Dutch lap slate roof in Vermont (C) Daniel Friedman How to Inspect and Repair Slate Roofs - The Basics

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How to inspect a slate roof: this article assists readers in evaluating the condition of slate roofs and planning for the maintenance, repair, or replacement of slate roofing as we discuss these concerns. 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 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.

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How to Inspect & Repair Slate Roofs - Detailed Procedures, Repair Methods, Slate and Replacement Sources

by lan Carson & Dan Friedman

Every slate roof looks terrific when it's wet - unless you're seeing leaks inside. There are, fortunately, some better ways to look at and think about this material. Slate roof failures result from breakdown of the material itself, from poor installation, or from poor maintenance.

Yet too often we see people choosing the abandonment of good slate roofs which should have been repaired is a financial shame and the destruction of a valued asset.

[Click to enlarge any image]

Photographs of an actual slate roof installed using the pattern shown at left are are at the top of this page and included in our slate photo library. This was, in keeping with the spirit of the Dutch, an economical slate roof but not necessarily a better slate roof.

Some slate roof installations such as that sketched at left and installed in a Dutch Lap or Dutch Side Lap slate roofing pattern, relied on the roofing felt underlayment - material much less durable than the slate itself.

Evaluation of the condition of slate roofing, and estimating its remaining life considers at least these factors:

  • Type of slate installed on the roof - durability of the slate material itself depends on the quarry from which the slate was cut and even the location within the quarry
  • Age and condition of the slates - where are we in the expected life of the material and what damage is visible
  • Condition of the roof flashings - rusted leaky roof flashing may require removal of some good slates in order to repair the flashing
  • Slate Roof Installation quality, fasteners used to secure the slates to the roof; the slate installation pattern used may determine the resistance of the roof to leaks and wind-blown rain
  • The slate roof's repair history, quality of maintenance work - if improper or poor repairs have been done the slate roof may have been damaged or its future life reduced
  • Roof Leaks - are a telltale which could point to any or all of the above problem sources
  • Roof Structure - on uncommon occasions slate roofing may be installed on a structure not designed to carry its weight

Other very important factors in evaluating the condition of a building roof such as the condition of roof decking or sheathing, and roof structure and framing, and condition of valleys and other flashings. Many of the slate roof defects discussed below are illustrated in our online Slate Roof Photo Library included below in this document.

Advice About Walking on & Inspecting Slate Roofs

Slate roof repair and access ladders (C) Daniel Friedman

Inspectors should be cautious in evaluating any roof condition to avoid failing the roof material itself when leaks are confined to flashing areas.

Watch out: our own experience is that it is absolutely impossible to walk directly on slate roofs without damaging them, particularly if the slates are worn, loose, damaged. And walking on such surfaces is unsafe.

Roof access hazards are discussed at ROOF INSPECTION SAFETY & LIMITS

On some other fragile but not totally fragile roofs such as slate roofs, cement tile, cement-asbestos, fiber cement, and hard-fired ceramic clay tile roofs, contractors suspend a ladder over the roof surface, hanging it from the ridge (as shown in our photo of slate roor replacement underway in Duluth, MN), and cushioning it off of the roof surface using foam or insulation padding, or contractors work from scaffolding.

See ROOF INSPECTION SAFETY & LIMITS where we describe roof safety and roof damage issues when inspecting, repairing, or otherwise walking on other roof materials such as asphalt, slate, wood roofs. As stated in stated in Best Practices Guide to Residential Construction (printed text), and in our online article at FLASHING, CLAY TILE ROOFS, to prevent breakage, walk on clay and similarly fragile roof tiles with extreme caution or best, not at all.

We've found good Vermont slate roofs which have been "roofed over" with asphalt (and ruined) when the leaks were located in and only in metal valleys. Metal flashings may be deteriorating from slate particles washing off of the roof.

Dutch Lap Slate at Vassar College (C) Daniel FriedmanOur photo at left of an open lap slate pattern (a Vassar College slate roof) and at page top of a Dutch Lap slate roof found in Vermont, and the sketch at above left show what was sold as a "better" roof but was actually a leak prone slate installation pattern that covered as much area as possible with as little slate as possible. The minimal head lap and side lap invite slate roof leaks in windy rainy weather.

Don't confuse the open lap pattern (left) with the minimal head and side lap roofing slate pattern in the sketch above.

We read repair suggestions in the Old-House Journal involving use of roofing cement and felt for temporary purposes, as well as the preferable soldered or replacement repairs.

We've found severe corrosion, particularly on copper, when asphalt roof cement has been used for temporary repairs. In our opinion this is a poor short-term repair which causes increased damage.

On an historic restoration project involving slate roofs with copper flashings the worst corrosion we found on the copper was where roof cement had been used to patch leaks. Other patches and materials over similar leaks caused much less apparent damage.> are not considered in this discussion but must indeed be evaluated by the inspector.

What to Do if the Slate Roof Needs Repairs

  • At SLATE ROOF REPAIRS we describe common repairs for all sorts of slate roof defects.
  • At SLATE ROOF REPAIR, WORN OUT we describe how a roofer will decide to recommend to a building owner that the slate roof is beyond economical repair
  • At page left you will find a series of articles that treat roof flashing problems individually

An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal - the content has been edited and updated with extensive text corrections and additions, and numerous photographs have been provided for this online version - July 2007, updated April 2014. Copies of the ASHI Technical Journal are available from the American

Society of Home Inspectors - ASHI at ashi.com.

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