Ok, so long story short I found out they were redoing the roof on a school about an hour and a half from here. They were selling off 2.5" 4×8 sheets of EPS for $3 a sheet so I hooked up the trailer and got enough to do under my slab. The plan was to stagger the joints since some of the edges are a little beat up and go two layers thick for a total of 5". Well, yesterday the inspector came for a different inspection and asked what I was using all the foam for. I told him and he says it wont pass. "Has to be XPS under slab" he says. I showed him that EPS is good for under slab but he doesn't think it's a high enough density EPS. In the end he says he'll pass it (cause this guy doesn't seem to know or care too much) but he thinks I'll have problems. So how can I tell? The foam does compress about 3/16"-1/4" if I squeeze it between my finger and thumb as hard as I can but I can stand on a 5 foot high stack and it feels solid as a rock. I figured being used under stone on a flat roof it would have to be a higher psi rating but it isn't as "stiff" as on of the Login blocks from my foundation. I think it will be fine but is there a way to know for sure?

Take a sheet (that hasn't been buried or soaking in a pond) and weigh it, then do some 5th grade math on it.

A cubic foot is 1728 inches, a 4' x 8' x 2.5" sheet is 11,520 cubic inches, so you're looking at 6.67 cubic feet.

Type-II EPS is 1.3 lbs per cubic foot minimum, somewhat higher nominal, and has pretty much the same load characteristics of Type-II XPS. So if it weighs 1.3 x 6.67= 8.67 lbs or more you should be just fine. I'd expect it to come in between 9-10lbs per sheet.

If it's more like 6-7lbs rather than 9-10 lbs it's Type-I EPS. That could be a problem if they also poured the slab too thin or the slab reinforcement wasn't up to snuff. If it's Type-I and you're worried about it, a quick consult with an engineer about what (if anything) needs to change on the slab construction to deal with it is cheaper than buying virgin-stock XPS.

What if I didn't make it past the 4th grade Lol!

This is great advice as always!

I'm the designer, contractor, concrete man. You name it its me. We're trying to do this mortgage free so I'd rather stay away from the Engineer if possible. I'll do the math and if I need to, I know where I can get some reclaimed 3" XPS for $25 a sheet.

I guess you get to be the engineer too? :-)

For some peace of mind, see:

http://insulfoam.com/wp-content/uploads/2014/04/pdfUnder-Slab-Design-Considerations.pdf

Here they are analyzing Type-I EPS under a 6" slab with 8000 lbs of forklift load (just like you have in YOUR house, right?):

http://www.structuremag.org/?p=1812 (<
With a 4" slab you can have a polka party with a dozen 200lb Czechs dancing while carrying kegs on their shoulders without having to sweat this one. About the only time you really need to do the real math on it is if it's carrying a substantial fraction of the weight of the house (like under grade-beams or footings.)

I buy reclaimed foam all the time. The easiest way to determine the density is cut a 12"x12" piece and weight it. I use a small scale I bought at Walmart. Same math as Dana said but more accurate by using a smaller piece. I've never had any reclaimed material that was over 1.5 lb. it's too expensive for roofing. If you find some Helix , you can pour a 3" slab that will out perform a 4" with standard mesh.

I've seen reclaimed polyiso that was pushing 2lbs density, but reclaimed EPS is usually sufficiently over 1lb that I haven't worried about it.

A lot of virgin stock Type-II EPS is really only 1.3-1.4lbs density, closer to the specified minimum than the "nominal" 1.5lbs. I'm sure there's a lot of Type VIII EPS (1.25lb nominal, 1.15lbs min) goods in the reclaimed mix of roofing foam too, which is also fine under any reasonable residential slab, and noticeably more rugged than Type-I. Bare Type-II is more "walkable", and won't leave deep permanent footprints unless you're really trying to. Type-I is more fragile.

OK so I put a sheet in the garage to dry overy night and just weighed it. It came in at 6lbs almost exactly. Maybe a little under. But it does have some weathered edges and corners. I had to use my bathroom scale and weigh myself, hold the sheet and do the math so probably not the most accurate.

I also measured the foam which I hadn't done before and it's 2.25" thick not 2.5". Thw guy said it was 2.5" and I never checked. I'm 200lbs and I can walk on a sheet without leaving footprints. If I bounce up and down with my weight on one foot, It leaves a print you can see but it's at most 1/8" deep. Probably close to 1/16". Even just one sheet feels "solid" to walk on and doesn't feel like your going to break it. I used to do a lot of commercial concrete work 12 years ago so I know what walking on under slap EPS feels like and it feels OK to me.

What do you think?

Given that this will essentially be a "floating slab" (no floor or wall load) and the highest load it will ever see would be possibly a pool table at some point, I'm going to use it unless the inspector changes his mind.

Thank you

You really don't have much to worry about here as long as the slab has the required thickness. Even a 2" rat-slab would be walkable, but might crack during an earthquake with a 50 gallon water heater resting on it. But a real 3"+ slab would be fine. The pool table is going to be fine. (I have a 50 gallon water heater resting on a <2" rat slab poured directly on clay soil. No cracks have formed but would expect it to in an earthquake.)