How to do it.

The proper way to reseal a vent is to remove it and reinstall it. Doing this also allows you to see the frame inside the vent area which may give you an idea of the amount of damage that has already been done. Remove the interior screen and bezel first. Then get on the roof and remove all the old junk that's been used to seal the vent. Some if this may come off more easily than you think, or it could just be a complete PITA. A lot depends on the type of sealant that's been used. My rig had the silver stuff on it and came off pretty easily. If someone's spread tar around up there, that could get messy. I just, very carefully, used a putty knife to scrape the old stuff off. Once you see the vent flange and have the screws exposed, remove all the screws. Rusty screws indicate water leakage and it's best to replace those. More on screws later. Using the putty knife, Carefully slide it between the roof and the vent flange while Gently prying it up Slightly as you go. Don't try to get it off all at once as you may break or distort the flange. Work your way around with the putty knife prying just a little at a time. Eventually you'll get the vent loose enough to remove. Once removed, clean the roof surface real good. Likewise with the vent mating surface. Be sure to get all the old junk off. Check the vent for cracks if it's a plastic model, or excessive distortion if it's metal. If you have a plastic flange and it has any cracks in it, I highly recommend you replace it. Once a crack has started, they tend to migrate with temperature changes. You can get replacements for less than $35.00 for a whole brand new vent. If you do replace it, I also recommend a metal vent over a plastic one. These are much more durable and will last forever. If you feel the need for letting light in while the vent is closed, then you'll have to go with a plastic version. If the existing vent is a metal one, these sometimes get distorted in the areas where the screws hold them to the roof. These distortions can be removed by laying the flange upside down on a solid flat surface (a concrete step works well for this) and hammering them flat again with a small plastic hammer. In the event you replace the vent, it's a good idea to compare the new one with the old one to see if the screw holes are in the same locations. If they are, then no problem. If they are different from each other, I feel it's wise to place the new vent on the roof in it's desired location. Then, with a marker of some sort, mark the new screw hole locations on the roof by using the screw holes in the vent as a template. After marking the roof, remove the vent and with a 1/8 inch drill, drill each marked location for about 1/2 to 3/4 inch deep to provide pilot holes at these locations. DON'T drill too deeply, 3/4 inch is more than enough. Pilot holes are a good idea because they'll keep the wood from splitting. Once the roof is thoroughly cleaned and the vent is ready to be reinstalled, apply butyl tape to the mating side of the vent flange. Butyl tape comes in rolls and can usually be found in better hardware stores or your local RV shop. Don't try to smear plumbers putty around the flange, just get and use the tape, it's so much easier. I like to apply two layers of butyl to the flange so it's about 1/4 inch think. This gives a larger amount of 'wiggle room' if your roof surface has some irregularities and the vent doesn't sit completely flat on it. You'll have to use your own judgment on that but it won't hurt to have extra. After the butyl has been applied, it's time to set the vent back into place. Do this carefully to get it as close to the original position as possible. You don't want to have to move it around too much once the butyl has made contact with the roof. Once you have the vent in place, it's time to screw it down. Now as I mentioned earlier, I'd talk a bit more about screws. You can use the old screws if they're not rusted, if they are, you should replace them with new ones. I'd normally recommend stainless steel screws for this application as they won't rust like the original ones likely have, but, there's even better. Tin roof screws! These screws have a built in washer and rubber gasket arrangement that seals water out once they're tightened down. Regular screws won't do this and will require you to add extra sealant over the heads of them all once the vent is secured in place. The tin roof screws will save you this extra step and will give a more attractive installation (even though no one will likely see them except you). Whatever type of screws you intend to use, start one in each vent hole and tighten them down just enough to contact the vent flange. Once in place, proceed by tightening them evenly all around a little at a time. Don't try to tighten them all at once or you'll crack a plastic flange or warp a metal one. As you tighten them, the butyl will ooze out from under the vent. When the screws have reached a point where they feel as though they're just starting to get tight and the butyl has oozed out evenly from under the flange, stop tightening them. You don't need to sock them down real tight, snug is good enough. This ensures there's an ample amount of butyl in contact with both mating surfaces to keep the vent sealed well and allows for expansion and contraction. The butyl will remain soft which is a good thing. For the finishing touches, take a credit card or other plastic card or use the back side of a butter knife and trim off the excess butyl that has oozed out around the flange. If you've used the tin roof screws for the installation, you don't need to bother with sealing them, otherwise you'll need to seal the screw heads. Eternabond is probably the best thing to use for this, but it's a little expensive. If you choose to use it however, get it in a width that will allow you to cover both the screws and the seal and reach to the roof as well. When the Eternabond is in place, the job is done. Dicor or Dap sealant is the next best choice followed by silicone RTV (don't use silicone on rubber roofs, ever!). You'll have to apply a blob to the head of each screw and then run a good bead around the the outside of the flange to seal in the butyl. If you used the tin roof screws, all that need be done is to seal in the butyl around the flange. Whatever you use, the parts to be sealed must be free of any oily substances. That's it!

Original (rough) draft: YoDude 7.11.06