Historic slate in Philadelphia should be repaired rather than replaced whenever possible, according to National Park Service guidance. The Philadelphia Historical Commission reviews visible roof work in historic districts and on the register. Slate replacement is considered only where deterioration requires it.
Key takeaways
- National Park Service guidance says historic slate should be repaired rather than replaced whenever possible.
- Slates common in the late 1800s and early 1900s lasted roughly 60 to 125 years, according to the NPS.
- Non-ferrous copper or stainless nails are the right choice on slate.
- The Historical Commission reviews roof work visible from the public right-of-way on protected properties.
- Confirm the status of the address before the job is scheduled.
Why repair slate rather than replace it?
National Park Service guidance says historic slate roofs should be repaired rather than replaced whenever possible. Repair keeps the roof's original material and character, and it is usually the better course where the slate is still sound.
The NPS's Preservation Brief 29 on historic slate roofs notes that the slates common in the late 1800s and early 1900s lasted roughly 60 to 125 years. That is a reason to work on the slate that is still there.
A repair replaces the individual slates that have cracked or slipped, checks the nails and flashing, and leaves the rest of the roof in place.
What does Historical Commission review cover?
The Commission's Roofs Guideline favors roof work that is inconspicuous from the public right-of-way. Its staff reports treat slate replacement as contemplated only where deterioration requires it, with exceptions for mansards, turrets and character-defining features.
A roof that can be seen from the street gets closer review than one hidden behind a parapet or a rear slope. Flat roofs hidden by a parapet are generally treated more simply.
The Commission confirms the status of each property. A permit may wait on that review, so check before the job is scheduled.
Which nails and materials are right for slate?
Non-ferrous copper or stainless steel nails are the right choice on slate. Iron and ordinary steel nails corrode and can split the slate around them.
A roofer resets and secures a slate with the correct nail, and reuses a slate in good condition when one is removed. Matching old slate is part of the repair decision.
Flashing at chimneys and valleys should be checked at the same time, because a sound slate roof can still leak through a failed flashing.
What does a slate repair visit involve?
A slate repair visit walks the roof, marks the slipped and cracked slates, checks the nails and flashing, and writes a scope the owner can review before work begins.
The roofer records the slates to be replaced, the nails to be changed and the flashing to be checked. A photograph of each area helps the Commission review, if review is needed.
Keep the written scope with the permit paperwork and the contract.
Frequently asked questions
Can slate be repaired instead of replaced?
Usually, yes. National Park Service guidance favors repair for historic slate wherever possible, and a roofer replaces only the slates that have failed.
How long do historic slates last?
The NPS notes that slates common in the late 1800s and early 1900s lasted roughly 60 to 125 years. The life of a specific roof depends on its condition.
Does a slate repair need Historical Commission review?
It depends on the property and on whether the work can be seen from the public right-of-way. The Commission confirms the status of each address, so ask before the job is scheduled.
Who should I call about a slate roof in a historic district?
Call (833) 751-2102 to arrange an inspection. Ask the roofer to put the scope in writing, with photographs, so it can be reviewed by the Commission if needed.
Sources
- National Park Service - Preservation Brief 29, historic slate roofs
- City of Philadelphia - Historical Commission
This guide is general information for Philadelphia County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.