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ROOFING INSPECTION & REPAIR
AGE OF ROOFING
AMERICAN CEMWOOD ROOFING
ARCHITECTURE & BUILDING COMPONENT ID
ASBESTOS IDENTIFICATION IN buildings
ASBESTOS CEMENT ROOFING
ASBESTOS MATERIAL REGULATIONS
ASPHALT ROOF SHINGLES
ASBESTOS ROOFING / SIDING DUST
ROOF ICE DAM LEAKS
BEST CONSTRUCTION PRACTICES GUIDE
BEST ROOFING PRACTICES
BUILDING SAFETY HAZARDS GUIDE
BUILT UP ROOFS
CATHEDRAL CEILING INSULATION
CATHEDRAL CEILING VENTILATION
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY INSPECTION & REPAIRS
CHIMNEY FLASHING Mistakes & Leaks
CHOOSING A ROOFING CONTRACTOR
CLAY TILE ROOFING
CLAY, CONCRETE, FIBER CEMENT TILE INSTALLATION
COLD WEATHER ROOF TROUBLE
COOLING LOAD REDUCTION by ROOF VENTS
DEBRIS STAINING on ROOFS
DECKS, ROOFTOP CONSTRUCTION
DEFINITIONS of ENGINEERED WOOD OSB LVL etc
DISASTERS: BUILDING INSPECTION & REPAIR
DISPUTE RESOLUTION on ROOF JOB PROBLEMS
ENERGY SAVINGS in buildings
EPDM, RUBBER, PVC ROOFING
EPDM ROOF LEAK REPAIRS
EXTRACTIVE BLEEDING on SHINGLES
FELT UNDERLAYMENT REQUIREMENTS
FIBER CEMENT & FIBERBOARD ROOFING
FIRE RATINGS for ROOF SURFACES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLASHING, ASPHALT SHINGLE VALLEYS
FLASHING, CHIMNEY Mistakes & Leaks
FLASHING, CLAY TILE ROOFS
FLASHING MEMBRANES PEEL & STICK
FLASHING for METAL ROOFS
FLASHING ROOF WALL DETAILS
FLASHING ROOF-WALL SNAFU
FLASHING SIDING DETAILS
FLASHING WALL DETAILS
FLASHING WINDOW DETAILS
FLASHING WOOD ROOF DETAILS
FLAT ROOF MOISTURE & CONDENSATION
GALVANIC SCALE & METAL CORROSION
Green House or Solarium Roof Leaks
GUTTERS & DOWNSPOUTS
HAIL DAMAGED SHINGLES
HEAT TAPES & CABLES on Roofs for Ice Dams
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP INSTALLATION DETAILS
HUMIDITY LEVEL TARGET
ROOF ICE DAM LEAKS
INSECT INFESTATION / DAMAGE
INSULATION IDENTIFICATION GUIDE
INSULATION INSPECTION & IMPROVEMENT
LEAD POISONING HAZARDS GUIDE
LEAKY ROOF DIAGNOSIS & REPAIR
LEED GREEN BUILDING CERTIFICATION
LOW SLOPE ROOFING
MASONITE WOODRUF FIBERBOARD ROOFING
MEMBRANE & SINGLE PLY ROOFS
MODIFIED BITUMEN ROOFING
NOISE / SOUND DIAGNOSIS & CURE
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROLL ROOFING, ASPHALT
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
ROOF JOB PROBLEMS, RESOLVING
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOF SLOPE DEFINITIONS
ROOF VENTILATION SPECIFICATIONS
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
ROOFING UNDERLAYMENT BEST PRACTICES
RUBBER, EPDM, PVC ROOFING
SADDLE CONSTRUCTION at CHIMNEYS
SLATE ROOF INSPECTION & REPAIR
SNOW GUARDS & SNOW BRAKES
STAIN & BIODETERIORATION AGENT CATALOG
STAINS on & in BUILDINGS, CAUSES & CURES
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
STAIN DIAGNOSIS on ROOFS
STAIN DIAGNOSIS on STONE
STANDARDS for ROOFING
STONE CLEANING METHODS
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
Thermal Expansion Cracking of Brick
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
THERMAL IMAGING, THERMOGRAPHY
THERMAL IMAGING MOLD SCANS
THERMAL MASS in BUILDINGS
TRUSS UPLIFT, ROOF
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WIND DAMAGE to ROOFS
WOOD SHAKE & SHINGLE ROOFING
WORKMANSHIP & ROOF DAMAGE
ZINC METAL ROOFING
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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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. 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.]
The 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 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.
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.
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
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.
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
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
The 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.
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SLATE ROOF INSPECTION & REPAIR