Solid cinder block foundation for shed?

Hi, I'm considering building an 8x12 shed (or getting a nice used one plus a delivery service costs about half of materials to build one), and elevating it up on a solid cinder block foundation.

Basically what this picture shows except mine would be on the ground instead of dug down, so it'll be elevating the shed above ground.
https://permaculture4peace.org/2014/...ouse-step-one/

What I mean is laying a whole bunch of free/cheap cinder blocks tight side by side with the solid side facing upward and the open sides facing sideways, basically making a concrete slab with them. And then coating the whole thing with an inch+ of mortar/resurfacer so the rain/moisture that gets under the cinder blocks doesn't evaporate up into the shed. The perimeter blocks will have the shed frame on them, so I'll put those blocks with the open ends upward and fill with concrete so they're stronger and so the perimeter blocks flat side are facing out to help prevent rain from entering the cinderblock foundation.

I know concrete is porous, but do you think this is enough to keep a mower and tiller etc from rusting? If the concrete layer on top cracks from settling, I can just caulk it or trowel concrete patch. Even though the sill plates will be treated, I'll even paint them with coppercoat wood preserver.

Or Maybe I should skip mortar-coating all the cinderblocks and instead put plastic sheeting on them and then plywood over that? The shed will have gable vents (and soffit vents if I build it) to exit moisture. Walls would be T1-11 PT with no tyvek behind. I'm thinking tyvek will trap moisture in the shed even though it's meant to breathe. T1-11 will be primed and painted (basically sealing inside whatever moisture gets in, but the roof will be proper sheathing with felt paper and shingles which I guess lets moisture exit upward regardless of tyvek or T1-11 essentially be sealed by paint) and possibly first coating the T1-11 with coppercoat wood preservative.

One other thing, considering that about moisture exiting up through the roof sheathing, would it be wrong to use already-painted repurposed 1x5" decking boards as sheathing and then felt paper and shingles over that, or does the roof need to breathe more?




Other option besides cinderblocks is to build a treated deck under the shed to elevate it about a foot off the ground. But it would be cheap and easy to just lay cinderblocks which also omits the need for floor joists and floor plywood flooring.

 

It seems you are blogging more than you are asking questions. Is there a question in your post that we can help you with?

 

Block are weakest when set in that position!

 

Marksr, I know cinderblocks are weaker in that position but not much weight will be on them. Perimeter blocks would put correct way and filled with concrete for the shed walls.


Chandler, I had a question which was if a 1-2" mortar layer over cinderblock keeps enough moisture out to prevent rust/deterioration. Now I have a lot of questions/ideas here:

I can omit the mortar layer and instead put 6mil plastic sheeting on the cinderblocks and lay a plywood floor (doesn't need to be pt with plastic under). I know plastic will shift and rip but can easily be replaced. I could also omit the mortar and instead put a sand/dirt layer over the cinderblocks and plastic sheeting over that and then plywood, so the plastic isn't rubbing against cinderblock to rip as easily vs sand/dirt.

I will tamp the ground before placing the cinderblocks, so hopefully they don't sink (but hopefully would only need a quick patch repair if it's mortar'd), but when it rains enough, this area puddles up a bit. So I think moisture will still wick up through the mortar and cinderblocks if no plastic is used. I can use concrete sealer on the mortar layer but plastic over dirt/sand seems better. A salvaged pt deck frame is possibly easier and stronger though.



After seeing the puddles in the plot area today, I'm leaning more towards using a pt deck platform because air flow under it should dry puddles out fast enough. Plus the chance for cinderblock settling. If I do the cinderblock, I think I would do the method with putting a dirt layer over them and then plastic sheeting and then plywood. However, on the perimeter, I would need to put the shed wall frame on the cinderblocks as opposed to on dirt/sand under the wall sill plates which would 'wash' out. In other words, with the plastic sheeting over dirt/sand, I would still need to address putting the sill plates directly on the perimeter cinderblocks and worrying about moisture wicking up through them. Do you think if I use pt sill plates and coppercoat wood preserver them, they would be fine?

another thing though is if I get the shed prebuilt used and delivered, I should probably go with a wood foundation vs putting the prebuilt frame sill plates directly on the cinderblock. (in other words, I could put the prebuilt frame on plastic but then I'll have no way to replace it if ever need be from deterioration. And if I wanted to seal the underside of the prebuilt frame, it would have to be while it's lifted with the shed-moving machinery. Not something I really wanna do.

For the deck, most people put it on single cinderblocks about every two feet.
but I don't think that gives enough height (air flow) because the area gets puddles.

If doing a wood framed platform, I might do double 12" high cinderblocks with concrete in them to lock together. And dig the first cinderblock down so the height to bottom of deck frame is about 15". 15" gives good air flow and I can crawl under if need be for whatever reason.

Or maybe I'll make ~15" cast piers for a deck frame. The precast piers are only about 8.5" tall.


I know people usually just frame a pt deck over cinder blocks few feet and put pt plywood on that and call it a day, but I'm worried moisture will come up through wood and rusting the mower/tiller etc. So,

do you think I should frame a deck box, put plastic on top of that and then plywood (or salvaged 1x5 pt decking) over that? Or should I crawl under the platform and tack plastic sheeting to the underside of the joists so moisture doesn't even get to the joists at all? How long do you think 6mil plastic would survive in either situation? I think putting plastic over the joists would be better than crawling under. I know not to do both ways because that'll lock moisture into the frame.

 

Like I said.....678 words is a bit much to convey simple questions. Is there a way to shorten your questions. Absolutely no one will read these diatribes. It is too much like blogging which we don't allow on the forums.

 

aite maybe someone will answer just this:

do you think I should frame a deck box, put plastic on top of that and then plywood (or salvaged 1x5 pt decking) over that? Or should I crawl under the platform and tack plastic sheeting to the underside of the joists so moisture doesn't even get to the joists at all? How long do you think 6mil plastic would survive in either situation? I think putting plastic over the joists would be better than crawling under. I know not to do both ways because that'll lock moisture into the frame.


1-2" mortar layer over cinderblock keeps enough moisture out to prevent rust/deterioration?

 

ah one last simple question, sorry I don't have Google Chrome to edit posts...


Suppose I have a 10" high concrete slab in an area with only 2" puddles. I want to put a shed on the slab but want to prevent moisture from emitting up from under the slab even though it's 8" above the puddles. I put a 1" dirt layer over the slab, and then 6mil plastic sheeting over the dirt, and then plywood floor over the plastic sheeting and that would keep moisture out (and the dirt will prevent the plastic sheeting from rubbing and ripping on the concrete)? I can check up on the plastic and re-do if needed if it does happen to rip or deteriorate.

 

anddddd, as mentioned, I am wondering if there's a max thickness a roof should have. I googled that but found nothing. Too thick prevents moisture from existing up? I said I might use repurposed 1x5" pt decking as roof sheathing, felt paper and asphalt shingles over it. I know I made some really long posts a few days ago but this oen wasn't so bad. But now the thread is a mess I don't think anyone will help. TY

 

to answer my own questions and finalize this:
I'm not at all considering the cinder block foundation anymore. If they were free and very close, maybe, but still probably not.

The puddling area the shed is going, I guess the best I can do is fill it with free fill dirt and make sure it's not too sandy. If it's clay-like and not sandy, it should help sweep the water away. The area is on side of house purposely graded to let rain flow out to street, so there's no avoiding water passing through.

I'm not attaching plastic sheeting under the bottom of the foundation frame. Don't want to trap moisture in there. Someone said it would be fine and air will take the puddles away fast but I noticed, on a paver patio with a picnic table which is often a good 3' off the ground (good air flow), the cracks in the pavers under the table (often clear glass which lets sun through to help dry out), still there's often much more moss/weeds in the paver joints under a table meaning that the moisture does just evaporate onto the bottom of the table and falls back down and repeats. I'm just hoping with filling in some dirt before building the shed that it will be good enough to not pool and potentially cause a rust problem in the shed. I'll have vents in both gables and about 4 in the soffits.

And, about using free used prepainted 1x5 decking boards as the roof sheathing to save some money, I don't think it would be too thick and prevent the roof from 'breathing' because the asphalt shingles would not let it breathe either way most likely. I might just use osb anyway since it's pretty cheap and easier.

 

Clay will often absorb water while sand will drain well.

Is it feasible to dig some sort of ditch to funnel the rain water past/around the shed and not at it?

 

If you pour a slab, you should have the plastic under the pour. If you have bare earth, you should have the plastic on the ground, not under the joists. It keeps the moisture from eking up and will allow air flow to keep all above it, including the joists and flooring drier.

I agree that OSB is a good choice for the roofing. 5/4 x 6 planks are really heavy and you can only get them in Pressure Treated, which may tend to warp with the heat that will be captured under your shingles. Be sure to use a drip edge and an underlayment prior to applying the shingles. Leaving a ridge vent, although you don't have a ceiling in the shed will still equalize the moisture inside the building by allowing the heat to escape. One drawback would be it also allows the heat to escape in the winter, so you will need to weigh the pros and cons of that.

 

I think sand would just let water seep and still pool. it's like gravel. I will make a channel like you suggested though.


I might ridge vent. Or two gables. Either way, the soffits will be vented. I have to google to compare those. I think ridge would be much better though. I don't think I'll use decking as the roof since OSB is so cheap.

 

A continuous ridge vent is almost always preferable to gable vents.