Inspecting Slate Roofs - Class on how to inspect slate roofing for condition, damage, leaks

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Slate roof inspection class, free, online: this is a classroom presentation on the inspection, diagnosis, and estimate of remaining life of slate roofs. It was made to home inspectors in New York and has been used in other states in the U.S.

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ASHI home inspection education class notes on how to inspect slate roofs

by Daniel Friedman
Hudson Valley ASHI - HVASHI Seminar - Kingston, New York
9 September 2003 class for home inspectors

This course outline reviews key considerations in evaluating slate roofing on historic or other buildings. Its presentation is intended to be accompanied by a collection of photographs and drawings. As time permits the author will place in this document links to representative samples of those illustrations.

Readers of this page should see Slate Roofs by Alan Carson and Daniel Friedman, for a detailed description of slate roof inspection procedures, slate roof materials, slate roof defects, slate and slate replacement sources.

Key Questions to ask about slate roofs

  • Is it slate?
  • How is it inspected?
  • How much life remains? Stratford-on Avon Saxon chapel, 1100 years +!
  • What repairs are needed?
  • What will it cost to repair or maintain?

Identifying Slate Roofs

  • Standard style, one or many colors
  • Textured style, varying thickness & texture
  • Graduated Slate, varying size, smaller, thinner at ridge
  • Not-Slate
    • asbestos-cement shingles
    • slate look-alikes and replacement materials

Inspecting Slate

  • Safety of the inspector comes first - Do not walk-on it
  • From ground - unreliable
  • Ladder at edge - reliable
  • From nearby windows/surfaces - good
  • Binoculars - useful, incomplete
  • Document inspection limitations & implications (hidden slopes often differ in materials, condition, and may not even be slate!)

Slate Roof Life

  • Quality of Slate (Vermont-NY, Pennsylvania, Virginia Buckingham)
  • Level of maintenance (repair history, competence)
  • Material failures (quality, age, condition, leaks)
  • Fasteners & fastener failures (common)
  • Flashing failures (most common)
  • Installation patterns (uncommon)

Quality of Slate

Slate is stone, unique to quarry where mined

Color and appearance are clues but not sure

  • Black
  • Blue-black
  • Purple
  • Mottled-purple & green
  • Red
  • "unfading" vs. "weathering" for each of above, not a durability factor

Slate Colors (continued)

  • Green, purple, black, red also avail - Vermont, most common, lower in lime than PA = 100-200 yrs.
  • Gray, gray-black - Vermont & New York lighter than PA slate, may include purple, green.
  • Blue-gray - Pennsylvania - best known, "Pennsylvania black" - less durable - 40-50 yrs. White efflorescence forms rings on 3 exposed sides. Unfading PA gray is soft-gray, longer-lived; Unfading PA black is rougher, longer-lived; Blue-black "hard-vein" PA slates darken with age.
  • Blue-gray - Virginia - tough, >100-200 yrs.
  • Red un-fading - Washington County NY

Quality of Slate

  • Variations in thickness - more is better

  • Variations in stone chemistry - quarry-unique

  • Imperfections and inclusions - iron & calcite

  • Ribbon slate - impurities in bands, shorter life, may vary depending on what minerals make up the color bands

Slate Maintenance

  • replacement slates - how many?
  • replacement fastening methods -hooks, tabs
  • temporary patches - with metal or other
  • tar or roof mastic - "the bigger the blob the better the job?"
  • loose or missing slates - how many?
  • valleys or ridge caps worn, rusted, leaky

Material Failures


  • delaminating
  • scaling along cleavage planes
  • science: slate becomes thin or soft and spongy: mineral impurities (calcite, iron sulfides) + alternating wet/dry cold/hot form gypsum which expands and delaminates the slate. Slate is stone, it does not "rot" but it does get soft.
  • white mineral salt rings may telltale degree of aging, some slaters opine that the area of the un-stained center defines the % remaining life - no science given

Fastener Failures

  • Many missing slates, many patches
  • Nail pops - vibration, high nails -> holes
  • Over-nailing - too tight -> cracks
  • Iron vs. copper/stainless nails - fastener failures - many slates may be about to fall

Flashing Failures

  • Mineral roll roofing valley liners
  • Copper or steel valley liners
    • corroded
    • tarred
    • leaky
  • Chimney flashings - usually tarred
  • Inspect leak history in attic - flashings & ice dams

Installation Patterns

  • "Cheap" patterns more likely to leak
  • Dutch Lap (smaller slates 10x6"?)
  • French Method
  • Open Lap - good for barns
  • Standard lap patterns, solid or mixed
    • sizes up to 24x14", square ends, uniform color & exposure
  • Textured Slate - look for on Tudor's, rough surface, varied thickness
  • Graduated Slate - graduated size & exposure

Sketches of Slate Patterns

  • Dutch Lap
  • French method
  • Open Lap
  • Standard Pattern

Slate Inspection Mistakes

Don't "Pass" a Worn Out Slate Roof: Criteria

  • more than 25% of slates are sliding down - fastener failure
  • more than 25% of slates are worn out - big replacement cost

Don't "Fail" or Replace a Good Slate Roof

  • many repairs, few current loose/bad slates
  • bad flashings, good slates
  • asphalt-shingle roofer sells owner on avoiding maintenance cost, removes 300-year material, installs 30-year material

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