Nailing down a roof vent


Hello,
I am repairing a small section of my three-tab composite roof, and installing a ducted roof vent for my bathroom fan.
What is puzzling me is that in addition to the two holes at the top of the roof vent (which will be covered by the top course of shingles), there are also two holes at the bottom of the roof vent.
If I put nails into these holes, they will be exposed, which is something that I know should be avoided.
Here is a picture of the roof vent:
http://jonnycats.home.comcast.net/~jonnycats/RoofVent.jpg
I'm hoping that the people who make the roof vent know what they are doing, but what exactly am I supposed to do here?
I'm guessing I am supposed to run the lower course of shingles to where they are butting into the left and right hand side of the section of the vent that rises up from the bottom plate (flange), so that the nail/nail holes are covered by them.
In other words, should the shingles (represented by the mustard colored overlay) cover the flange as in this picture?:
http://jonnycats.home.comcast.net/~jonnycats/RoofVent03.jpg
In all of the other roof vents that I have seen, the entire width of the bottom end of the vent flange is treated like a shingle, and is just left exposed without any holes or exposed nails, which is why this configuration is puzzling me.
Thanks for any help on this,
Jon
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wrote:

There are a number of ways to do it, and the picture is one of them. Another way is to use some metal roofing screws with the rubber washer under the head, or you could use some self-sealing membrane under the vent so if water did get by the nail head it wouldn't follow the shank down to the wood sheathing.
Here's another way to do it if you are replacing a section of the roof.
http://www.sgsconstructions.com/yahoo_site_admin/assets/images/DSC01208.142124105_std.JPG
Do not use roof cement to glue down the shingles over the lower section of the flashing as that will trap water. I don't particularly like covering the lower part of the flashing as any water getting by the shingles above is directed under the shingles below, and that's a bad situation. Here's a variation that I like better (and the drawing, too): http://farm4.static.flickr.com/3392/3205874325_6577ab58f7.jpg?v=1232293184
Frankly, though, most roofers install the vent flashing with the bottom edge exposed and just put a dab of roofing cement or silicone caulk on the nail head. Generally works, but since you're doing it yourself, and hopefully not settling for a "generally good" job, go with the belt and suspenders approach - a piece of self-sealing membrane below the vent and under the shingles where you'll be nailing, and then the gasketed screws or roofing nails with sealant over them.
R
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RicodJour wrote:

http://www.sgsconstructions.com/yahoo_site_admin/assets/images/DSC01208.142124105_std.JPG
Thanks Ricod, I appreciate it. I looked around at a few other installations today, and I did indeed see most of them with exposed (but caulked) nails, and the full bottom width of the flange exposed.
I'll go ahead and use a little goo on the nailheads on the flange.
Thanks again,
Jon
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Here is an excellent video on how to install a vent on a roof. I followed this when I had to put in a range hood vent.
http://www.youtube.com/watch?v=gee3itzjOG0

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Mike rock wrote:

Aye, thanks Mike, that's a great video. I see he used some nails on the bottom edge of the flange, and just put some caulk over them.
Jon
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