Every hole in the roof
A roof plane sheds water without effort. Every place something passes through it is a hole that was cut on purpose and then closed again, and those are the places that open first.
- 1 The ridge. The highest, hottest course, and where a shared roof usually begins and ends.
- 2 The field. Tile or shingle above, and the underlayment that actually keeps water out below.
- 3 The valley. One channel carrying what two slopes shed, and where debris settles between rains.
- 4 Wall flashing. Where an addition or an enclosure meets the house, and where neither trade owns the seam.
- 5 Penetrations. Collars, curbs and skylights: holes cut on purpose, then sealed. They open first.
- 6 Eaves and metal. Drip edge and fasteners. Near the water they end the roof; twelve miles west they do not.
Why penetrations lead the list
Because at a penetration the waterproofing layer has to turn a corner, climb a little, and stay sealed against a different material. On a tile roof it is the underlayment doing that work, and when heat has made it brittle the seal around a collar is where it gives out first.
The rubber collar on a plumbing vent has its own, shorter clock. Sun hardens it years before the covering around it is finished, it splits, and the daily summer rain does the rest.
Skylights and mechanical curbs
A skylight is a hole with a frame in it, and its curb flashing is a detail that either was built or was sealed. Older units also fail at the glazing seal itself, which produces the same ceiling stain from a different cause and invites the wrong repair.
Air handling equipment sitting on a curb adds vibration to the list. Anything that moves against a seal, all day, every day, eventually opens it.
What a quote should say
That every collar and curb is replaced rather than reused, and with what. Reusing hardware around a penetration is the most common reason a new roof leaks inside its first years, and it is a small line on a large invoice.