Dumping the effluent onto the deck would be out of the question, in my view. At the roof, instead of a radial fan in a housing blowing air down the roof to the deck, use an "upblast" fan housing with compatible fan. Broan may still make such a unit (NuTone did) or search on the web. Even Greenheck may make one small enough. The upblast housing roof interface will have to be the correct pitch for the roof, and, if snow builds up very high in that location, high enough to keep the top out of the snow. kas

Thanks, the Home Depot unit has a large flange (flashing) about 2" wide and a good looking spring door. In real cold weather I'd stuff something in the outside just as I do with the cloths dryer and tell my wife not to use unless we remove the block. I have never cut through an exterior wall but believe all there is, is the drywall on the inside, insulation between studs with the vapor barrier against the drywall (inside) and plywood sheathing with tar paper or other wrap on the outside, plus the clapboard siding. I believe it would be correct to remove enough siding to open the sheathing for the rectangular hole. Then to place the aluminum assembly through the hold with the flashing caulked (with an adhesive silicone caulk) around the hole under the assembly flashing. Then reinstall the siding to fit the assembly sticking though, with most of the flashing covered by the siding. Sound right?

I tried my hood for the first time the other day and it seemed to be quite good. While my cooktop is on an outside wall, venting to that wall or even up through the roof would have meant having a vent thingy right at the gable in the front of my house or right at the top of the roof in front, which would not have been too attractive. So we did one 90 degree turn so we could hide it on a part of the roof that wasn't as visible.

You are correct, the ducting should be rigid. Also, if the walls are open, and you have the opportunity to install new exhaust ducting, it should be 8" in diameter, not 6". People who have 6" ducts have them because they are stuck with them already-behind walls when they moved in, because builders just put in 6" ducts as the easiest thing. If you are installing new ones, they should be 8" in diameter. You create issues with a 6" duct, mostly a lot of noise as the air is pushed through too small of a space, and less efficiency of the exhaust, because of the smaller duct. See if you can switch to an 8" duct at the same time as you switch to a rigid duct. I am concerned about your installer. First because AFAIK, flex ducting for an exhaust is against Code everywhere due to fire hazard. Second because he is unable to provide a wall vent cap. These are available at places like Home Depot - they are hooded or lidded, and have a grate at the opening to prevent critters. They are also usually available from the hood manufacturer. You may want to stop with this contractor, and find another one who knows what he is doing.