What’s That Under My Slate Roof?

By: Bill Kibbel , Contributing Writer

In response to Slate Roofs and Worn slate roofs - repair or replace, I’ve received many specific questions.  Several of the questions ask about what’s under a slate roof.  Although not common practice throughout the US, older slate roofs in my area are frequently installed over spaced wood battens rather than solid planks.  These narrow strips of wood are nailed on, perpendicular to the rafters and each course of slates are secured to each batten.  With this type of installation, the underside of the slate is exposed and visible.  More recent slate roofs installed on spaced battens often have roofing felt (tarpaper) under the slate.

There’s some in the roofing trade, as well as many inspectors, that believe felt underlayment is necessary for the performance of a slate roof system.   When this felt fails, at around 40-50 years, they pronounce the entire roof as in need of replacement.  I will always disagree.  A majority of the old buildings I inspect have slate roofs.  These roof systems were installed before the manufacture of roofing felt and perform just fine without it.  The only disadvantages are a small amount of wind-driven snow can blow “up” between the joints of the slate, when conditions are just right and slate dust on stored items in the attic.

Some of the questions I receive are about failing mortar under the slates.  Although mortar under slate roofs is not all that common, I’ve found 3 different types of mortar application.  The most rare is the coating of the entire underside of the roof system.  This application, also called rendering, is likely an attempt at insulating. Another application involves bedding the top of each slate in mortar.  This “lifts” the top of each slate so it lays flat over the slates on the course below.

The most common use of mortar under slates is called torching.  This consists of a thin line of mortar trowelled across the top edge of each coarse of slate.  The purpose of this mortar is intended to stop the very small amount of wind-driven precipitation, common with slate roofs.

Failures of the felt underlayment or mortar under a slate roof does not pose a significant concern.  This is proven by the satisfactory performance of the multitude of slate roofs installed without these appurtenances.


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  1. 8 Responses  to “What’s That Under My Slate Roof?”

  2. Eric Blaise
    Feb 13, 2015
    That wood looks bare, and just ready for termites to start attacking if they haven't yet. Please tell me I am wrong, because termites are the last thing you would want to deal with. It looks like the standard for the industry now at days is to build a frame from chip wood, and then attach everything else later on. It makes for a pretty solid structure, and pretty much environmental friendly and conscious. the slates are laid on sheets of chipped wood, making it a pretty easy and straight forward procedure. Eric | http://karbellaconstruction.com
  3. Mar 27, 2014
    Many older slate roofs fail is due to the kind of slate – some varieties wear out before others and once they wear out they can not be saved. Many minerals make up slate, for instance quartz and muscovite, biotite, chlorite, hematite, and pyrite to name a few. All slate roofs ought to have felt fabric beneath the slates overlapping by four inches sheet on sheet once more 1st sheet going halfway into the guttering thus any rain that penetrates the coating can run down the fabric and into the gutter ( make certain the felt border goes and folds well into the guttering and not short) onto the felt fabric long lengths of batons area unit nailed on at areas to mend the slates to.
  4. Julia
    Jan 31, 2013
    Ok we had this exact problem with our garage! We're trying to find someone to do our new roofing in Kansas City, MO and put in something of a higher quality. Thanks for the info!
  5. Aug 29, 2011
    Interesting article. I have worked with slate for a number of years and I love it I think I'd say its my favorite roofing system and probably the best quality. I'd also say felt was never used in the past on old building that I work on in Dorset in the the Uk.
  6. Aug 29, 2011
    The only disadvantages are a small amount of wind-driven snow can blow “up” between the joints of the slate, when conditions are just right and slate dust on stored items in the attic.
  7. Aug 29, 2011
    We always choose metal roofing instead of ceramic roofing because we believe that metal roofs last longer.*';
  8. Aug 29, 2011
    i think the best roofing are ceramic based because it is a very good insulator.,-,
  9. Aug 29, 2011
    Great article, it is refreshing to see some one using logic in their inspections. Tom Camp Tom Camp Inspection Services, LLC