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How to Evaluate Slate Roof Condition - other factors InspectAPedia®  -     Other factors can assist in evaluating the condition of a slate roof - we discuss them here How to inspect, evaluate & repair slate roofs - the basics

This series of detailed slate roof inspection and repair articles describes procedures for evaluating the condition of slate roofing. How to inspect, identify defects, and estimate remaining life of slate roofs are addressed. The article series also references slate repair procedures, repair slate sources, and slate quarries.

An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal - the content has been edited and updated for this online version - July 2007. Copies of the ASHI Technical Journal are available from the American Society of Home Inspectors - ASHI at ashi.com.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Other factors in slate roof condition

Roof slope, as with all roof systems, is a big factor in shingle wear. The steeper the slope, the more durable the roof. Slates have been used even on flat roofs, as ballast, and slates were used to line the reflecting pool at the US Capitol in Washington DC.

Condensation in attic interiors can be a problem. All slate roofs need ventilation to equalize the temperature between inside and outside the roof surface. Elimination of unwanted moisture in winter and heat in summer will both extend roof life. Serious damage from interior moisture is likely to be to roof sheathing and framing, but high interior moisture will shorten the life of most roof coverings as well.

The open lap slate pattern (left) was probably intended for use on buildings with high interior moisture - perhaps a cow barn.

Most normal slate roof installations, as originally built, may have had adequate ventilation. In our opinion, serious moisture problems may begin when modern renovations insulate between rafters and add interior finishes in attics. For those designs special provisions may be needed to cool and dry the roof cavity.

Mineral deposits found on the interior surface (attic view) of slates, when none are visible outside, are a sign of possible attic moisture problems. Where no outside staining or efflorescence match inside findings, the inspector may suspect that sun exposure is drying the outer surface leaving longer moist conditions inside. If these conditions are not consistent on all roof surfaces you may find the most mineral deposits on the north or shaded roof surfaces.

Moss on slates (see our photo at page top) should be considered as damaging as on any other type of shingle. The moss retains moisture against the slate and its roots may actually penetrate and damage the material. On older roofs with heavy moss the growth can actually lift and separate the shingles. Chemical treatments may help with moss as it does on wood roofs. Extreme care should be used in working with chemicals.

Our photo at page top shows a mossy and lichens covered slate roof on Phillips Road in Poughkeepsie, NY - this slate barn roof is at the end of its life and is leaking.

Roof framing for slate roofs is often stronger than similar framing for houses of the same age for which lighter roof materials were used. A slate roof can weigh as little as 700 pounds per square up to 8000 pounds where 2" thick slates were used on a graduated slate roof. On a residential building the maximum weight per square you'd probably find would be 2500 pounds. The addition of slate to a house not framed for this purpose will require an analysis of the roof framing system.

(One roofing square is 100 sq. ft.)

Clues about the quality of a slater's work may be found in hip and ridge design. This topic is not discussed here. Flashing details, valley flashing methods, weather exposure, mechanical damage from falling limbs, foot traffic or improper workmanship, are examples of other factors which affect roof life. They are not discussed here. -- See Alan Carson's "Slate Roof" presentation notes used at several ASHI Seminars.

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ROOF INSPECTION SAFETY & LIMITS
SLATE ROOF INSPECTION & REPAIR
How to Inspect & Repair
Types of Roofing Slate
Slate Colors, Chemistry
Slate Roof Aging and Durability
Ribbon Slates
Slate Roof Installation Quality
Other factors in Condition
Slate Roof Repair history
Slate Roof Leaks
Slate Holding actions
How to Repair Slate Roofs
  Copper Tab Method
  Slate Hook Method
SLATE ROOF PHOTO LIBRARY
Slate References
Slate Sources, Repairs
SLATE ROOF INSPECTION CLASS

More Information on Building Diagnostic Inspections and Repairs

• How to Inspect & Repair Slate Roofs a detailed article about evaluating the condition of slate roofing and repairing worn or damaged slate roofs.
• Slate Roofs: a photographic library of slates, slate roofs, roofing styles
• Roofing: Defect Recognition, Repair, Prevention for Roofing & Flashing main web page
• Inspecting and Evaluating Slate Roofs Hudson Valley ASHI seminar content
• Choosing a Roofing Contractor How to find and select a roofing contractor - (thanks to Jessie Schrader)

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