with this kind of setup? Any advice? I also looked at piers. I have the equipment to do piers, just don't know if it's worth the effort and added cost in this case.
And yes I know that concrete slab might be the optimal choice, but a properly done slab is going to cost more than the entire rest of the structure according to the prices I've looked at. Plus I want flexibility to easily add plumbing later.
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Brettny
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# Posted: 26 Jan 2024 04:35pm
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6in frost line I would dog down to below the frost and place 8x8x16 solid concrete blocks there. That's prety shallow frost line but with 3 skids you may never get a chance to re level if needed. I would skip the gravel pad, it's not really needed.
Have you thought about just doing 2 skids and doing 2x10 or 2x12 floor joists?
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karamazov
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# Posted: 26 Jan 2024 04:42pm
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Thanks for the response!
I had considered it, but I really don't want my floors to be bouncy/springy. I also figured that distributing the weight across more blocks/larger surface area would reduce settling problems.
I had considered digging down a bit and packing gravel, placing blocks, mortaring them together, etc. If I'm going to dig down anyway though I figure maybe I'll just do piers.
One other thing I was trying to figure out is how close to the edge of the structure I need the skids to be. I've seen some people say that they need to be squarely under the load bearing walls, others say up to a foot of overhang is fine. Any thoughts there?
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spencerin
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# Posted: 26 Jan 2024 08:05pm - Edited by: spencerin
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Are you building the entire structure yourself? Or buying a pre-built 14' x 32' gambrel-style shed? I can tell you those types of sheds come on 2 runners/skids all the time, so the walls overhang the skids, without issue. That should answer your question about that.
Brett's foundation suggestion is probably the easiest and lowest cost. Why would you want to do piers? Are you talking about solid concrete piers, like sonotubes?
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karamazov
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# Posted: 26 Jan 2024 08:25pm
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Quoting: spencerin Are you building the entire structure yourself? Or buying a pre-built 14' x 32' gambrel-style shed? I can tell you those types of sheds come on 2 runners/skids all the time, so the walls overhang the skids, without issue. That should answer your question about that.
I looked at some Sturdi-bilt ones. To be honest I really wasn't impressed with their build quality. I can't justify paying $13k for something like that when I know I can build it far better for less.
For what it's worth all the ones I saw had 4 skids, with the outside ones pretty close to, but not exactly under, the walls.
Quoting: spencerin Brett's foundation suggestion is probably the easiest and lowest cost. Why would you want to do piers? Are you talking about solid concrete piers, like sonotubes?
Yes I was originally planning to do rebar reinforced concrete piers w/footings dug down a couple feet.
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gcrank1
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# Posted: 26 Jan 2024 08:42pm
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Fwiw, I was told the inner 'runners' were for carrying it on the delivery trailer/truck; the outer ones are where the building support needs to go.
That said, I can't think of a downside of 'blocking' the inner ones too except for the extra expense and difficulty in do it.
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ICC
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# Posted: 26 Jan 2024 08:54pm
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Quoting: gcrank1 I can't think of a downside of 'blocking' the inner ones too except for the extra expense and difficulty in do it.
IF or when the building requires re-leveling those center supports will create difficulty.
Yes, I would want a very solid floor in my shop. I would also want the floor to be as level as possible and to stay that way. That makes it easier to build things that are square, level, plumb, and so on. I did spend a fair amount on a poured concrete monolith slab and foundation to get that though.
It is possible to use a roto-tiller to loosen the earth and dig down to a 12 to 18-inch depth. Then pour a concrete perimeter foundation, reinforced with rebar, using a portable mixer to mix your own sand, gravel, cement and water. Lay at least a single course of concrete block on that and you will have a better foundation than piers of any kind. That would be all tied together whereas piers are almost always individual units with each one free and able to move in different directions.
-izzy
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karamazov
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# Posted: 26 Jan 2024 09:14pm
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Quoting: ICC It is possible to use a roto-tiller to loosen the earth and dig down to a 12 to 18-inch depth. Then pour a concrete perimeter foundation, reinforced with rebar, using a portable mixer to mix your own sand, gravel, cement and water. Lay at least a single course of concrete block on that and you will have a better foundation than piers of any kind. That would be all tied together whereas piers are almost always individual units with each one free and able to move in different directions.
I appreciate the insight. I've had some others suggest doing a perimeter foundation. At one point I really liked the idea of doing a sort of conditioned crawlspace with a block perimeter. For DIY it seems a lot more doable than a mono slab.
I don't see a lot of those (or maybe any) down here in TX. Is that because they can have trouble with the expansive soils we have?
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Brettny
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# Posted: 26 Jan 2024 10:13pm
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You definitely dont want to till up the soil then expect it to not move.
Can you get a concrete truck to your site? A concrete floor in a shop is very nice to work on and with 6in frost depth I would scrape off a few inches and make a form.
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909
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# Posted: 26 Jan 2024 10:13pm - Edited by: 909
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I did what you're planning, but used 18" x 18" x 3.5" footing blocks.
I also trucked in 7 loads of pit run ( road base ) on a tandem dump truck (14 - 16 tons per load ) totalling over 100 tonnes of material .
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karamazov
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# Posted: 26 Jan 2024 10:34pm
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Quoting: 909 I did what you're planning, but used 18" x 18" x 3.5" footing blocks. I also trucked in 7 loads of pit run ( road base ) on a tandem dump truck (14 - 16 tons per load ) totalling over 100 tonnes of material .
Ah well I'm interested to hear what your experience has been since! How has it held up?
You must have either gone deep with the gravel or have a much bigger pad because the gravel pit nearby told me I'd only need one load (about 15-16 yds)!
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darz5150
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# Posted: 26 Jan 2024 11:08pm
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I put this 12 x 24 on this set up.
We did the outer perimeter with pre cast mafia blocks, then filled it with CA 6 unwashed rock and packed it with a track hoe. That was about 4 years ago. Did get some settling in one corner. But all in all it still is very solid.
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ICC
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# Posted: 26 Jan 2024 11:30pm - Edited by: ICC
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Quoting: Brettny You definitely dont want to till up the soil then expect it to not move.
Sorry if I wasn't sufficiently explicit. The roto-tiller loosens the earth so it is easier to remove with a square-nose shovel. Remove the dirt and you have a trench much like you would have if dug with a backhoe. However, this way you don't need any heavy-duty equipment. The cost is more muscle power but fewer dollars from the budget.
-izzy
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karamazov
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# Posted: 27 Jan 2024 02:29pm
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Quoting: darz5150 We did the outer perimeter with pre cast mafia blocks, then filled it with CA 6 unwashed rock and packed it with a track hoe. That was about 4 years ago. Did get some settling in one corner. But all in all it still is very solid.
Interesting, so you basically buried foundation blocks inside the gravel pad.
Are you concerned about moisture with the floor that close to the ground or is drainage good enough that it isn't a problem?
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darz5150
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# Posted: 27 Jan 2024 04:44pm
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No. We Put the blocks on the outer perimeter.. then filled the void with the rock. Actually overfilled it and packed it. Then added more to bring it above the grade of the surrounding ground. You can kind of see in the first pic, the whole building and pad sits about 4 or 5 inches higher.
Don't seem to have any problem with drainage. The unwashed rock sets up like concrete.
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darz5150
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# Posted: 27 Jan 2024 04:51pm
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Quoting: karamazov Are you concerned about moisture with the floor that close to the ground
The floor was painted with water/stain proofing inside and out. The ends of the skids, I nailed hardware cloth on to them and buried it somewhat. So it still has some airflow underneath. It also has the foil bubble insulation between the joists and the floor.
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spencerin
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# Posted: 28 Jan 2024 01:12am
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I stand corrected. It looks like most of those 14' wide sheds are in fact on 4 skids/runners, not 2. FWIW.
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Brettny
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# Posted: 28 Jan 2024 11:32am
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That dosnt mean you need to support all 4.
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spencerin
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# Posted: 28 Jan 2024 02:28pm
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But, isn't 14' too wide a span for most dimensional lumber? I would think support in the middle would still be needed.
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karamazov
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# Posted: 29 Jan 2024 08:33am
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Quoting: spencerin But, isn't 14' too wide a span for most dimensional lumber? I would think support in the middle would still be needed.
The joist span calculator I used says you can go 14' with 2x10 SYP spaced at 2' on center, L/360 deflection. Then again I dont know how much sag our bounce the floor would end up having.
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Brettny
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# Posted: 29 Jan 2024 09:54am
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I wouldnt go anyless than 16in spacing. You have to think about how much deflection your going to get in 3/4 plywood.
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909
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# Posted: 29 Jan 2024 10:49am - Edited by: 909
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I ended up using 2 x 10 PT spanning 10' spaced 16" on center .
3/4 SPF ply ontop. 1/2 PT ply on the bottom. R24 insulation. Resting on 6 x 6 PT beams about a foot from the ends.
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karamazov
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# Posted: 29 Jan 2024 11:02am
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Oh I'm definitely planning on 16", I was just saying that's what the calculator showed for 2x10.
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909
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# Posted: 29 Jan 2024 03:35pm
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Quoting: karamazov Ah well I'm interested to hear what your experience has been since! How has it held up? You must have either gone deep with the gravel or have a much bigger pad because the gravel pit nearby told me I'd only need one load (about 15-16 yds)!
It's a brand new build. Perfect so far.
https://streamable.com/xw7ihf
We did an entire driveway plus the base for the cabin.
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