I am trying to figure out what is the most cost effective or least expensive way to build a roof for a two level ICF house.

Here are the details:

- Approximate roof area 1500 sqft
- Zone 5 (Pacific NW)
- Hurricane area: no
- Target R value of the roof assembly = R60
- Roof deck = Hambro with 3" concrete
- Architectural requirement - either modern looking flat roof (0.5:12 pitch) or traditional gabled roof.
- Contractors specialized labor requirement - at least as possible if at all.

After evaluating various options such as wood trusses, ICF built gables with beams and rafters or simply a flat roof, I am inclined to believe the easiest an also the cheapest option is to place EPS tapers on top of the concrete deck, glue them down and add some TNG OSB or plywood on top.

After that I can either apply a vinyl deck membrane (such as duradek) or some other coating.

Having the OSB on top will also allow re-roofing with a standing seam metal in the future.

From my research it looks like I can find 1 lb/cft EPS geofoam or flotation billets on craigslist for under $100/cubic yard so I should be able to cover the whole roof with 10" thick EPS for less than $2000.
There won't be any crickets - it will be sloped in one direction only. Some special attention will be needed to the edge where the gutter will be attached in order to avoid thermal breaks.

I am also thinking that I may have to cut the EPS blocks myself to create the tapers so I have budgeted about $1000 for a 48" hotwire.

How does this whole plan sound to you? Do you see anything wrong here?

If you're doing a flat roof, do a flat roof.
Use the EPS geofoam to lay out your water control countouring, waterproof the surface and put down some form of ballast.

OSB and standing seam are contraindicated on such a low/no-pitch roof. And they'll flatten out any sort of water control you have in place and won't shed water properly.

If you're planning on using the foam to simulate another type of roof pitch atop a flat roof, you're going to need some sort of support structure other than the foam. Because you have to mechanically tie that back into the roof deck. You can't just drop a foam sandwich on top of the flat roof and call it done.

Thanks for the suggestions.

My plan is to use foam blocks glued down to the concrete deck. I am thinking the glue alone should be good enough and I hope to be able find thick enough foam panels - at the highest point it will be something like 18 or 20 inchex so I hope I ca put only one piece of foam instead of multiple layers.

Now that I am thinking - the glue may give up over time so the glue alone may not be enough. Not sure how to attach it to the deck otherwise. Maybe multiple layers of foam fastened to each other may be a good idea in addition to the glue.

No offense, but talk to an engineer before you trust your roof to nothing more than construction adhesive.

Ditto.

I am not stuck on having a flat roof but I think that is the easiest.
I get that attaching several layers of foam only with glue is not a good idea so some kind of long screws or fasteners will be needed. I am still trying to figure this out.
The main question still remains -is the flat roof the better option?

I am not stuck on having a flat roof but I think that is the easiest.
I get that attaching several layers of foam only with glue is not a good idea so some kind of long screws or fasteners will be needed. I am still trying to figure this out.
The main question still remains -is the flat roof the better option?

The most cost effective roof on ICF or any other residential building is a standard wood truss structure with asphalt singles. That’s why contractors typically use this approach on tract housing projects. Why do a concrete roof if you don’t have a real need to have one. A flat roof is always a high maintenance headache. Having a vented attic space is way better than anything else. Architects are usually best avoided because they often design expensive structures without adequately addressing construction and engineering requirements...lots of corners and high ceilings and other dumb things...all which become problematic to owners.

I am totally with you on avoiding the architects
The reasons for the concrete roof are the air tightness of the house and the ability to expand and add a new floor on top in the future. Also the concrete roof will be mostly leak and rot proof.

Hey Pacificstart: Is there a reason that you want to use “Hambro” instead of AMVIC Amdeck, Quad-lock Quad-deck, Lite-form litedeck, etc? Have you compared the prices? I am working to build an ICF house in Florida, with a shed ICF roof (needs to be rated for >130 MPH wind). I don’t want to use any asphalt shingles and will be gluing a white color EPDM to the concrete roof (similar idea as using duradeck). I could always add EPS inside (to the ceiling) to increase the R value. You end up with a continuous EPS when transitioning from the walls to the ceiling (roof) by using ICF.

I am not suck on Hambro but one of the main reasons for using a truss / open web joist system is the 32 ft span. Using insuldeck or a similar icf system would require additional structural reinforcements such as concrete pillars and beams which will need to be integrated in the first floor as well. That will require additional forming and also shoring for the pour of the floor itself.
As for the outside of the roof I will still need to add additional foam or some kind of insulation up to R60 in order to achieve passive house certification. Either way - Hambro or Insuldeck the outside of the concrete will need to be insulated to prevent thermal bridging propagating into the walls. So in my view, the insulation on the outside of the concrete is more important than the inside insulation.

The advantages I see with Hambro are:
No shoring required
Large spans
Room for MEP

Main disadvantage I see with Hambro vs an ICF floor are:
Additional furring channel required for ceilings
More work required to achieve proper acoustical insulation between rooms.

Hey pacificstart, take a look at this YouTube video (https://www.youtube.com/watch?v=0_fq4Fah_Ow ). I think, you can get the 32 ft span using ICF. The span depends on the rating of the c-channel (truss) inside the ICF (plus the re-bars used on top, inside the concrete). You will not have any thermal bridging by using ICF on the roof because the EPS (at the top of the inside wall) will be touching the EPS on the ceiling (roof). The roof (ceiling) concrete will be on top of the EPS.

Posted By z_zk_z on 13 Aug 2017 12:14 AM
Hey pacificstart, take a look at this YouTube video (https://www.youtube.com/watch?v=0_fq4Fah_Ow ). I think, you can get the 32 ft span using ICF. The span depends on the rating of the c-channel (truss) inside the ICF (plus the re-bars used on top, inside the concrete). You will not have any thermal bridging by using ICF on the roof because the EPS (at the top of the inside wall) will be touching the EPS on the ceiling (roof). The roof (ceiling) concrete will be on top of the EPS.

You'll still see thermal bridging from the top surface of the roof into the concrete walls (bypassing the exterior insulation layer) if you don't insulate the exterior roof surface.

Posted By pacificstart on 12 Aug 2017 07:06 PM 
The reasons for the concrete roof are the air tightness of the house and the ability to expand and add a new floor on top in the future. Also the concrete roof will be mostly leak and rot proof.

You can achieve air tightness with truss construction more easily. Concrete will be rot proof but don't count on it being leak proof. In fact concrete will crack and create leaks. As for adding a third floor later the cost of destruction to get started might even be cheaper with a truss roof. In fact rot proof shouldn't be a concern. That presupposes the structure getting wet, which is exactly what you want your design to avoid. A sloped roof vs a flat roof means you only need to design it to shed rain water, not be waterproof.

If you don't intend to actively use the attic space I would guess the most economical way to go is with a truss roof using heel trusses. Spray foam a couple of inches on the ceiling and then dump cellulose to your heart's content, up to R60 or even R80!

Some things are better done not DIY. Putting up a roof is one of them IMO!

My thoughts exactly Dmaceld.

yes, that is true, but my ICF shed roof will have overhangs (ICF extending beyond the walls), and my thermal bridging will be due to the re-bars connecting the walls to the roof. see attached photo, in my case the roof EPS will be on top of the walls and beyond it (overhangs), so no roof concrete will be in contact with the concrete inside the walls (thermal bridging will be via the rebars from the walls, through the EPS, to the rebars on the roof.

We did a flat shed style roof with overhangs and used 1" fiberglass rebar to connect the roof to the wall and prevent thermal bridging. We used a water proofing additive (vl2020) to the concrete so we have no membrane. Spanned 32' with no load bearing interior walls. Talk to quad deck or an engineer familiar with the product.

Hey ksandjs, Thanks a lot for the information. I didn’t know about the fiberglass rebar. People with KIO (fish) ponds add XYPEX admix / poly urea to concrete for waterproofing. Is VL2020 the same? Can you please post photos of your ICF shed roof? I want to see if it looks okay, due to having a thickness from the EPS & concrete. I want to build an ICF based shed roof myself, and not worry about having roof shingles with >130 MPH wind. I also want to collect rainwater from the roof.

ksandjs - how did you break the thermal bridge between the concrete itself in the walls and the roof? Can you post some pics or drawings?

Hey pacificstart, did you see my last reply to you? yes, that is true, but my ICF shed roof will have overhangs (ICF extending beyond the walls), and my thermal bridging will be due to the re-bars connecting the walls to the roof. see attached photo, in my case the roof EPS will be on top of the walls and beyond it (overhangs), so no roof concrete will be in contact with the concrete inside the walls (thermal bridging will be via the rebars from the walls, through the EPS, to the rebars on the roof. here is the photo: https://www.google.com/imgres?imgurl=https%3A%2F%2Fbeopt.nrel.gov%2Fsites%2Fbeopt.nrel.gov%2Ffiles%2FQuad-Deck%2520Roof%2520Overhang%2520-%2520December%25202013%2520Final%2520hi-res.png&imgrefurl=https%3A%2F%2Fbeopt.nrel.gov%2Fnode%2F411&docid=ANy99vFryJmF2M&tbnid=nnZWT14FlE8DcM%3A&vet=10ahUKEwj06YvDm93VAhWCw4MKHQC4DC0QMwhjKBMwEw..i&w=1530&h=1055&bih=569&biw=1280&q=icf%20roof%20options%20photo&ved=0ahUKEwj06YvDm93VAhWCw4MKHQC4DC0QMwhjKBMwEw&iact=mrc&uact=8