Bathroom exhaust fan installation

Due to logistical reasons in my attic I need to route the new bathroom fan exhaust duct up through the roof instead of out the side wall. It's also a shorter run if I go vertically instead of horizontally. I'll be buying 4" aluminum ducting so when I buy a hole saw to cut the roof hole do I buy one that's 4" round, 4 1/4", or 4 1/2"?
Anything else I should do or watch out for?
Thanks. Walter
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If you want to do it with a hole saw, use a 4 1/2", but it'll cost you. You could do it with a sawzall or similar.

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No - a hole saw is $4 and $10 for shipping on Ebay. I don't have a sawzall. I only have a jig saw.
Walter

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A jig saw will work fine. Use a blade with coarse cut.
Don't cut any holes until you get the roof fixture. Check instruction recommendations first. Easier to make a hole bigger than smaller.
I assume you mean the 4" rigid aluminum and not the foil stuff.
Good part going through roof: Shortest run is usually best. If you go by code there is often a max run length (for dryer vent anyway). Each bend shortens the max length.
Bad part: May be harder to insulate. Regardless, you MUST insulate or you will have water dripping out of the fan in the bathroom.
Going out the gable end the wall fixture usually has flaps that are closed when not in use. Keeps wind from blowing back into the bath. Going out the roof you may just have an upside down J type with a screen. I would think wind could get in there. Also, snow buildup could block it if the exit is low and you are in that climate. I hear there is a higher neck rise type but I could not locate one locally.
Just some thoughts to keep in mind.
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Red Green wrote:

Snow melt has been a continuous nuisance for our roof vent, at least in terms of contributing to a little ice dam problem. I've been tempted to replace it with a furnace vent to get it a couple of feet off the roof.
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Here's an excellent video on how to install a roof vent by Tim Carter. After I saw this , it made installing mine easier.

http://www.youtube.com/watch?v=gee3itzjOG0

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Thanks Red!
Walter

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For size measure the flange on the vent fitting you will be using on the roof and add about a 1/4 to 3/8 to allow the clamp to fit into the hole.
I would not use a hole saw to cut through the roof. The tar in the shingles will grab the hole saw of that size and tear the tool out of your hands, the grit in the shingles will destroy the teeth on the hole saw. Take a jar lid or some other template of the correct size and a Magic Marker to the roof, use a drill to start the hole and cut along the line with a very coarse tooth jig saw blade -- the blade will most likely be destroyed cutting the hole and you will probably toss it when finished, but they are cheap.
I have installed several this way.
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i would prefer to run exhaust thru sidewall..........
be certainto well insulate any exposed line in attic so condensarttion doesnt cause water damage
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Walter Cohen wrote:

Make sure you insulate the duct. If you don't, the moist air will condense in the duct (because of the cooler air in the attic) and run down the sides and drip in the fan.
a
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I use a 4 1/4" hole saw in my angle drill at slow speed. You don't want to go too fast with a puppy this big.
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Thanks John. Some have suggested NOT to use a hole saw but instead a jig saw or sawzall. Mostly because of tearing through roof shingles once it pokes through. Thoughts on this?
Walter

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The hole through the roof will not be round, but an oval due to the slope of the roof. The higher the slope, the longer the oval. I just hold a piece of pipe vertcally in position on the underside, and trace the outline with a magic marker, this will give a nice clearance. You can start with a hole saw, but you will need a drill with a side handle to hold one that large. I only use a heavy duty angle drill for holes this large. Better to use the jigsaw.
Drill a couple of locating holes from the underside, then go on the roof and mark the oval by placing a piece of ductwork vertically and tracing again. Cut away the shingles with a razor knife, than cut the hole with the jigsaw. The hole throguh the shingles will be larger than the one through the roof, so that the flange of the flashing will fit.
--
Dennis


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On Feb 10, 10:33am, snipped-for-privacy@SPAMwowway.com (DT) wrote:

Let me understand - when outlining the hole to cut (whether inside the attic or outside on the roof) does the traced item (pipe, coffee can, etc) lay flat with the angle of the roof against the wood/shingles or do I need to hold the traced item vertically against the roof slope and then trace it. The first way gives me a round hole but not at the correct angle. The second way gives me the correct angle with an oblong hole (but just a bit more difficult to trace). I think that if I trace the outline flat against the roof with an extra 1/4" or so then I'd be ok. After all, the roof cap would cover any minor "mistake" I make on tracing/cutting.
Walter
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Here is Broan's installation guide for a bath roof vent http://www.broan.com/ImageLibrary/broan/pdf/InstallGuides/99041929.pdf It says to cut the hole 1/2" larger than the duct size. This is what I did when I installed mine. Then I filled the gap around it with low expanding Great Stuff foam. I also drove 2 screws from the inside securing the duct to the roof plywood. It turned out great, no leaks. So in general, just cut the hole 1/2" larger. This will compensate for the roof slope.
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I'll probably use a jig saw as that is what I have on hand right now. From inside the attic I'll nail a hole up through the roof and then measure a bit more than 4" diameter and then start cutting through the roof itself. Then cut the shingles away. The outside cap (as it is square) will fit over any inconsistent cutting I do.
Walter

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