What shed base on top of large gravel please?

[BJul]

Hi

I had an area of my garden prepared by builders recently for a shed and later a summerhouse, which now has hardcore and then about 100mm deep of gravel (about 10-20mm size, not the pea gravel). They said when the time to lay a shed came it would be more cost effective to lay concrete fence posts as shed bearers on the gravel than make a concrete slab. Now I'm talking to shed companies who don't agree and say the gravel, and bearers may move. I get their point.

Now I'm confused what would be a suitable base for a small 8x4 shed. Should I build a timber frame and use metal corner stake kits to make above the gravel? will that be firm enough not to move on the gravel? Help!

 

[MarineBob]

I built an 8 x 10 shed a couple of years back. What I did was to use the sort of truncated pyramids (technically called a frustum I think) for the corners and one in each long side middle, although I am not sure those were needed. I tried to disturb the soil underneath them as little as possible but laid an inch or two of gravel to make for a bit of drainage under each one. For the floor frame I used 2 x 6's 12 in on center to make the floor solid with the 2 x 6's. I used them instead of 2 x 8's to keep the floor as low to the ground as possible. I put the pyramids into the ground a bit so the bottom of the PT 2 x 6's are a couple of inches above grade. If I were to do it again, I would have put some heavy screening on the inside of the framing down to the ground to keep critters out. After a couple of winters and such, no settling and everything stayed square and plumb. I think once everything is built not much is going to move anything. For what its worth, I have a 3/4 plywood floor with prefinished hardwood. I looked at a paint for $45 a gal and found a Craigslist posting where a guy had just the right amount of leftover flooring from a big job for my shed. Cost me $90 and now I got 3/4 ply and 3/4 oak floor. Kind of over kill but I subscribe to the idea of any job worth doing is worth going way over board.

 

[He who knows]

What about one of those interlocking plastic grid systems - eg (one of several on the market) https://www.shedbase.com/probase/ ?

 

[MarineBob]

Not familiar with those systems but another idea from my distant past came back to my bourbon infused memory. An old time carpenter I knew and worked on a couple of projects with used to do this. Instead of a more traditional approach, he used pipes for supports at the corners of a shed or room addition. Depending on how you want to attach to the bottom of your framing might need to weld some plates but here is what he swore was the best/easiest approach. He'd go to a scrap yard and find several pipes.....with two different OD's. One would fit sort of snugly, but not tight, inside the other's ID. He'd grease up the interface and use the larger pipe as a sort of frost protection slide over the pipe that actually went between the footing and the framing. The idea was the outside slide pipe would keep frost from moving the inner pipe so long as the inner pipe was footed below the frost line. Hope that makes sense. If frost is not an issue in your area, then this is probably not worth the effort. Looking at the link, I am not sure I'd do that for a shed. I like the idea of corner posts so if you do get some settling, You can always pry up the corner and shim back to level and plumb. I think sometimes people think a shed is too heavy to move but a good lever will lift about anything.

 

[AC Power]

If your detached structure (from the house) is simply a 8x8 shed, just built with 2x6 floor joists and normal 2x4 framing. Period.

What is appears you have there is either 3/4" or 1" washed gravel, that is, there is no sand in the gravel for proper pre-construction compaction.

As comments, pouring an 8x8 floating concrete slab (4" thick, above grade) is ok.....but it might be an overkill its only a garden shed.