Inspecting Slate Roofs - how to inspect slate roofing for condition, damage, leaks
RoofAPedia ^©

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

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

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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left on most of our website pages show where you are in our document & website.

Key Questions

�      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 effloresence 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

�      Weathering:

�   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 bad roof

�    more than 25% of slates are sliding down - fastener failure

�    more than 25% of slates are worn out - big replacement cost

�      Don't Fail a good 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

References

�       "Slate Roofs," Alan Carson, Dan Friedman, ASHI Tech Journal Vol1 No1 - from ASHI, www.ashi.com &
The Construction and Building Failure Diagnosis Website - www.inspect-ny.com

More expert information on this topic

More Information on Slate Roof Inspection, Diagnosis, & Repair, & Other Building Diagnostic Inspections and Repairs

• How to Inspect and Repair Slate Roofs - Detailed Procedures for Inspection and Repair of Slate Roofs - ASHI Technical Journal, 1991, Updated 2006
• Slate Roofs: a photographic library of slates, slate roofs, roofing styles
• Roofing: Defect Recognition, Repair, Prevention for Roofing & Flashing main web page
• Choosing a Roofing Contractor How to find and select a roofing contractor - (thanks to Jessie Schrader)

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