Worn out leaky slate roof (C) Daniel Friedman Slate Roof Wear, Slate "Rot", Slate Roof Life

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Signs of slate roof wear & deterioration from close-up inspection allows one to assess remaining roof life: the abandonment of good slate roofs which should have been repaired is a financial shame and the destruction of a valued asset. At the same time, careless optimism about a bad slate roof which is at the end of its life risks an angry inspection client.

This article describes various indicators of wear and aging found on slate roofs as an aid to assessing the overall roof condition, its risk of extensive leak points, and its probable remaining life.

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Aging and Durability of Slate Roofing - How Long Will a Slate Roof Last?

Tar coated slate roof (C) Daniel Friedman

The slate roof in our photo at left was coated over all of its surface with roofing tar that later began to wash away.

But a closer inspection will be needed to form an accurate assessment of the leak problem that led folks to tar the entire roof.

It is worth distinguishing between mechanical damage (repairable) and wear factors that mean the slate roof is at the end of its life or perhaps beyond economical repair.

Close-up Inspection of Slate Roofs is Necessary to Evaluate Their Condition

Does slate "rot"?

We were warned by an experienced slate roofer and we have confirmed by field inspections of slate roofs that determination of slate condition other than by direct up-close inspection is highly unreliable.

Slates may look fine from the ground, but be found soft and at end of life on close inspection.

Photograph of soft roofing slates that are worn out Photograph of worn out rotted roofing slates Photograph of worn out rotted roofing slates

By direct inspection we mean looking at slates from a few inches, either from a ladder or some other point of view such as an attic window. We strongly advise inspectors not to walk on slate roofs. Risks include falling - very slippery, and extensive damage to the slates. [Trapasso, personal communication.]

Surface delaminateion on a slate roof (C) Daniel FriedmanThe photographs provided here show how soft roofing slates can become. These slates might, from a distance, look pretty good, but we were easily able to crush the slates with my fingers.

We do not agree that we should call this "rotted roofing slate" as did Mr. Trapasso, but on seeing how soft and delaminated this slate can become, we can understand his loose use of that term.

It is useful to contrast the above "soft crumbling roofing slate" with the surface delamination of the roofing slates shown at left. The slates in our photo (left) may look questionable from the ground but a closeup inspection may show, as this one did, that the slate body is still thick and hard, and it is surface delamination of the slate that was visible from the ground.

These slates are not yet at end of life.

These photographs make clear that it can be tough to accurately evaluate the condition of some slate roofs from ground level - a close-up inspection is needed to distinguish between the slate roofs in our photos shown here. The first is or really past end of roof life and the second slate roof has good life remaining.

Life Expectancy / Durability of Different Roofing Slates

Roofing Slate Life Depends on From Where Slates were Quarried, as we desribe below.

Life expectancy of Virginia Buckingham roofing slate

Darker gray "Buckingham" slates from Virginia are considered a durable high-quality material which, properly maintained, can have a near indefinite life. Average life of these slates is 175 years.

The Old House Journal, April, 1983 p. 57

Life expectancy of Vermont roofing slate

Vermont slate roofs, with proper maintenance, can last indefinitely.

Slate in good condition (C) Daniel FriedmanThe Old House Journal, April 1983, p.58, and Slate Roofs, National Slate Association, p. 71.

By other sources, Vermont slates have life expectancy of 100-200 years. Our photo at left shows a slate roof in good condition at Vassar College in Poughkeepsie, NY. We do not know the origin of this particular slate, but it's wear condition and age indicate a durable roof.

Steven Trapasso, personal communication. Also the Old House Journal April 1983 rates Vermont slates as having a 175 year life.

Such roofs should be maintained, not replaced.

With proper maintenance such an asset may be continued for an indefinite period. Inspectors who do not know the composition and source of slates would not want to condemn, except in the worst of circumstances, a Vermont slate roof.

Life expectancy of Pennsylvania roofing slate

Typical life for Pennsylvania slate roofs is 40-50 years. These life expectancies are general experience with the material and do not consider installation or environmental factors which vary from one building to the next.

Life Expectancy of Ribbon Slates

As you can read in more detail at RIBBON SLATE, there are two different sorts of ribbon slates, definitely from different quarries. Some ribbon slates whose ribbons or lines of color through the slate demonstrate color lines comprised of soft minerals, sometimes even with small voids, often of a red or rust-brown color. In our experience, in the Northeastern U.S. where these slates were found, often these were a low-priced and short-lived roofing slate.

But other ribbon slates, also found in the Northeastern U.S. as well as in other areas, prove to be as durable as other high quality roofing slates, with lifetimes from 20 to 80 years, perhaps even longer. At below left, the ribbon-slate roof in Poughkeepsie, New York was about eighty years old when we took the photo. It remains in good condition. On a different home in the same city, the mansard style slate roof at below right employs a mix of ribbon slates, some of which were quite worn and appeared to be at the end of their life.

Ribbon slates, Poughkeepsie NY (C) D Friedman Ribbon slate (C) Daniel Friedman

Life expectancy of British and Welsh roofing slate

Earlier British experimentation on slate durability and composition emphasized that variations in the tendency of slates to absorb water may be the principal cause of deterioration seen by modern inspectors. Slates which are soft, delaminating, and thus are at or near end of their useful life may have been damaged by frost.

"That sort of slate", said Dr. Watson, the bishop of Llandaff, "other circumstances being the same, is esteemed the best, which imbibes the least water; for the imbibed water not only increases the weight of the covering, but, in frosty weather, being converted into ice, it swells and shivers the slate." Watson continued to suggest that slates, being less porous than clay roofing tiles of the day, were more durable.

The New Practical Builder and Workman's Companion , Peter Nicholson, 1823, reissued 1861, republished with companion modern opinions in Handbook of Building Crafts in Conservation , 1981.

Watson's opinion that slates are more durable than clay tiles is arguable as both materials are found with considerable variation in composition. Indeed unglazed clay absorbs more water (up to 10%) than slate (usually 1%) but tiles are not necessarily damaged by these freeze-thaw conditions. Watson's comments pertain to use of slate in Britain.

The extent of color change in Pennsylvania [and possibly other] slates often indicates the extent of deterioration. More white, more efflorescence probably means more deteriorated.

Reader Question: comparing the properties of Canadian Slate & Welsh Slate for Roofs

5/5/2014 john owen said:

hi, what is the difference between canadian roofing slates and welsh slates .


John you must know that the names refer to where the roofing slates originate.

Canadian roofing slates are made from slate quarried in Quebec or possibly Newfoundland, and sold by a variety of companies. According to the National Slate Association, (quoting)

Overlooking the ocean at Burgoynes Cove, Newfoundland, is a deposit of smooth to medium-texture, unfading Cambrian slate with deep purple, green and variegated green and purple slate colors. The purple slate is very similar in color to Welsh Purple from England. Production of roofing slate from this quarry has been intermittent since the early 1900s.

The Glendyne Quarry in St. Mark du Lac Long, Quebec, Canada, is the largest roofing-slate producing facility in North America and one of the largest in the world. Their unfading black slate is currently produced for the European and North American market and is marketed in North America as North Country Unfading Black. The first production from this vein of slate occurred in the early 1900s. This smooth-textured slate is easy to cut and has a consistent black color with subtle vertical shade markings. -

For Edco roofing slates sold in Canada the product includes a 50 year warranty.

Welsh roofing slate is described by which I quote as well, noting that this is also marketing literature mixed with useful facts:

Natural slate from Welsh Slate’s Penrhyn, Ffestiniog and Cwt-y-Bugail quarries produces the finest roofing slates in the world. Its durability, which sets it apart from other slates, is derived from its density and the geological formation found in that part of North Wales.

Welsh Slate roofing is commonly removed from roofs after 100 years not because they have failed but because the roof timbers holding them up have failed. The Welsh slates may then be re-used. Penrhyn slate, for example, was removed from St Asaph Cathedral after 400 years in use and reused and this practice also applies to civic buildings as far away as Australia.

This blend of quality and aesthetics, as well as the large format sizes available, makes Welsh Slate roofing slate much desired for heritage and prestigious projects including Buckingham Palace, Number 10 Downing Street and St Pancras and King’s Cross Stations. -

Finally, when comparing slate properties it is worth remembering the location of the slate quarry and considering the shipping costs of alternative slates. Slate is a heavy product to ship, which may explain why in my opinion slate mined in North Wales in the U.K. is not likely to be found on many buildings in the U.S. or Canada.

Contact information for these companies and slates can be found at SLATE ROOF SOURCES & TOOLS

What is the normal rate of deterioration of roofing slates?

The rate of slate deterioration on roofs depends on the source quarry and the depth from which the slates were cut. These determine the density and composition of the slate. Site conditions, such as the roof pitch, exposure to sun, building moisture and ventilation and exposure to mechanical damage also affect slate life.

It is interesting to note that several references indicate that slates actually harden after being quarried and exposed. We'd presume that observation pertains to the early or initial state of use of the slates, and we found no reference which claimed some chemical or molecular process of permanent or continuing hardening over life.

Signs of deteriorating roofing slate

Slate effloresence (C) Daniel FriedmanDeteriorating Pennsylvania slate shows a mottled appearance, with white faded patches resulting from exposure to sunlight and moisture.

The white areas generally begin at the three exposed sides of a slate, growing towards the center of the slate as a function of time and exposure.

The white visible in Pennsylvania slate is evidence of efflorescence - lime deposited on the slate surface. If the whole slate is white in color and scaling you should consider the slate to be at the end of its life.

However as long as the slates are physically intact the roof is serviceable. If a roofer recommends tearing off the whole roof you should ask to be shown examples of failed slates.

Roofing slate fading versus mottling

Fading versus mottling: It is important for an inspector to distinguish between the mottled edges-in whitening (efflorescence) and fading. By contrast to the efflorescence process, fading slates generally fade monotonically across the slate rather than from edges in. Faded slates will not show an obvious whitish powder of surface deposited mineral salts as thick as that left by efflorescence. All slates will fade from weather exposure. Those which have only minimal color change are classed as "permanent" or "unfading". Those which change more markedly are classed as "weathering".

Monotonically means that the color change is even across the material rather than showing up as a splotch, or a ring of white around darker color, or in other variations. It's a nice word, not recommended for inspection reports.

We've heard several explanations for the mechanism by which Pennsylvania slates deteriorate, of which the most interesting was the opinion that lime in the slates in the presence of moisture reacts to break down organic components in the slate. For slates higher in carbon or carbonaceous materials this may be indeed the most accurate explanation.

Pennsylvania "Black Bed" quarried slates were high in carbonaceous impurities and were recommended by the Slate Association only for temporary buildings or for chalkboards. By the 1920's this slate was recognized as not as durable; you'd not expect to find it surviving on a building in 1991.

The National Slate Association reference does not detail the mechanism of slate deterioration. It does point out that the slate roof on the Saxon Chapel at Stratford-on-Avon was built 1100 years ago - still in good condition.

By Mr. Trapasso's account the decomposition of Pennsylvania slate may be understood as a process similar to decomposition of wood. The lime which was in the slate, when the slate was mined, had not caused deterioration because the slates were protected from sunlight.

When the slate is split, cut, punched for use, the lime is in an inactive or dormant state. Following application on a roof surface the slate material is exposed to ultraviolet rays (sunlight) and moisture. These components cause the lime to break down the organic materials in the slate (shale), making the slate increasingly porous and causing the white efflorescence. Lime and other mineral salts are dissolved and deposited on the surface of the slate when moisture evaporates.

Slate Roofs, National Slate Association, p.71-76.

Calcium Oxide - Sources of white minerals and oxides in roofing slates

Examining quantitative analysis of slates from nineteen quarries we observe that while oxides of silica (SIO2) and aluminum oxide (AL2O3) are the dominant components of slates, almost all have measurable levels of calcium oxide (CaO) and other carbonates and oxides. There are dramatically higher levels of these materials in Pennsylvania slates (3%) than in slates from Vermont (.3%-.7%). Clearly there is more material available to form calcium carbonates and efflorescence in the shorter lived materials.

Slate Roofs p. 73

Carbon, which would be broken down in the process described by Trapasso, occurs in slates from only a few quarries, and then in usually small amounts of less than 1%. This would seem to argue against the "slate rots" explanation of deterioration.

Brown staining in roofing slate

Brown slate stains (C) Daniel FriedmanThe brown color of some slate may be from oxidizing iron content in the slate which is following a similar reactive path. we did not find references to brown in Virginia slates.

The known chronological age of slates may be available from an owner or other documentation. As with all materials, experienced inspectors rate the apparent wear age of the roof, not the actual age.

This article series reviews types of slate, common defects, inspection topics, and some repair tips. We also provide slate sources and where to buy slate roofing materials and slate roofing tools and products.

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.

Information in this article is taken from the references shown below, from inspection experience, from opinions shared at ASHI and other education seminars, from the authors' personal, limited experience repairing and rebuilding of slate roofs, and from very helpful Bill Markcrow and Doug Sheldon at Vermont Structural Slate Co. We also paraphrased from a presentation to NY Metro ASHI Members by Mr. Steven Trapasso in October, 1990. Mr. Trapasso has 40 years experience in working with slate, and has a clear love affair with the material, particularly when it comes from Vermont.

Slate Roofing Articles


Continue reading at SLATE ROOF PATTERNS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SLATE ROOF DEFECTS where we describe a variety of problems and leak points on slate roofs that are not necessarily a function of the age or wear condition of the roof.

Suggested citation for this web page

SLATE ROOF LIFE EXPECTANCY at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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