Definition: sarking

Definition: sarking

sarking, n.

More fun with the old 2,300 page, unabridged dictionary

I’m going to disagree with my old 2,300 page unabridged Webster’s and say that sarking is properly the asphaltic felt used beneath roof shingles, derived from the shirt, or sark, defined just above. Here’s why…

Asphaltic felt is a great shirt for a house. Felt is simply matted wool, and asphalt would have been some bituminous coating (tar) that it was soaked in. Supposedly pre-industrial craftsman stocking undershirts were black so you can see how close the two definitions would be. (Although I haven’t discovered an authoritative source for this supposition yet.)

Architects used to specify this felt beneath exterior cladding systems. This layer would:

  1. prevent rain soaking and absorb incidental leakage for drying afterwards
  2. channel moisture behind the cladding down and out
  3. block air/wind blown through cladding cracks
  4. slow humidity drive
  5. add a little bit of insulation value, (R-2?) given the felt and intermediate layers of air space

Every proper building envelope system should manage:

  1. bulk water
  2. moisture
  3. air
  4. vapor
  5. thermal flow

So this undershirt really is a great idea for a house.

Historically, the paper was specified by weight, with 30 pound felt being 30 pounds for a roll, which equaled one roofing square, or 100 square feet. 15 pound felt was also used, sometimes in double layers to equal the 30 pound felt.

But unfortunately, you can’t get this kind of felt today. In my adult lifetime of working with buildings, the derived product has become a very lightly coated paper. The old symbol “#” which mean pounds no longer implies the weight. I distinguish #15 “number 15” paper from 15-lb “15 pound” felt as completely different products.

And of course, these changes mean the product has reduced performance in all five categories compared to the original material. So it has been replaced by more sophisticated products for vapor control, moisture barriers, rain screens, drainage mats, pressure relief, venting channels, furring, coatings, and flashings.

Still, despite well-performing new tech products, ye ol’ asphaltic felt is still one of the more viable building products in a modern envelope strategy.

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