Dutch Lap Slate at Vassar College (C) Daniel Friedman Slate Roof Installation Procedures & Slate Roofing Job Quality
     


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Slate roof installation workmanship: this article explains various factors that help determine the quality and thus life expectancy of a slate roof. 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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Slate Roof Installation quality, Fasteners Used & Slate Pattern

Slate roof cheap installation pattern was not leak proof (C) Daniel FriedmanThe following may all be indications that the roof is failing from the condition of the slate nails:

[Click to enlarge any image]

  • many loose slates sliding down and often, pieces of slate on the ground around the building
  • numerous repairs showing old slates re-secured in place (could also indicate other problems)
  • improper tar and flashing materials used to secure loose slates

In our opinion, galvanized flashings (or too-thin thin copper or painted steel) are a mistake on slate roofs. The flashing is very likely to wear, rust, and fail before the slates. While replacing flashings is quite possible and appropriate, it's often expensive and if not done by an experienced slate roofer, there is risk of costly damage to the roof.

We had a client this year whose slate roof was damaged by the mason working on the chimney. He accepted a low-bidder for roof repairs. We found asphalt shingles nailed over and around the area of damaged slates. To say that the roof leaked was the least aggravating observation concerning this work.

Inspecting from the attic interior may also give clues to fastener age, type, and condition if some nails are visible. You may see tips of nails, depending on the length of fasteners used.

Slate Roof Installation Pattern as a Factor in Slate Roof Life

[See sketch above]

Some slate companies advertised A slate roof that cannot leak, yet [was] inexpensive, easy to apply, beautiful..., durable as time," using a design which was soon found to be a disaster: 12" slates were placed with 9" exposure, leaving 3" for headlap and 6" which was backed only by a cap sheet of 32# felt interlaced with the slates.

Roofs were also installed following this poor design, using 14" slates with 10"-11" exposure. Felt is not functional as a permanent roofing material: even where it is not exposed directly to sunlight, as the organics dry out the felt cracks, disintegrates, and leaks.

We have reports that inspectors have been the subject of legal actions following their failure to identify this defect in slate roofs. See the illustrations above. -- Personal communication, Doug Sheldon, Vermont Structural Slate, December 1990.


Side lap slate pattern (C) Daniel Friedman Open lap slate pattern (C) Daniel Friedman

Side lapped slate Vassar Campus (C) Daniel Friedman

Slate installation pattern: as we introduced at SLATE ROOF INSPECTION PROCEDURE, some slate roof installation patterns "stretched" the material by installing slates with minimum head lap or side lap.

The proper minimum side lap for roofing slates is shown in our sketch at above left. An "open lap" slate roof is shown at page top (Vassar College Campus, Poughkeepsie, NY) and in our sketch at above right.

In some climates (blowing rain) this may lead to a leaky roof that was fine on a barn but not so nice on a house, in particular if the side lap of the slates is less than 3". Notice that 3" is called for in the right hand sketch and that a 50% head lap is also shown.

The side-lap pattern of the Vassar College campus slate roof at left shows that the slates are overlapped 50% - this slate roof pattern should be fine provided that there is also adequate head lap (which we cannot see). .


Slate Nails (C) Daniel Friedman


Slate nails:
Proper installations use copper, stainless, or hot-dipped galvanized nails for fastening slates. Very early slate roofs were secured using wood pegs.

Later ones used tie-wires in some applications. Slaters' nails have a thinner head than conventional roofing nails, avoiding damage to the covering slates.

We've found many slate roofs installed with steel nails in the Northeast.

We've also found lots of roofs in that area which are losing good slates from nail failures.

Slate nail holes: When a slate is punched (usually at the quarry) the hole is driven from the back of the slate, leaving a ragged pit at the front (exposed) surface of the slate so that the nail head can be countersunk flush with the top of the slate. This avoids damage to the next course of slates which overlay the nail heads and which may crack under load.

Slate hammer Hand punching of nail holes in slates: Because most roofs have at least some custom slate fitting (for example at valleys), some slates are hand-punched and may be fractured around the nail hole.

Usually only two holes are punched, 1.25" to 2" from either side and about a quarter of the length of the slate down from the top.

More holes and more nails may be used to hang thicker heavier slates. Where battens or spaced roof sheathing are used the spacing of the battens will affect where the holes are punched in the slates.

Slates punched too close to their centers or too low in the slate are more likely to leak when water seeps down in a fan-shaped pattern from the vertical abutment of the sides of the slates in the next course up the roof.

If you are inspecting a roof which frequently leaks following prolonged rains, and if the slates and flashings look pretty good, and if leaks are everywhere, you might look for improper punching or nailing errors. Don't rush to condemn the roof - how often and how badly does it leak? Under what conditions? What is being damaged? Where is water going?

Reader Question: how tight should you lay the roofing slates?

(Oct 23, 2011) mike said:

how tight should you lay the slate ?

Nail tightness: Slates should hang loosely on the nail. Nails too tight may break the slate as they're pulled through it as wood shrinks. Nails not driven fully are likely to break the slate above.

Other signs of poor installation include inadequate side laps. Side Lapped slate pattern.

Side laps on slate roofs should be not less than 3" and each side-joint should be as near as possible to the center of the slate below. Particularly where improper repairs have been done you may find violations of this rule. Inadequate side laps risk leaks as a result of wind-driven rain.

Inadequate side laps might be suspected on roofs which use varying-width or graduated width shingles. However some experienced roofers and inspectors commented that graduated, random-width, graduated width slate roofs were more costly than other slate systems and were often installed by more skilled roofers.

Do not mistake graduated or random slate sizes for necessarily improper application.

An earlier version of this article appeared in the winter 1991 issue of the ASHI Technical Journal - the content has been edited and updated for this online version - March 2010, April 2014.

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