Slate Roof Repairs, Restoration and Replacement Options
Good slate lasts a very long time. The National Park Service's Preservation Brief 29 puts Vermont and New York slate at about 125 years and Buckingham Virginia slate at 175 or more, and the NAHB study gives slate in general more than 50. Most slate work is keeping that stone on the roof.
Replacing Broken and Slipped Slates
A cracked or missing slate gets replaced one piece at a time. The repair the Park Service describes runs like this:
- A slate ripper slides under the broken slate and cuts or pulls its nails.
- A matching slate slides into the gap.
- It's fastened with a copper nail.
- A copper bib, a small bent strip of sheet copper, slides up over the nail head to cover the new hole.
Some roofers use a copper tab nailed to the deck and bent up over the bottom of the new slate. NPS warns against it for northern climates, because snow and ice fold the tab open and the slate slides out. In Ann Arbor's winters, ask for the nail-and-bib method or slate hooks.
When many slates are slipping at once, the cause is usually the original fasteners. NPS explains that iron nails corrode while the slate stays sound, and the fix is to salvage the slates and re-lay them on new copper nails, not to buy new stone.
HomeGuide puts broken slate replacement at $50 to $300 per slate. Michigan's code wants two fasteners per slate on a solidly sheathed deck at 4:12 or steeper, so a repair that leaves slates hanging on one nail isn't finished.
Copper Flashing and Valleys on Slate
"Flashings are the weakest point in any roof," the Park Service brief says, and on slate they usually fail decades before the stone. Its recommendations:
- 16-ounce copper is the minimum weight for flashing; lighter sheet wears through under grit carried by rainwater.
- 20- or 24-ounce copper for valleys, gutters and spots that are hard to reach.
- One metal throughout. Mixing copper with galvanized steel or aluminum speeds corrosion where they touch.
Michigan's code sets the floor: sheet metal flashing and counterflashing on slate, valley flashing at least 15 inches wide, and no thinner than 0.0179-inch G90 galvanized steel. Copper at the weights above is the traditional choice on historic slate.
On a slate roof, a leak at a chimney or valley calls for new metal worked in under the slates, not roofing cement smeared over the joint. Brick chimney flashing in detail is on our chimney flashing repair page.
Restoring vs Replacing a Slate Roof
NPS gives a short test for deciding between repair and replacement. If less than about 20% of the slates are broken or missing and the roof is generally sound, evaluate it for repair. Other points from the same guideline:
- Don't assume the slates leak. Gutters, valleys and flashing are more likely, and attic condensation can pass for a leak.
- Tap the slates. A sound slate gives a full, deep ring; a worn one gives a dull thud and feels brittle under hand pressure.
- Check slope by slope. Often one face of the roof has weathered and the others haven't, so one slope can be replaced and the rest repaired.
Restoration means treating the whole roof at once: re-flashing, re-nailing slipped slates, and replacing the bad ones. HomeGuide prices it at $2,500 to $10,000 or more, and says a roof is a candidate if it was installed correctly and the slate has at least 15 years left. Full reslating runs $20,000 to $60,000.
If new slate is needed, plan for the wait. NPS notes delivery can take 4 to 12 weeks, and up to a year for special orders.
Synthetic (Composite) Slate
When the stone is too far gone or the budget won't stretch to new slate, synthetic slate is the usual substitute. These are molded composite tiles shaped and coloured like slate. HomeGuide prices synthetic slate at $9 to $12 per square foot installed, against $10 to $30 for natural slate.
Two points before choosing it. Michigan's code doesn't allow new roofing to go over existing slate, so the slate comes off first (the layer rule is on our asphalt shingle roofing page). And in a historic district, synthetic slate replacing natural slate is a change of material, which takes it out of the staff-approval route described next.
Slate Roofs in Ann Arbor's Historic Districts
Ann Arbor has 15 local historic districts, from a single property up to 344. Work in them is reviewed by the Historic District Commission and by city staff on its behalf. What decides the route is the material:
- Staff can approve a roof replaced with material exactly matching the existing top layer, or with the original material or a replica of it (Staff Approval List, item 8). Staff review costs $35.
- Everything else goes to the Commission, including replacing slate with asphalt, metal or a synthetic.
The city's design guidelines call repairing historic roofing where it can be repaired the appropriate approach. Replacing roofing that is still repairable, changing the shape of a roof, or covering built-in gutters are listed as not appropriate. Where the original material is "not technically feasible", the guidelines say compatible substitutes "may be considered". The city assessor's online records show whether a given address falls inside a district.
Our view: in a district, get the slate roofer's written repair scope before anyone mentions replacement. A repair to match needs no change of material.
Why Ann Arbor homeowners call Ann Arbor Roofing Co.
- Written estimate, free of charge
- Shingle line, underlayment and ice barrier named in the price
- Deck repair priced per sheet before the tear-off
- One independent local roofer per request, not a call list
- No fee to the homeowner for the referral
What Does Slate Roof Repair Cost in Ann Arbor?
HomeGuide's national figures put most slate repairs at $500 to $2,100, with about $1,400 typical. A few broken slates sit at the low end; re-flashing a valley or chimney in copper is where it climbs. Their ranges by job:
| Slate job | National range (HomeGuide, 2026) |
|---|---|
| Replace broken slates | $50 to $300 per slate |
| Replace flashing and fasteners | $400 to $3,000 |
| Restore the whole roof | $2,500 to $10,000+ |
| Reslate (new slate) | $20,000 to $60,000 |
What raises a slate repair price: access (scaffolding on a three-story gable), slate that has to be matched by colour and quarry, and copper weight. Our view: also ask how the roofer gets around on the roof, because every slate cracked on the way to the repair adds to the bill.
What a Slate Repair Estimate Should Say
Slate is specialist work. In our view, a roofer who mostly does shingles is more likely to recommend a tear-off the roof doesn't need. Ask the estimate to name:
the fastening
: copper nails with bibs, or slate hooks, never steel nails or exposed roofing cement;the flashing metal and weight
, such as 16-ounce copper, and that it matches what's already on the roof;the number of slates
to be replaced, and where the replacements come from (salvage or new, and which quarry colour);whether a historic district review applies
, and who files it.
Storm-broken slates are often an insurance matter; see storm damage roof repair. For an overall condition check before buying an older house, see roof inspection in Ann Arbor.