I’m in the design/permitting phase of building a detached MIL unit in my back yard (22’ x 15’), and I want to get some thoughts on slab foundation design. I’m currently going back and forth with my architect and engineer about spread footing and stem wall vs. monolithic slab w/thickened edge. From a code perspective, either is acceptable. Other context: this will be a heated structure with a frost-protected foundation; the grade is flat; and my intention is to do it myself.

I was initially inclined toward monolithic slab, because it’s a single pour instead of three pours (each of which will have a short load charge and a service charge). I see this as a savings of both time and money. Also, the forms seem simpler.

The counterargument, I’m being told, is that placement of utilities is easier with a traditional spread footing. I don’t fully understand why that is. If the forms are in place for the monolithic slab prior to the utilities being placed, how does the presence or absence of actual concrete make a difference? And how much of a difference is it, really? It seems like using ABS and PEX gives you all the flexibility you’d really need, but I would love for someone to challenge my view on that.

I've also just learned about prefabricated ICF foundation forms, which could possibly invalidate my argument about forms being simpler for a monolithic slab.

I also understand the monolithic slab requires somewhat more concrete, but by my rough math, the cost of the additional concrete is outweighed by the savings of multiple pours.

Thanks!

 

How deep is the frost line?

Slabs

That site has a lot of info on slabs. It might help.

 

Frost line in Seattle is 18". However, the municipal code is clear that frost-protected foundations for heated structures don't have to go below the frost line.

Thanks, also, for the slab resource!

 

You may not be required, to have the footings below the frost line but I would do it anyway.

 

I agree, especially with the frost line only being 18" deep. If I remember correctly when I lived in central fla they required all footers to be 16" deep [and they hardly have any frost depth]

 

Ok, noted about frost line--seems like sound advice. Do you have any thoughts on the monolithic slab vs. spread foundation question? Specifically regarding how hard it is to locate utilities?

 

As a painter, I've been around a lot of new construction and for the most part the only time I've seen them use monolithic slabs [for residential - in the southeast] was when the ground wasn't solid enough to support a traditional foundation. Hopefully some of the others that know more about foundations will chime in and give you a better answer.

 

I don't see how utility lines can be accessible once the utility lines are buried in any slab. I recently started a thread called Pex in plain sight. A plumber had run Pex along the top of a wall to avoid excavating a slab, to repair the original pipe. My job is to hide it.

I don't see how a monolithic slab can be as strong as a slab with spread footings or ICFs. Bridgeman would be able to clarify that, if he is still around. Stadry seems to have disappeared completely.

 

They usually pour the concrete around a PVC pipe chase to house electrical wires or HVAC lines.

 

For what you are doing I simply don't see any reason not to do a monolithic slab. It can be just as strong and as you mentioned is much easier to do. The forms as you mentioned are much easier to do, all you need to do is a perimeter form. Utility feeds are not any harder either IMO. I would put all of my supply lines inside of larger PVC sleeves so you could move the PEX and replace it in the future if we need arose.

 

Thanks, this is helpful. I have no structural concerns whatsoever about the monolithic slab--the engineer and architect have both signed off on it. My real concern was about opening myself up to a greater chance of plumbing errors. Appreciate the reply!

 

Some building inspectors and city codes limit the size of pad and foundation that you can pour in a monolithic fashion. I'd suggest you check with your local agency if you go that route, just in case. Generally up to 24x24 is not a problem, but you never know. I would think your architect wouldn't steer you wrong.

 

City code is not at issue here; we can legally go either way (I have a licensed engineer doing my foundation plan). My question is, really, just why would a plumber prefer one system over the other? Either way, the foundation forms are in place so you know exactly where wall is going to be and therefore where the plumbing should be stubbed up. It's not like the spread footer gives you any more wiggle room for locating plumbing, right? You still have 5.5 inches within which to locate a 2.5" OD pipe. And for the toilet and shower drain, where it's not in the wall, you still have to measure just as carefully. I just don't really understand why that preference exists.