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How to Evaluate Slate Roof Condition - other factors

Slate roof life and leak factors:

Besides the obvious problems you may see at a slate roof such as broken or missing slates or leak stains in the interior, there are other critical slate roof factors that affect its durability, remaining life, and leakiness - here we describe them.

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.

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Other Slate Roof Factors that affect the slate roof life or slate roof leak risk

Open lap slate pattern (C) DanieL Friedman

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 (above 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. I

n 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. C

hemical 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.

An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal as cited below. That original content has been edited and updated for this online article series - March 2010, December 2023. Copies of the ASHI Technical Journal are no longer available from the American Society of Home Inspectors - ASHI at ashi [dot] com.

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