What a pipe boot is, and why it is the weak point
Every house has plumbing vents: open pipes that run up from the drainage system, through the attic, and out through the roof, so that water leaving a fixture does not have to fight a vacuum. Each one leaves a hole in the roof about the size of a fist, and each hole is sealed the same way. A flat metal or plastic flange is laid on the deck, woven into the shingle courses so the shingles above lap over it and the shingles below tuck under it, and a moulded rubber collar in the middle of that flange grips the pipe and closes the gap.
The flange is fine. Sheet metal on a roof lasts a long time. The rubber collar is the problem. It sits at the top of a roof in full sun, gets no shade at any hour, and is made of a material that ultraviolet light degrades. It hardens, loses its grip on the pipe, shrinks away from it, and then cracks, usually radially, from the pipe outwards. The split is often narrow enough to be invisible from the ground and entirely obvious from three feet away.
No photograph on this page, deliberately
There is no picture of a failed pipe boot in the Keys photograph library. There is a photograph in the set with the word PipeBoot in its filename, and it is not a pipe boot at all, it is a large metal furnace flue with a storm collar, and it has a member of the public standing in the frame. Rather than illustrate this page with something that is not the thing it describes, or with a stock photograph of somebody else's roof, this page carries no photograph. If a roofing site shows you a dramatic close up of a cracked boot, it is worth asking whose roof it came from.
Why it fails here early
Two reasons, and they compound. The first is heat. A dark shingle field in a Missouri July runs far hotter than the air temperature, and the rubber component sitting in the middle of it is being baked for a large part of the year. The second is movement. In winter the same component is repeatedly frozen and thawed, and rubber that has already hardened in the sun does not flex through that cycle, it splits.
The practical consequence is that the boot is very often the first thing on a roof to fail, and it fails while the shingles around it still have plenty of life. A homeowner who has been told that a leak means a new roof, when the leak is a perished boot, is being told something that is not true.
How a boot leak behaves
- EntryThe split in the collar Water runs down the outside of the pipe, reaches the collar, and instead of being shed onto the shingles it goes through the crack and onto the flange, then off the edge of the flange onto the deck.
- TravelDown the pipe, then along the framing Very often the water simply follows the pipe itself down through the attic, which means the first evidence is a wet patch on the pipe or a rusty ring where the pipe passes a framing member, several feet below the roof.
- ShowsA stain near, but not at, a bathroom Vents serve bathrooms and kitchens, so the stain frequently appears in or beside one, in a ceiling or at the top of a wall, and the homeowner reasonably suspects the plumbing rather than the roof.
What a proper repair replaces
Not a bead of sealant around the pipe. Sealant on a perished collar is a way of buying perhaps one season, and it makes the next repair harder because the material has to be cleaned off before anything new can be bedded down. It is also, from the ground, indistinguishable from a real repair, which is why it gets sold.
A real repair takes the boot out. That means carefully lifting the shingle courses above the flange without breaking them, removing the fasteners, taking the old unit off the pipe, fitting a new one, weaving it back into the courses so the laps run the right way, and re-bedding the shingles. On a roof with brittle shingles this is the part that decides whether the repair is possible at all, because shingles that snap when lifted cannot be woven back.
The upgrade worth asking about
Where a boot is being replaced, it is usually sensible to fit a unit with a metal collar and a replaceable seal, or a lead sleeve dressed down into the pipe, rather than the same moulded rubber that just failed. It costs slightly more at the time and it removes the component that is most likely to fail again in the same place.
If one has gone, look at the others
The vents on a house were installed on the same day, out of the same box, and have been in the same sun ever since. When one collar has split, the others are the same age and have had the same exposure. It is normal for a roof to have three or four vents and for two of them to be visibly cracked once somebody actually looks. Replacing all of them while the ladder is already up is very often cheaper than three separate visits over three years, and a roofer who checks them all and tells you which ones are still sound is doing the job properly.
This is also the clearest example of the argument this site is built on. The stain was near the bathroom. The failure was on the roof. The two are related but they are not in the same place, and the repair belongs at the failure. If you are not sure which of your vents is the problem, that is exactly what a trace is for.