This research paper should allay your concerns. While polyiso is less resistant to thermal and impact degradation than other materials, the tested differences are "relatively minor."

(Of course, if there's a Bourne-style footchase across your roof, all bets are off.)

Select a convenient location for those who neglect to have sufficient equipment to reach the roof from the ground and apply three layers as a designated access area.

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This post was edited by Renovator8 on Sat, Aug 10, 13 at 13:40

12 to 15 times a year on a roof is a lot, but I can speak to how durable polyiso is as I have it in my attic and I was just up there this morning. Bear in mind that my polyiso is 2.5 lb density (i.e, 1 cubic foot is 2.5 lbs). Some polyiso goes up to 5-6 lb density and would be much harder.

Anyway, walking on it is no problem. Crawling on hands and knees I can feel very slight deformation with my knees. I was also lying down and supporting myself on one elbow and I remember thinking I could feel the tip of my elbow denting the foam perhaps 1/4 inch. Sorry, but I have no idea if the dent remained and I could not find it now if I tried.

I'm only up in the attic maybe once a year and obviously don't have to worry about water pooling. If you are walking on the same area of your roof as many times a year as you say, I would take the safe route and lay down a thin sheet of something when I was up there to help prevent denting, at least in any area that gets repeatedly walked on. If you are always accessing different areas of the roof and walking across it randomly, then you probably would be okay with just walking on it.

Thanks much RMTdoug. Just as I suspected. I know the material is not resilient, so a dent will not recover. I believe the 2.5 lb density you mention is about what most roofers use, and what you buy in the 4x8 sheets at the big box store. I am amaized roofers use this polyiso and that my research has not found any complaints.

Sounds like the higher density material is the way for me to go,since it probably has a much higher compression strength. but I have not seen it in roofing applications and suspect it is expensive. I will also think about using the 2.5 lb density product and covering it with something that will not dent, such as a thin plywood or densdek roof board. The contractor I hire will probably think I am nuts. Either way would probably add a lot to cost.

Has anyone done this type of thing? Would like to hear about experiences.
Thanks

The polyiso used in roofs is typically 20-25 psi.

If you have regular traffic--and it seems you do--cover or walkpads may be necessary to avoid damage to the roofing membrane.

This post was edited by worthy on Wed, Aug 14, 13 at 10:45

"The polyiso used in roofs is typically 20-25 psi, not 2.5 psi."

Worthy, in order to not confuse anyone, we are talking about two different things here. In the study you cited, the polyiso they tested had a density rating of 2.65, which gave it a compression rating of your 25 psi (see Table A). The reason I cited the density of my foam is because that is what I know it is and is how most polyiso is sold.

At least we now know that 2.5 lb/cf density polyiso has a compression rating of around 25 lb psi.

Worthy ��" Thanks for the link (walkpads) This quote from the link summarizes my concern.

âÂÂIt is a common misconception that cover boards are not needed and that the polyisocyanurate insulation has a high enough compressive strength to support the roof loads. Roofing manufacturers don't typically require cover boards, and so they're often eliminated from the roofing system to reduce costs. Cover boards should always be considered in roofing systems, specifically for protection of the insulation and for the benefits of prolonging the roof membrane itself.âÂÂ

The web site also says that when walk pads are used, workers tend to walk in random patters anyhow. Walkpads are not practical for my residential application.

RmtDoug ��" This tells me my situation is borderline as to whether I need a cover board. My walking will be OK, but I never know what someone else might do. According WorthyâÂÂs research paper (This) the small deflections you are causing in the attic will have only tiny, almost negligible, impact on thermal performance, so you are OK. However, on a roof, the dents could become puddles.

MorkigTenSur ��" This is good news that you do maintenance on the roof with no deflection. I also feel confident when I am up there I would have no problem, since I am aware and careful. However, who knows if a contractor in the future (plumber, painter, window cleaner, satellite dishâ¦) is not careful, etcâ¦.

Also, Intuitively, I donâÂÂt see how having some kind of cover board between the polyiso insulation and the membrane would cause the problem your contractor spoke about. Maybe this would not apply to TPO or PVC membranes? In fact, WorthyâÂÂs link says that cover board should be considered.

Now I need to figure out what type of cover board to use. It needs to be as thin as possible, since I need clearance for a short door step. I could use ü inch plywood. I am not sure if roof board comes that thin. Any thoughts?

Also, how is it installed? I assume long screws with those 3-inch plates. Can these same screws be used and go through the insulation into the deck also be used to hold the insulation down? Or do you first need a separate set of screws to attach the insulation?

Worthy, in order to not confuse anyone, we are talking about two different things here

Thanks. Correction made in post.

The National Roofing Contractor's Association has this (pdf) bulletin on Polyiso roofing.

The cover boards go between the insulation and the covering membrane. The walk pads rest on top.

According to the NRCA bulletin,

This post was edited by worthy on Wed, Aug 14, 13 at 11:03

Thanks everyone. This all helps.
A related question: Is Mule Hide TPO a good product?
(It's a shame I cannot get a contractor to bid on PVC around here, except for one that is extremely expensive)