Pmullen503 wrote:If the plywood is well adhered to the internal bracing, you'll be fine. It may flex a bit if you were kneeling in the center of the unsupported area. But mostly the weight will be spread out in use.
Before the roof is on, you can test it by standing in that area. Give the glue a good week or two to fully cure.
Worst case, you could bolt a beam to the underside of the floor to stiffen it up.
QueticoBill wrote:Is the foam glued to the ply top and bottom? Glueing foam to ply forms a very strong "beam".
Pmullen503 wrote:You definitely want the top and bottom ply securely attached to the middle layer. Gluing the foam layer helps too, because of the large surface area.
KCStudly wrote:...... Definitely level this with a long board or other technique so that the second ply skin lays evenly on foam and frame. Don't assume that the foam will compress under weight or clamps, it won't; there's just too much surface area of foam and it does a good job of resisting compression in this scenario.
Pmullen503 wrote:Gorilla glue will safely glue foam. You need very little, it expands as it cures. Squirt a little in the foam cavity and spread, then scrape it off with an old credit card (playing cards work well too.) The little film that is left is enough.
Spritz it with water and put the foam back into the cavity. Weight it down with whatever you have. The white (CLEAR) GG sets in less than an hour, the brown takes a couple hours.
You'll probably want to use the brown GG on for the top for the longer working time.
QueticoBill wrote:In simple terms, the foam is the web and the ply are the plates, just like an "I" beam.
KCStudly wrote:I would use a non-expanding glue. In my part of the world summer humidity causes expanding glues to react too much.
Epoxy would be best because it doesn't need moisture or air to cure. Perhaps thickened slightly to better grab the texture of the roughed up foam and fill any slight voids in fitment.
TB2 or 3 would be easiest and least expensive, but won't cure well do to lack of air.
The PL300, intended for foam, absolutely needs air and would not be a good choice. Also, it has a thick consistency and doesn't flow out well when squeezed, even when applied with a notched trowel (which also hinders air reaching the middle).
Based on my experience, I would not recommend the 3M green contact adhesive. It just didn't work well for me. In hind sight I would have used epoxy.
The foam doesn't have super tight tolerance on its thickness and might stick up some after gluing into the wooden frame and first side of ply. Definitely level this with a long board or other technique so that the second ply skin lays evenly on foam and frame. Don't assume that the foam will compress under weight or clamps, it won't; there's just too much surface area of foam and it does a good job of resisting compression in this scenario.
23Sojourney45 wrote:P, I've used gorilla glue before to glue some odds and ends but nothing to this extent. Squirt a little and spread? For example, 12" x 12" square cavity, spread GG like paint in the cavity? I was thinking of putting a few glue "dots" and just putting the insulation on the top hoping that it will spread but that might not be the way to do it.
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