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Community and Q&A

Minimum EPS density (compressive strength) for roof/wall external sheathing

Jose Castro | Posted in Green Building Techniques on

Do you know if it would be possible to use low density EPS on top of a roof sheathing?

I suppose the critical thing here is the EPS being able to support either the plywood, OSB or 2×4’s that lie over it. In other words, we need a minimal compressive strength .

I would bet that with plywood or OSB you would not have any problem using any low density EPS, because the force exerted by the roofing weight is spread through all of the plywood/OSB board’s area, but if you use 2×4’s spaced 24 inches on center, to hold the rigid foam and nail, for example, a metal roofing over it, I’m not sure that a low density EPS will hold without being squeezed by the 2×4’s holding the weight of the roof.

I think the lowest density you guys get in the US is Type I, which is 1 pcf. But here in Chile the cheapest and most commonly used EPS is 0.62 pcf.

Do you think a 0.62 pcf EPS would be ok using 2×4”s on a light metal roof? (I think we are talking of a compressive strength near 4 psi)

Would you recommend a higher density EPS?

What about a wall? what compressive strength would I need for an external EPS insulation sheathing installed with bolts, 1×4″ furrings, and holding wood siding?



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  1. Expert Member
    Dana Dorsett | | #1

    >Do you know if it would be possible to use low density EPS on top of a roof sheathing?

    Even without plywood 1.25 lbs per cubic foot "Type VIII" EPS is considered "walkable" on flat membrane roofs on commercial buildings. With a plywood nailer deck the 0.62 lb stuff would be fine.

    In the US 0.70 lb "Type XI" is available- more often used in protective or thermal packaging products for shipment, and definitely not "walkable" on a roof without permanent deformation. With plywood to distribute the load it would be fine though.

    It's not clear if it would be good enough for mounting steel for 2x4 purlins, but it should be OK with 2x6 or 2x8 purlins. I'm not sure how you'd mount the purlins without damaging the foam though.

    On wall sheathing it should be fine with 1 x 4 furring as long as the screws for the furring penetrate the studs by at least 1.5". In the US it's common to use pancake head timber screws for attaching the furring, but any high quality wood screw rated for outdoor use would be fine, with fender washers to avoid splitting the furring.

    1. Jose Castro | | #2

      That's great Dana. You wouldn't believe how helpful you've been.
      Answering your question, the purlins on the roof would be mounted just like the furring on the walls, using long screws that penetrate at least 1.5" of, in this case, the rafters. I've done this before, on both walls (in which I did use the 0.62 pcf, but I wanted to check an external opinion just in case), and roofs (in which I formerly used 1.87 pcf foam panels and 2x3"s purlins). I think both solutions worked ok, but maybe the 1.87 pcf on the roof was overkill, and this material is not cheap here.
      Thanks a lot!


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