Under tile the underlayment is the roof, so it deserves more attention than the tile on top of it.
At a glance
- Thirty pound felt is the old baseline and the shortest lived.
- Synthetic sheets lay flatter and survive sun during the job.
- Self adhered membrane belongs in valleys and at eaves.
- Two layers under tile is the Los Angeles standard for a reason.
Asphalt felt
Felt is paper saturated in asphalt, sold in fifteen and thirty pound weights. It is cheap and familiar. It also wrinkles when it gets wet, tears in wind, and dries brittle in about twenty years of Los Angeles heat.
It still has a place as a low cost layer under shingles on a budget job. Under tile it is the reason so many roofs need a relay at year twenty five.
Synthetic underlayment
Synthetic sheets are woven polypropylene. They weigh less per roll, cover more, resist tearing, and can sit exposed for weeks during a long job without failing.
They cost more per square than felt and less than a self adhered sheet. On most reroofs today they are the sensible middle.
Self adhered membrane
These peel and stick sheets bond straight to the deck and seal around every nail that goes through them. That self sealing behavior is what makes them the right pick in valleys, at eaves, around skylights and on low slope sections.
A full deck of self adhered membrane under tile is the premium detail. It costs more and it is the closest thing to a fifty year underlayment we have.
What to specify under tile
The common good spec in Los Angeles County is two layers, often a base sheet plus a cap sheet, or one self adhered sheet where the budget allows.
Ask what the crew plans to use in the valleys specifically. Valleys carry the most water on the roof and get the least attention on cheap jobs.
Common questions
How long does synthetic underlayment last
Thirty to fifty years under a covering, and most brands allow months of sun during construction.
Is felt ever the right choice
On a low budget shingle job where the roof will be replaced in twenty years, it still works.
Can underlayment be replaced without new tile
Yes. That is exactly what a lift and relay does.