Repair and Restoration of Slate Roofs
Domain 4 - Repair and Restoration of Slate Roofs
This domain tests diagnosis: knowing why a slate roof leaks before you touch a slate.
Diagnose the failure
On a well-built roof the slate often outlives the fasteners. Corroded or fatigued nails are the most common cause of failure in a 60- to 100-year-old roof, showing up as slipped, sagging or slid-down courses, especially on south-facing slopes. Other causes: broken slate from foot traffic or ladder placement, spalled or delaminated faces, pyrite inclusions popping out, rust stains from steel nails, moss and lichen holding moisture, and - frequently the real leak source - failed flashings, valleys and counterflashing rather than the field slate.
Repair methods
- Slate hook: a copper or stainless hook slid under the slate above to hold an intact but slipped slate.
- Replacement: a slate ripper shears the nails of the broken slate, pieces come out, and a matching slate is cut narrow enough to pass between neighbours, then held with hooks or a slanted nail plus copper wire.
- Copper bibs and soldered laps for small cracks and flashing laps.
- Epoxy is a temporary fix, not a restoration method.
Matching material
Salvage original slate wherever possible and match thickness, grain, color (weathering versus unfading) and grade. Too thick a replacement lifts the course above; too thin and it cracks.
Restoration decisions
Separate repair (fix one defect), restoration (return the roof to watertight condition with compatible materials) and replacement. Historic buildings may require review under the Secretary of the Interior's Standards and local approval. Flashings and fasteners can add decades at a fraction of replacement cost.
Worked example
One slipped slate mid-field, intact but unheld: check that neighbouring slates are sound, lift the two slates above, slide a copper hook under the lifted slate and over the slipped one, hooking its butt; seat the hook with a setting tool so the slate lies flat; nail nothing through the courses above; verify headlap before leaving.
Traps
Steel replacement nails; pressure washing or harsh biocides; mastic instead of flashing; cold-weather foot traffic over the field; replacing good slate when only flashings failed.