I'm building a 16x24 shed/garage. I have done most of the site work and I have a concrete guy starting tomorrow on the forms in preparation for a pour on Friday. He's done insulated slabs, he has 30 years experience, I trust him, but I don't quite know if he's up to date on the exact designs of FPSF slabs. I just read a PDF, I'm not an expert, so I could use some help too.

enter image description here

Below I have questions on vapor barriers, rebar, compaction, an extra footing "rib" or two, and pipes.

We're doing a monolithic FPSF because the ground underneath is moist in the spring. One of my concerns is moisture coming through and causing rust on whatever is inside the shed. The location of the vapor barrier came up - he suggests under the insulation. I'm worried about water seeping from the sides, but he has assured me that all I need is a french drain around the perimeter and it'll be the best thing ever for wet ground. I don't disagree, I'd just like to verify. I am worried about the french drain negating the insulation value of the ground on top of the foam insulation. I have space to add another 2 inches of topsoil around the perimeter if that drain means less ground insulation. Do I even need to worry about moisture leeching through the sides?

My next concern is rebar and mesh. He says to use fiber concrete and I don't need mesh on a slab so thin. He agreed to four #4 rebars around the perimeter though. I think it'll cost only an extra hundred or three if I lessen up the fill, make the slab 5 or 6 inches thick, and use mesh. I've been forewarned that all concrete cracks anyways, but I'd prefer my slab not breaking in half and continuing to separate.

Then I thought about weight bearing on top the insulation. To properly bear the weight of the slab, do I need to compact the fill on top of the foam? Won't this damage the foam? Should I start with sand and stones on top the foam since it's self-compacting?

We're going to need entry for electrical, an exit for electrical, an entry for data lines (likely optical) an entry for irrigation well water, and an one exit for well water. Is there anything I should know about pipes through FPSF foundations? I'm assuming I want the pipes under the insulation to keep them stable. I don't know if a pipe through the insulation is a "cold bridge," I considered making insulation sleeves for the pipes with layers of hole sawed insulation. The electrician says I need a fitting between the slab and the ground to allow movement, but I don't know how that would work.

There's a structural need for a post next to the stairs to bear the weight of the second floor. It's sort of a loft setup I'm going for. I assume that I want a bit of a "divot," where the concrete under the post goes all the way down to the same depths of the footings, but how should I attach the post? Just put a bolt there perhaps and then some sort of a mechanical fastener? Or can the post just rest on that spot, with some shear support from being attached to the stairs?

While I was thinking of that, I thought about rigidity of the slab. I don't know what it's called, but some photos I've seen have "ribs" under the concrete, kind of like footer type things throughout the slab. Instead of pouring the concrete floor on a level surface, there's a coarse grid pattern to keep it rigid. Should I do things like that here? Instead of doing just the perimeter being over a foot thick, there would be the perimeter and a plus sign shaped grid of thick concrete inside the shed.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.