The folks who used to own my house (Inspector507 calls them the Dreaded Previous Owner aka DPO) left a gap about 2" wide between the house roof and the breezeway roof. The house roof is about 2' higher and overhangs the breezeway roof about 18" so most of the time it's OK, but if the wind is in the right direction water will get through. I want to use the expanding foam to fill this gap. I thought of using flashing, but the expanding foam would be cheaper, easier, and faster. It won't be visible from the ground, so it doesn't have to look great.
It sounds like it would work fine. The foam isn't structural, but it will do a great job of keeping out the wind. I prefer a minimal expansion foam since the tendency is to use too much and the high expansion stuff can easily get out of hand. Also, the high expansion seems more prone to air bubbles.
When you're done filling, let the foam dry. Then, if you have room to work, get one of those sanding disks that fits in your drill. I prefer the kind that looks like it's made out of a furnace filter with grit glued on. It will help you smooth the foam down for painting, etc. You can run right along a board and grind the foam to the same heighth as the board with no problem. (Be sure to wear a dust mask, the dust will leave you feeling off for a few days).
If you have that big of a space, you will need several cans of foam, whether it is high expansion or not. The waterproof bond stuff should work. If you want, you could leave the surface rough for now and grind it flat in the spring, when the weather is more conducive to painting.
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