Grace Ice & Water or Select not recommended for tongue & groove boards
A follow-up to this discussion: https://www.greenbuildingadvisor.com/question/rebuilding-roof-on-deck-house
Our contractor discussed adding the self-adhering Grace Ice & water or Select™ to to the tongue and groove on our ceiling. Grace’s rep response advised strongly against using of the self-adhering Grace products (The Ice & Water Shield and the Select). He informed us that there can be a reaction between any knots in the tongue and groove boards that causes the membrane to sort of melt, and then can leak down through the knots.
Has anyone heard of this issue? Our contractor is very hesitant to go this route and is recommending a synthetic underlayment like Triflex.
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Replies
I don't know the best membrane to use, but you absolutely want a fully adhered membrane. Triflex is mechanically attached. Check with Henry and the folks at https://foursevenfive.com/.
Thanks for the reply. There seems to be a limited amount of products that fit the bill and the manufacturers are hard pressed to recommend their product as this proposed usage is not typical. Henry told us that the proposed usage would fall outside of their warranty. They've heard of people using it as we propose, but haven't studied the matter themselves.
I looked a bit and didn't find anything that was an obvious match. I would still give https://foursevenfive.com/ a call. Grace recommends putting at least 1/4" sheathing down over the existing boards in your application. A new layer of roof sheathing might not be a be a bad way to go after all. The key with this sort of approach is paying attention to how to seal the roof to the top of the walls in an air tight manner. What do the overhangs and perimeter of the roof look like? If the pitch isn't too shallow, you could use taped sheathing, roofing underlayment and Ice and water where needed. Use a blower door after sealing things to confirm air sealing before proceeding. Then stack foam on top.
If sealing to the walls is a real challenge, you may be able to use exterior spray foam to accomplish this. Install 2x3 or 2x4 exterior furring and apply spray foam against the ceiling boards. The foam could be shaved flush with the furring and additional layers added on top. I think you will likely need a layer of sheathing on top of the spray foam though.
Thanks! Foursevenfive.com recommends Adhero, but it is a vapor permeable membrane (Vapor Permeable 7 perms), which I understand is not ideal for our purpose.
Grace has also mentioned sheathing before the membrane and I'll discuss that option with our contractor. Not sure why they don't want to go that route.
Our other option is probably to forgo the membrane and tape all the seams religiously, which I understand can be a successful approach, but I do worry about on the perimeter sealing.
For whatever its worth, out of ignorance I did do self-adhered Ice and Water (not grace brand) over my 3/4" whitewashed knotty pine shiplap before laying 2 layers of rigid foam (10" total) and then Zip sheathing on top. This was all sitting on top of a timber frame structure. I have yet to see any negative consequences after discovering my mistake. The roof assembly is 1.5yrs old at this point and has been through one winter. Northeast Connecticut.
Was the building structural design use the T&G boards for the roof diaphragm? Usually it is easier to make the diaphragm connections if placed over the T&G.
@Tim R, as far as I understand your question, yes, the tongue & groove is part of the structural roof. Right now it is tongue & groove, plywood sheathing and shingles. Zero insulation. We are building up the roof structure in order to add 3 layers of rigid insulation.
So then you will not place the ice & water shield directly on the T&G?
It will be on the plywood. If that is the case, you could tape the plywood seams and not worry about the ice & water shield oozing or melting thru the t&g.
That's my understanding about Grace's recommendation. The roof stack would look like this:
1) Tongue & Groove
2) 1/4" sheathing
3) Grace Select™ (air/vapor barrier)
4) 3 layers of rigid foam board with a bonded OSB on last layer
5) Grace Ice & water shield
6) Shingles
I do wonder though if we could forgo the 1/4" sheathing. Rigid foam seems to be a suitable deck materials per Grace's website:
1) Tongue & Groove
2) First layer of rigid foam board
2) Grace Select™ (air/vapor barrier)
4) 2 final layers of rigid foam with a bonded OSB on last layer
5) Grace Ice & water shield
6) Shingles
Usually only modified bitumen products have issues with sap from wood or plasticizers from vinyl. Heat makes it much worse, under insulation I don't think it will get hot enough to flow. You can do a test piece at home and let it sit for a week than peel it back, if it turns gooey then you'll have problems.
If you ware worried, can also go with something like VP100 which is doesn't have bitumen in the adhesive.
Thanks for the suggestion. Henry told us that roof usage is not endorsed/tested by the company. I believe over slip risks. But it sounds like if it can be installed safely, it would fit the bill.
What are people's thoughts on applying the Grace Select™ membrane to the first layer of rigid insulation instead of the tongue & groove?
From Grace's Chemical Compatibility Technical Letter, rigid insulation is a suitable : "GCP’s self-adhered underlayments are applied directly to a clean, dry, continuous roof deck. Some suitable deck materials include plywood (including fire treated plywood), oriented strand board (OSB), wood plank, metal, concrete, exterior gypsum board, glass mat surfaced gypsum board, and rigid roof insulation board. "
https://gcpat.com/en/solutions/products/grace-ice-water-shield-roofing-underlayment/tl-0005-chemical-compatibility-0
That's not really buying you anything there. The reason for the peel and stick is to seal up T&G, you can seal up foam with tape which is much cheaper.
My structural comment is for lateral forces, Forces from wind or seismic forces. It is best to add sheathing over the T&G so you can nail it directly to the framing thru the T&G. It seems you T&G is at least 2x material, so thick enough to walk on. Then make your air barrier on top of the sheathing with tape or full Ice & water, then your insulation layers with a final OSB layer to screw the entire assembly down to the framing.
Sheathing would be better to use 3/8" rated sheathing - I think 1/4" is not rated for structural applications.
I think you need to decide where your air barrier is located.