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Question DetailsAsked on 6/7/2016

why do my cedar shakes get more black near metal objects

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1 Answer


Several common reasons:

1) copper, brass, or steel fasteners or pipes or such corrode in contact with cedar oils (and from water runoff with salts or pollution in it), causing black (or occasionally green or blue near copper/brass) staining. Using stainless fasteners and sleeving or caulking a gap around penetrating utilities and such will slow or eliminate this. When installing new shingles/shakes, a top-notch installer will coat the penetrating pipes and such with a sealer or metal paint to eliminate this contact, and use stainless fasteners.

2) the penetrations open up the wood to evaporative/wicking of moisture in the shingles to the surface, causing staining from the oils and other organic materials that tag along with the moisture. Caulking penetrations will slow this.

3) moisture from the wetted shingles evaporates primarily to the front or outside of the shingles, especially if unsealed - so that remains a damp place longer after wetting, and this occurs more readily at penetrations (including fasteners) so mold can form there, commonly as a gray or black ring on the wood around the penetration/fastener.

4) rarely, in colder areas, the fasteners will frost up at night as the outdoor temperature drops (especially on shakes/shingles with insulation under them) putting wetness at the penetration point, which then promotes/supports mold growth.

With existing shingles, scrubbing or light sanding to remove the staining or mold combined with caulking (mold-resistant exterior long-life caulk) around penetrations like conduit and pipes can reduce this, as can changing fasteners from downspouts and such to non-corroding fasteners (cad-plated for moderately long life, stainless for basically life of the house as long as you stay away from cheap foreign "stainless', which commonly is not and I commonly see coming in rusty just from storage and shipping.

Answered 3 years ago by LCD

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