How long slate lasts
HUD's useful-life table for capital needs assessments plans a slate roof at 75 years, the longest of any common roof covering (asphalt shingles: 20).
A single leak on a roof with years of life left is usually a repair.
Slate is never a base for a new layer
The residential code forbids a recover over slate. A new roof cannot be laid on top of an existing slate roof, nor over clay or cement tile (IRC R908.3.1.1). Replacing a slate roof with anything else starts with taking the slate off to the deck.
The same section matters on older houses where slate sits under later layers of shingles: if slate is found underneath, a recover is off the table. In the town pages this site covers, about a quarter of all homes in Pennsylvania and nearly a third in Massachusetts and Rhode Island were built before 1940 (Census ACS 2020–2024), and each town page gives its own share.
Reusing slate
Sound slate can be reinstalled; damaged, cracked or broken slate cannot (IRC R908.5). On a roof where the slate itself is good and the rest has failed, a contractor may strip, set the good slate aside, rebuild underneath and relay it. Ask for that option in writing alongside a full replacement.
Historic districts
In a local historic district, roof material is usually a matter for the district commission before it is a matter for the building permit. Providence's Historic District Commission guidelines, as posted by the City, say: "Asphalt roof shingles are not encouraged as a replacement material for slate." Philadelphia and Boston have their own commissions and districts. Check with the commission before choosing a material.
Synthetic slate
Synthetic slates are made of rubber, plastic or composite to look like slate. HUD's table has no line for them, so their life expectancy is what the manufacturer's tested data and warranty say; ask to see both.
Weight
Natural slate is heavy. If you are going from a lighter roof to slate, ask the contractor whether the framing has been checked for the weight.
What changes the price
Natural slate against synthetic or another material; reusing sound slate; copper or other flashings; the pitch and height; scaffolding; and in a historic district, the approval process. See materials compared and what sets the price.
Questions
How long does a slate roof last?
HUD’s useful-life table for capital needs assessments plans a slate roof at 75 years, the longest of any common roof covering; asphalt shingles are planned at 20.
Can you put shingles over a slate roof?
No. The residential code forbids a recover over slate, so any replacement starts with taking the slate off to the deck (IRC R908.3.1.1). The same holds when slate turns up underneath later layers of shingles.
Can old slate be reused on a new roof?
Sound slate can be reinstalled, but cracked, damaged or broken slate cannot (IRC R908.5). Where the slate is good and the rest has failed, a contractor may strip it, rebuild underneath and relay the good slate; ask for that option in writing alongside a full replacement.
How long does synthetic slate last?
HUD’s table has no line for synthetic slate, which is made of rubber, plastic or composite. Its life expectancy is what the manufacturer’s tested data and warranty say, so ask to see both.