You do not want to get the sealer between the rubber boot and on the plastic pipe.
Replace rubber boot roof vent.
Roof boots are built to maintain their water tight seal even in the most extreme weather conditions.
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I ve used this repair on several roofs over the years.
Make sure you need a new boot may just need a bit of bitumastic caulk if it is leaking a bit at the duct contact as they commonly do as the rubber gets older.
Item 1262068 model pbr 312 3n1 bk.
This old house general contractor tom silva shows how to create a watertight seal around a vent pipe.
About 15 30 per boot parts depending on whether you go with the cheapest all rubber or silicone which i do not recommend except inside bathroom kitchen vent hoods or metal cone flashing with rubber boot rim longer lasting especially in heavy snow ice country and also in areas with baking sunshine.
Labor assuming this is a shingle roof typically about 250 400 to do 4 boots on normal slope roof likely 100 or so higher for steep roof.
In simplest terms a new vent pipe boot flashing is slid over the top of the existing one and the top edge of the pan is lapped beneath the overlying shingle course.
See other blogs for a proprietary vent flashing repair kit and a repair using a boot flashing with the roof pan trimmed off.
If split then yes it needs replacing though a good wrap with waterproof duct tape or bitumastic caulk will keep it intact and dry until a roofer can fix it.
Apply a liberal amount of silicone sealer to the old rubber boot.
However over time they will begin to tear and gap.
Slide the new boot over top and make sure it is sealed completely between the old and new boot.
It is imperative at this point that you repair the roof boot.
When this happens there is a chance that water will enter your home through the damaged roof boot.
Slides over the existing roof vent boot preventing future leaks.
2 in and 3 in x 9 75 in plastic vent and pipe flashing.
I put in enough material that it squished out around the entire outside perimeter.
Existing pipe boot repair and preventative maintenance the perma boot is a gasket less two piece pipe boot repair system designed to permanently repair the most common type of roof leak the leak around the vent pipes that penetrate your roof.
Designed for all standard roof pitches 3 12 to 12 12.