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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 ROOF INSPECTION & REPAIR 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
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 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
An earlier version of this material appeared in the winter 1991 issue of the ASHI Technical Journal as cited below. In this article series we have made additions and corrections to the original text,and we've added many slate roof photographs and details.
Carson, Alan & Daniel Friedman, SLATE ROOFS [PDF] (1991) ASHI Technical Journal, Vol. 1. No. 1, January 1991
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Evergreen
Slate Co., & Hilltop Slate Co., both in Granville, NY
Buckingham Slate Co., Richmond, VA.
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724
ISBN-13: 978-0471331728
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008)
ISBN-10: 1405161035
ISBN-13: 978-1405161039
Handbook of Building Crafts in Conservation, Jack Bower, Ed.,
Van Nostrand Reinhold Company, NY 1981 ISBN 0-442-2135-3 Library of
Congress Catalog Card Nr. 81-50643.
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008)
ISBN-10: 0471788368
ISBN-13: 978-0471788362
Historic Slate Roofs : With How-to Info and Specifications, Tina Skinner (Ed), Schiffer Publishing, 2008, ISBN-10: 0764330012
, ISBN-13: 978-0764330018
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Slate Roofing in Canada (Studies in archaeology, architecture, and history)
In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.