Slate turns up on older Philadelphia houses: on pitched main roofs, on steep street-facing mansards, on porches and bays. 39.9% of the county's housing was built in 1939 or earlier (ACS 2020–2024), and in some ZIP codes the share is far higher, 71.6% in 19134 and 64.7% in 19148.
Slate always means a tear-off
The City's EZ re-roofing standard lists the cases in which every existing layer must come off before a new roof goes on. One of them is a roof that "is wood shake, slate, clay, cement, or asbestos-cement tile." The building code says the same: a roof recover (a new layer over the old) "shall not be permitted" where the existing roof covering is slate (IRC 2021 R908.3.1.1). Nothing is nailed over slate.
Old slate can go back on
The same standard says: "Existing slate, clay or cement tiles in good, working conditions shall be permitted for re-installation." On a mansard or a small slate section, sound slate is sometimes salvaged and relaid over new underlayment, with new slate matched to fill in. Whether enough is sound is a judgment made on the roof, and an estimate should say how many pieces are expected to be reused.
Flashing does not get the same allowance. Deteriorated, rusted or damaged vent flashing, metal edgings, drain outlets, drain collars and counter-flashing may not be reinstalled under the standard.
How long it lasts
HUD's useful-life table gives slate 75 years, against 20 for asphalt shingles. It is a planning number for capital budgets, not a measurement of your roof. On a slate roof the fasteners and flashing often fail before the stone; that is a question for the inspection.
When the Historical Commission decides first
If the house is designated, either individually on the Philadelphia Register or inside a local historic district, the Historical Commission's approval comes before the permit. The commission tells owners that they need its approval to "make changes to your property that require a building permit" or to "alter the exterior of the building," and the EZ standard does not apply to a historic building "without prior approval." Replacing slate with another material on a street-facing mansard is a change the commission will look at. Its staff says it "can often give their approval for your building permit application" and refers larger changes to the Architectural Committee and the commission.
How to check the address and what to bring: historic districts.
Choosing a material
The choice between new slate, salvaged slate, and a substitute is yours unless the commission's approval limits it. Ask each contractor for the material by name and type in writing, how they will flash the mansard into the flat roof behind it, and who files the permit and the commission application.