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moisture barrier under shed?
Posted by: mrbigstuff
Date: July 13, 2007 04:52PM
So, I'm building a shed... (wait for Lafinfil comment.... it will come...) and I'm thinking that, although I'm laying a bed of stone about 6" deep, it might also be good to put some kind of a barrier down below the rock. But, I'm also thinking.... since the rock will disperse the runoff from my driveway and short piece of grass yard (as I want it to do), it might just end up getting caught in the barrier and not be able to leech into the ground.

the shed is about 5-6" below the level of the yard (grass) and runs downhill at about a 4-5 degree decline toward the shed area.

What think ye?
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Re: moisture barrier under shed?
Posted by: bfd
Date: July 13, 2007 05:26PM
What kind of floor? Will the shed be completely above ground or not? What are you planning to store there?



Edited 1 time(s). Last edit at 07/13/2007 05:27PM by bfd.
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Re: moisture barrier under shed?
Posted by: lafinfil
Date: July 13, 2007 05:51PM
>
> wait for Lafinfil comment
>

: -D

When you build this imaginary shed I don't think you will need a vapor barrier

Here's what I would suggest - when you get your crushed stoned for the base get a finer crush
with stone dust in it and pack the hell out of it.

You want the water to run off of it, not through it or it will wash out

I helped my neighbor build a "shed" recently (we'll call it a shed for zoning purposes -
really a freestanding one car garage 12x20')

He laid a base of about 8" of crushed stone and packed it flat until it was a coupe inched above grade
using his SUV and my tractor to pack it down (the yard is really flat - no much runoff)

On top of that he laid out 18x18" cement pavers and then built a box base out of 2x10 treated lumber
on 16" inch centers and then built the floor out of two layers of 3/4" treated plywood that was
laid out so all off the plywood joints were staggered.

It is strong enough to park a Nissan SUV in it (which is what it was built for)

That's probably overkill for a smaller shed but gives you a good starting point
(maybe 6" of gravel and 2x6 or 2x8 for the base - paver under where the rim and floor joist meet)



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Re: moisture barrier under shed?
Posted by: slbett
Date: July 13, 2007 06:51PM
Unless it is above the rest of the grade in the yard it will only retain water. Raise the grade and have enough fall to let any water run off or skip it entirely. Moisture is going to form on metal if you are in a cold climate.
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Re: moisture barrier under shed?
Posted by: mrbigstuff
Date: July 13, 2007 07:47PM
the floor will sit above the yard, raised up by thefloor joists, which will will be nearly levelwith the rest of the yard. i dug out about 6" - 8" below grade in which to lay the stone. i also bought sono-tube and was prepared to pour footings (four of 'em) to anchor thewhole thing down. that might be overkill, though.

so, i really have two issues, i guess. to pour footings or do a box foundation of posts and what to do about the vapor barrier and the stone.
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Re: moisture barrier under shed?
Posted by: lafinfil
Date: July 13, 2007 08:03PM
If you have 6" to 8" of packed gravel you probably are fine without putting down a footer with the tubes
You might even create more of a problem if you don't get them deep enough below the frost line
and end up having it heave on you during the frost cycle. A small shed can float on the top.

>
> the floor will sit above the yard, raised up by the floor joists, which will will be nearly level
> with the rest of the yard.
>

I wouldn't have any wood below grade (even treated wood)

If you want the floor to be level with the rest of the yard better pour concrete - IMHO
(unless you're selling the house in the next few years)



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Re: moisture barrier under shed?
Posted by: mrbigstuff
Date: July 13, 2007 08:21PM
Quote
lafinfil
I wouldn't have any wood below grade (even treated wood)

If you want the floor to be level with the rest of the yard better pour concrete - IMHO
(unless you're selling the house in the next few years)


nah, i don't really care about the floor being level, i dug down a bit so that i could get some stone inthere and create room for drainage in heavy rains. actually, the shed will probably sit above the grade by about 4-6" depending.

yeah, i have reservations about footings, too.
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Re: moisture barrier under shed?
Posted by: lafinfil
Date: July 13, 2007 08:41PM
FWIW - my small shed is about 10x12 and it sits on what appears to be a slap ablout 6" thick and the walls
of the shed rest directly on standard 8x16" hollow concrete blocks on that slab with a sill plate
so effectively it floats with the freeze and thaw cycle

The shed is about 55 years old and is in fine condition (well it needs a roof) but the slab is
sound and level and this is in an area of the country where the frost line is about 42"

Pack the gravel and put a couple of support blocks down (pavers) to distribute the weight
and support the corners - maybe every 4' too. No vapor barrier needed



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Re: moisture barrier under shed?
Posted by: mrbigstuff
Date: July 13, 2007 09:03PM
'fil, thanks. my shed floor will be 3/4" ACX, I believe and have to support a couple of motorcycles, some power tools and various planting stuff. It will also be 10x12. And, maybe one day it will have a tree house or such for the kidlets. (it's under a couple of trees now)

The only issue is that i dug out some old bricks that were apparently part of a century-old garage and undermined a bit of soil, so i am a little concerned about the solidity of the gravel now.
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Re: moisture barrier under shed?
Posted by: lafinfil
Date: July 13, 2007 09:12PM
Just pack the gravel good and you should be fine

If you can get a vehicle to the spot do a back & forth over the gravel with the wheels to pack it
and the pavers will widen the foot print and weight distribution


If you had given me some notice I would have come over and drank beer and watched ....

I mean supervised !



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Re: moisture barrier under shed?
Posted by: viaripatti
Date: July 13, 2007 09:34PM
BEFORE you nail down the floor paint the underside with good elastomeric paint-2 coats.
After the floor is placed and nailed down paint the top side with good paint. Now you will
have the best moisture barrier you could hope for. I've done this with every shed I've built and, in the middle of the heaviest rainy season, there is no moisture in my sheds where I store some expensive designs. I actually leave them airtight with no vents; by that I mean every last potential air gap is caulked. Also, placing a large moisture enhancer inside would be helpful insurance.



Edited 2 time(s). Last edit at 07/13/2007 09:36PM by viaripatti.
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Re: moisture barrier under shed?
Posted by: bfd
Date: July 13, 2007 09:41PM
For what you're doing, it sounds like concrete's the answer. But it's probably more than you were thinking about doing. Wood might last 15 years - maybe - in the conditions you've described.
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Re: moisture barrier under shed?
Posted by: mrbigstuff
Date: July 13, 2007 11:44PM
concrete means that if I ever want to dismantle it, it's all that much more of a PITA. plus, to do a cement floor correctly, I'd have to mix a lot and that means renting a mixer, etc.

but, yes, I thought about it.
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Re: moisture barrier under shed?
Posted by: ajakeski
Date: July 14, 2007 06:15AM
concrete




I'm not a bad guy! I work hard, and I love my kids. So why should I spend half my Sunday hearing about how I'm going to Hell? -Homer.
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Re: moisture barrier under shed?
Posted by: billb
Date: July 14, 2007 07:01AM
There can be tax and permit issues if you start bolting this thing down.
Same with a concrete floor and foundation. Often higher taxes.
Also a "movable" structure can often be placed closer to a fence without incurring a six or eight foot from the property line rule.

6 inches of stone should be fine.
Remove the nice dark rich moisture holding topsoil first.


If your shed is above grade consider nothing or fencing to keep animals out from underneath rather than solid skirting which can help retain moisture and block drying winds.
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