Solar on SIPs

Sample section of SIP
The sample section of SIP, with the solar rack standoff mounted and ready to be tested for pull-through strength.
A Toggler Snaptoggle anchor
A Toggler Snaptoggle anchor is similar to a toggle bolt but with a stronger cross piece. (Note: Plastic straps are used only for initial placement.) Once in place, the anchor accepts a cap screw inserted from the opposite side of the SIP.
Sample section of SIP
A Toggler Snaptoggle anchor

A structural insulated panel (SIP) roof has no embedded lumber in the structure, and therefore nothing substantial for attaching PV array and solar hot water mounts. Single or double lumber splines could be put into the panels, or I-joists at 4-foot intervals could be added. However, the thermal bridging that they would create are at odds with the goal of an energy-efficient building. As buildings get tighter and more insulated, thermal bridging plays a relatively larger role in energy loss.

One solution for mounting solar equipment to a SIP roof is to drill completely through the roof, passing a threaded rod through to the bottom skin and placing a large washer under the nut. Since most SIP roofs are vaulted on the inside, though, few homeowners would be OK with seeing the nuts and washers on their ceilings.

On a recent SIP project in Hood River, Oregon, the PV contractor asked for test results for fastener pull-out so he could design an appropriate rack system. An independent test at Rigging Products in Portland, Oregon, provided the figures he needed. The first test, using hollow-wall anchors, gave a result of 405 pounds to failure. Dividing by three (which accounts for the industry standard safety factor) gives a working load of 135 pounds. Failure occurred as the hollow-wall anchor folded up and pulled through the anchor holes. In an area with 100 mph wind gusts, we did not think a hollow-wall anchor would hold up.

In the second test, two 3/8-inch, high-performance toggle bolts were used. Toggler brand bolts were used with cap screws instead of the original machine screws since the holes in the standoff were 3/8 inch. In hindsight, I would have used 5/16-inch bolts to give a little wiggle room for alignment. In this test, the failure of the SIP—when the OSB skin cracked—occurred at 1,105 pounds, or a working load of 368 pounds.

—Patrick Sughrue

Comments (13)

ART005's picture

Hello, I own a SIP home. Have been looking for a way to attach PV for some time. Using the Toggler Snaptoggle, how does the anchor beam portion seat on the skin beyond the hole when SIP foam is there?

Michael Welch's picture
It goes all the way through to the other side of the other skin. (Edit: I have come to realize that is not the case.)
ART005's picture

Then it's the same problem as bolting through which is not acceptable for vaulted ceilings. Am I missing something that there is some reason this is acceptable for vaulted ceilings? Plus the article says the failure was pulling the skin off. This would not happen with the anchor on the outside of the inside skin.

Michael Welch's picture
I sent a note to Patrick to see if he can help clear this up.
ART005's picture

I think you are pretty far off. The article is about mounting PV to SIP roofs inspite of them not having structural members to anchor to. The structural members don't exist to prevent thermal bridging through the SIP. The issue is not whether an anchor/attachment would ever contribute to thermal bridging. Not bolting through is an esthetic issue due to how often SIPs are used for cathedral ceilings.

Somehow the toggle part of the bolt used was able to open under the outside skin of the SIP. Then the glue between the OSB skin and the foam holds the anchor for over a 1,000 lbs.

Ben Root's picture
Sorry Michael, I think Art is right. The toggle only goes through the outer sheathing of the SIP...that is the point...avoiding thermal bridging, crazy long bolts, and unsightly interior fasteners. As for a solution to the original question, is it possible that the expansion of the toggle would just compress the foam around it? Or could you dig/crush an area with an L-shaped tool in the hole? Or chuck that tool into a drill, and clear out a slightly larger circle behind the hole? I "heard" once (dangerous hearsay, at this point) of a SIP contractor who would "drill" holes for wire runs in SIPs by heating up a large steel ball bearing with a torch, then just dropping it into the SIP. Gravity would make a perfectly plump, perfectly round hole as it melted through. So maybe a hot metal "L" shaped rod would work (try at your own risk on a scrap piece...don't burn down your house...I'm not SIP pro). Ben
ART005's picture

Great Ben, now we're back on track. The article says the test was done with the special toggle bolt and the results were over 1,100 lbs before the skin failed off of the foam core. So when the test was done, was it necessary to remove some foam around the hole? The article says they drilled a hole, put the toggle anchor in, pulled on the plastic straps to position the anchor, inserted the cap, then screwed in the bolt for the test. No mention of any additional prep work to the hole to facilitate positioning the toggle anchor. So the question is to the author, tester, or anyone that has used this if the hole needs additional prep than just the first drilling? I'll try to contact the toggle manufacturer to see if they are aware of what is done with SIP panels. If I hear anything useful I will add a comment here.'s picture

This test covers pull out strength, but what about compression? We need to be sure the panels can support 30 pounds per square foot snow load. With these attachment points, that load is going to be concentrated.

ART005's picture

I have a SIP roof with steel roof (not standing seam) and have been pursuing PV array installation for a couple of years. No anchor solutions found that have been acceptable to all parties involved. Compression has never been a concern. Wind and shear are much more powerful. The nature of the foam continuous support is extremely strong in compression. Enough rail supports with adequate anchor to survive wind shear will be more than enough for snow load compression. Remember the footprint of the rail support clip is bigger than the footprint of the anchor under the OSB skin. So the compression load is smaller, and the support footprint is bigger. Also materials rarely fail in compression meaning materials are stronger in Compression. So somehow the SIP would have to deflect in compression to create enough shear on the opposite side of the compression force to create enough shear to break the SIP. Like bending a stick until it breaks. With ~10" of foam to bend, acting like the web of an I-beam, it will never happen.

meaghannelson's picture

Hi, I just wanted to know whether is it successful? I am using SIPs of versiclad which is quite good.

Terrell Deppe's picture

A standing seem metal roof and S5! mounts on a SIP roof should also be considered. The metal will last at least as long as the panels, the load is distributed broadly across the roof without bridging, and it works well in high wind conditions (see S5! website for load tests since there are many factors). Of course, some people object to the aesthetics of a metal roof -- to each his own.

Justine Sanchez's picture

Hi Lawrence,
Thanks for posting. Yes, if you check out the caption to the second photo you will see that it is a Toggler brand Snaptoggle:

"A Toggler Snaptoggle anchor is similar to a toggle bolt but with a stronger cross piece. (Note: Plastic straps are used only for initial placement.) Once in place, the anchor accepts a cap screw inserted from the opposite side of the SIP."

Justine Sanchez
Home Power Magazine

lawrence tran's picture

Hi can you specify exactly what type of toggles used ? is it sanp toggle?

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