Slate Roof Leaks — how to find and diagnose leak points on slate roofs

Slate Roof Leaks - how to find and diagnose leak points on slate roofs

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How to find the source of leaks in a slate roof: all 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.

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.

How to Track Down the Source or Point of Leaks in Slate Roofs

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 Shield TM .

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.

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.

Reader Question: what’s the best way to repair a leaky valley on a slate roof

(Aug 17, 2011) lois said:

can someone tell me the easiest way to repair a leaking valley on a slate roof please? thank you so much.


Unfortunately because the valley has to extend back up under slates on either side of the valley itself, to make the best and proper and most durable leaky valley repair, one has to remove slates to replace or re-line a leaky slate roof valley.

You could make a temporary patch using stick-down materials or on some roofs I’ve seen roof mastic used in that application; beware of using asphalt roof mastic on copper flashings — it fixes the leak in the short run but corrodes the copper in the long run. Details are at SLATE ROOF LEAKS, TEMPORARY REPAIR

Once the necessary slates are removed, the valley is usually replaced with new, heavy copper or on some roofs folks use other flashing materials like aluminum (I don’t recommend that).

I’ve repaired complex metal roof flashings with torch-down modified bitumen — but there is a fire risk doing so.

I’ve repaired flashings temporarily when the leak was localized, using stick-down EPDM roof products too.

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