Seeking advice on storage shed construction.

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Forum topic by HuckD posted 11-21-2013 09:03 PM 4057 views 0 times favorited 13 replies Add to Favorites Watch
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311 posts in 1911 days

11-21-2013 09:03 PM

Topic tags/keywords: question moisture

Ok, I know this isn’t about the wood shop but having a 12×16 storage shed is going to get a bunch of junk out of my shop. :)

The construction crew is to be here next Monday. I’ve spent this week preparing the site by digging up all the sod and cleaning up the area. The shed will be sitting on 4×8x16 concrete blocks – only one layer as my site was mostly level. I’ve got all the blocks level. The construction will be 4×4 pt skids, then 2×6 pt joists on 12” centers. Once it’s built I’ll have no access to the under side.

In an effort to prevent moisture problems underneath I put a layer of 4 mil poly down under the concrete blocks.

My question: do you think the poly was a good idea? Does anyone have any experience or advice. It’s not too late to remove it completely, or I could cut around the blocks and put the blocks directly on the ground, leaving the rest of the poly in place but held down with bricks.

-- Visit my Youtube Channel:

13 replies so far

View luv2learn's profile


2871 posts in 2500 days

#1 posted 11-21-2013 09:18 PM

Hey Huck, I would not put poly under your shed. The moisture from rain and/or snow will pond on it. If you live in an area of the country where you experience freezing temperatures you might want to consider digging out under your concrete blocks and fill the space with 8 to 12” of gravel to help eliminate ground heave in the winter. It will also serve as drainage during wet weather. But hey buddy that is just my opinion and we both know what those are worth LOL.

-- Lee - Northern idaho~"If the women don't find you handsome, at least they ought to find you handy"~ Red Green

View crank49's profile


4032 posts in 3168 days

#2 posted 11-21-2013 10:02 PM

I would want the vapor barrier poly on top of the blocks.
Can you cut the existing poly between the blocks and put a new layer on top of the blocks.
Let the water run through under the blocks but still have a vapor barrier under the wood that way.
The gravel would have been a good idea too, but I think that would require starting over.

View GFYS's profile


711 posts in 3668 days

#3 posted 11-21-2013 10:37 PM

geo fabric might be a better idea.
a vapor barrier is pointless since its open anyway.
heaving probably wont be an issue unless you plan on heating the structure or have posts exterior to the structure penetrating the ground
I don’t think the plastic will be an issue either way although I wouldn’t want a swimming pool under the building..

View patcollins's profile


1687 posts in 3062 days

#4 posted 11-21-2013 10:43 PM

I recomend gravel under the storage shed for the simple fact that it will keep animals like ground hogs and skunks from digging burrows under there.

View GFYS's profile


711 posts in 3668 days

#5 posted 11-21-2013 10:47 PM

true pat…its kinda spendy but 3/4 crushed stone aggregate on top of geofabric extending out past the eaves is what I usually do.

View jdh122's profile


1052 posts in 3015 days

#6 posted 11-22-2013 12:30 PM

I don’t think the poly under the shed is a good idea – you need drainage under there. I don’t know where you’re located, but up north here it would be important to put gravel at least under the concrete blocks, ideally down to below the frost line.

-- Jeremy, in the Acadian forests

View NoLongerHere's profile


893 posts in 2873 days

#7 posted 11-22-2013 01:23 PM

If you could line the outside walls with more concrete blocks, it would be better at keeping the critters out. Chipmunks will dig under and make a mess out of it eventually. I used this method on my shed when I first built it to save money. Hindsight – I wish I had (the money to) poured a slab to begin with.

It lasted about 8 years before I had to replace the wood floor and pour a slab. See my story: The Tool Shed

I used 4 mil poly to keep the moisture down and prevent roots from growing. But I also poked holes in it every foot for drainage, if needed, and I put 3/4 gravel, 2” thick on top, just like I do under the decks I build. It keeps the gravel from sinking in to the dirt too.

I had a 4×4 foundation and the outside band sat on cinder blocks so it was sealed from big critters but the chipmunks turned it in to a maze of tunnels and started chewing on the wood.

View Dan Lyke's profile

Dan Lyke

1520 posts in 4322 days

#8 posted 11-22-2013 07:18 PM

Poly under slab, or on the bottom (ie: on top of the dirt) of a well ventilated crawlspace, is a good idea. Seems like if you can minimize rain ingress from the sides, that poly is exactly what you want, and where you want it.

-- Dan Lyke, Petaluma California,

View  woodshaver Tony C   's profile

woodshaver Tony C

6222 posts in 3550 days

#9 posted 11-22-2013 09:42 PM

The only thing you need to make sure of is not to trap water under the shed. Good air flow and drainage and like someone said here Stones to keep critters from digging holes is a good idea. If your land is flat I would mound the dirt in the middle of the shed a little bit to keep water from pooling under it. keeping water flowing is always a problem if you on flat land. Plastic is used under homes with crawl spaces and that’ ok, it’s a moister barrier and rain can’t get to it. but under a shed will just create more problems then it fixes.
Hope that helps!
Good luck whatever you do!

My shed is about 6” off the ground. All I did was kill the grass. The land is on a slight slope toward our pond so water will just runs off. I use PT 4”x4” posts and PT 2”x6” joist and for the floor I used 3/4” T/G subfloor ply. So far after 5 + years no rot at all.

-- St Augustine FL, Experience is the sum of our mistakes!

View Tooch's profile


1805 posts in 2073 days

#10 posted 11-22-2013 10:13 PM

I’m with tony, no need for the poly as long as there is some kind of drainage. Mine sits directly on concrete blocks and so far no mold/mildew/rotting (4 years old).

good luck!

-- "Well, the world needs ditch-diggers too..." - Judge Smails

View HuckD's profile


311 posts in 1911 days

#11 posted 11-23-2013 05:18 PM

Thanks for all the responses!!! I’ve decided not to use the poly and took it up this morning. Moving concrete blocks around on the ground is not something this 67 year old likes to do! lol.

I did a little shovel work to facilitate drainage in three spots so I don’t think I’ll have a problem there. There’s a slight slope from front to back.

The borg delivered all the materials this morning and the runners will be 4×6 pt instead of the 4×4 that was in the original info. I like that better. :)

Thanks again everyone.

-- Visit my Youtube Channel:

View  woodshaver Tony C   's profile

woodshaver Tony C

6222 posts in 3550 days

#12 posted 11-23-2013 05:38 PM

Great! Your on your way! I love having a shed. I made a work bend and put a small sink in my shed. I also put power in it so I could have a florescent light. I use it for garden tools , Potting plants, Plant food , lawn food and to store my Snapper lawn tractor.

67! I’m right there with you! Good luck with your shed build!

-- St Augustine FL, Experience is the sum of our mistakes!

View sean28's profile


6 posts in 1601 days

#13 posted 07-22-2014 07:37 PM

sorry have no idea. I am a beginner and this info really helped me. Thanks everyone.

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