Inspecting Slate Roofs - how to inspect slate roofing for condition, damage, leaks InspectAPedia® -
Course notes on slate roofing inspection, diagnosis, repair
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ASHI home inspection education class notes on how to inspect slate roofs
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.
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
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References on Slate Roofing
"Slate
Roofs," Alan Carson, Dan Friedman, ASHI Tech Journal Vol1 No1 - from ASHI,
www.ashi.com
The Slate Roof Bible, Joseph Jenkins, www.jenkinspublishing.com
"Slate
roofing: an old-world tradition," Professional Roofing, February 1993
"Special
Roof Issue", Old House Journal, April 1983
Handbook of Building Crafts in Conservation, Jack Bower, 1981
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