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CERTIFICATIONS for ROOFING CONTRACTORS
CHOOSING A ROOFING CONTRACTOR
ASBESTOS CEMENT & FIBER CEMENT ROOFING
ASPHALT ROOF SHINGLES
BUILT UP ROOFS
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY INSPECTION & REPAIRS
CHOOSING A ROOFING CONTRACTOR
CLAY TILE ROOFING
CONCRETE ROOFING
CORRUGATED ROOFING
ENERGY SAVINGS in BUILDINGS
FIRE RETARDANT PLYWOOD
FLAT ROOF MOISTURE & CONDENSATION
HEAT TAPES & CABLES on Roofs for Ice Dams
LOW SLOPE ROOFING MATERIALS
MASONITE WOODRUF FIBERBOARD ROOFING
MEMBRANE & SINGLE PLY ROOFS
METAL ROOFING TYPES
MODIFIED BITUMEN ROOFING
ROOF INSPECTION SAFETY & LIMITS
ROOF VENTILATION SPECIFICATIONS
ROOF VENTING ENERGY SAVING DETAILS
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
SOD ROOFING
SIDING WOOD
STAIN DIAGNOSIS on Building Exteriors
STAIN DIAGNOSIS on Roofs
STANDARDS for ROOFING
STONE ROOFING
THATCH ROOFING
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
THERMAL MASS in UPSTAIRS
TILE ROOFING
WARRANTIES for ROOF SHINGLES
WOOD SHAKE & SHINGLE ROOFING
WORKMANSHIP & WIND DAMAGE

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Slate Roof Repair History
InspectAPedia®  -    

  • How to determine and evaluate the repair history of a slate roof
  • How to inspect, find, & evaluate roof leaks & repair slate roofs
  • List of slate roof suppliers for roofing slates & slating tools
Our site offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at InspectAPedia.com/appointment.htm.

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 appearedin 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 the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Slate Roof Repair history helps diagnose the condition of a slate roof

Dutch lap slate roof in Vermont (C) Daniel FriedmanTar or roofing mastic is normally an indication of improper maintenance. It should be considered a temporary, unreliable repair. Tar or roofing cement should not be found at chimneys, sidewall flashings, plumbing vents, or elsewhere.

Our photo at left shows tar patching in the dormer valley and below the dormer. But further loose slates are falling off of this roof, possibly damaged during application of the valley tar.

The tar itself usually remains effective as a sealant only for a year or so. While in place, tar traps moisture within the slates, causing the slate itself to deteriorate. When flashings leak on a roof they should be repaired or replaced as necessary, without using roof cement.

Copper tabs and fastener failures on slate roofs: Watch for numerous copper tabs at the center of replacement shingles. These indicate that repairs have been made and they may be a clue about the overall condition of the fasteners. In a first-class slate repair you should not be able to see the copper tabs. The Old-House Journal April 1984 roofing issue describes an alternative method for securing slates which will leave a copper tab exposed. Expert roofers have a less visible method of fastening the replacement slate. [See "Repairing Slates" sidebar article.]

Our page top photo shows a perfectly good repair made by sliding a piece of copper flashing up under slates to cover a hole that may have been made by a nail pop from below.

Asphalt shingles applied over slate roofs: Using asphalt over slates is surprisingly common "re-roof" procedure as it avoids labor and disposal costs for the slate material. [In some areas of the Northeast it also represents the popularity of lower-cost asphalt and the dearth of experienced slaters who might have repaired the slate roof.]

When nailing the asphalt shingles it is common to find only a poor bond of new nails to original roof decking. The slates below tend to chip and bulge the new material resulting in a "peanut brittle" or "popcorn" effect. Since the comparatively large thermal mass of the slates retains heat, the asphalt shingles are "cooked" from their backside as well as from their front when exposed to sunlight. The life of such shingles is estimated by some to be half the normal span.

Leaks in Slate Roofs

Slate roof leak (C) Daniel FriedmanAll slate roofs probably have at least some flashing, slate, or other damage or mechanical installation errors. Some conditions such as a side lap error, damaged slate, or even flashing error may leak only in certain weather conditions such as windy rain storms from a particular direction, water backup behind ice dams, or prolonged rains.

Where slates are missing near valleys the adjoining slates may be damaged as well.

Openings may cause leaks or water to pass below the valley flashing even if the flashing looks intact. Where there were previous repairs it's common for the felt underlayment to be torn as well. If there are porous slates or openings above the tears in the felt, water may leak through.

Ice dams at roof eaves can be a serious leak source on slate as most other roof systems. Traditionally 30# felt was used at eaves as "insurance" against this problem. Some slate suppliers recommend this heavier felt for all underlayment, not just at the eaves. However two components conspire to reduce the effectiveness of felt as ice-dam protection: every nail at the eaves punctures the felt, and with age felts often dry and disintegrate before the slates have worn out.

Ice dam protection is improved in new or re-roof applications using the newer sticky membranes such as WR Grace's Ice and Water ShieldTM. However the preferred solution to this problem is proper attic ventilation. Good venting avoids the ice-dam problem and adds reductions in winter moisture and summer heat problems. Old houses whose attics have been converted to living space, particularly with un-vented ceilings following the underside of fully-insulated roofs are likely to be serious moisture and heat traps.

Slope requirements for slate roofs In conventional roofing design slates are used on roofs with a slope of at least 4" of rise in 12" of run, that is, on 4 in 12 roofs. A 3" head lap is used, often 4" when the slope is less than 8 in 12. So a 20" long slate, with a 3" head lap, would have an exposure of 8.5". For 18" slates the exposure is 7.5", and for 16" slates, 6.5". Roofs with less head lap or more exposure may be more leak-prone.

Flat slate roof (C) Daniel Friedman

But slates have been used even on dead flat surfaces such as balconies and decks, as we show here.

When we inspected this slate covered deck in Newburgh, NY in 1991 we found that the slates had been set in a bed of tar over canvas to form a flat roof that lasted without leaking for nearly 50 years.

But as you can see in our photo, the slates had now come loose from their bed of tar.

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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

About The Authors

Alan Carson and Dan Friedman are ASHI Members, home inspectors, educators and writers in Toronto and Poughkeepsie respectively. 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.

ROOFING INSPECTION & REPAIR
ASBESTOS CEMENT ROOFING
ASPHALT ROOF SHINGLES
FIRE RETARDANT PLYWOOD
ROOF INSPECTION SAFETY & LIMITS
ROOF VENTILATION SPECIFICATIONS
ROOF VENTING ENERGY SAVING DETAILS
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
SOD ROOFING
SIDING WOOD
STAIN DIAGNOSIS on Building Exteriors
STAIN DIAGNOSIS on Roofs
STANDARDS for ROOFING
STONE ROOFING
THATCH ROOFING
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
THERMAL MASS in UPSTAIRS
TILE ROOFING
WARRANTIES for ROOF SHINGLES
WOOD SHAKE & SHINGLE ROOFING
WORKMANSHIP & WIND DAMAGE


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