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Grade 1 is cut from heartwood, a clear (knot-free), completely edge-grained wood
that is more resistant to rot than other grades. Grade 2 is flat-grained and
has some knots as well as a limited amount of sapwood (sapwood is less rot-resistant
than heartwood); grade 2 is acceptable for residential roofing.
Use grade 3 shingles for outbuildings only. Grade 4 shingles have large knots,
and are acceptable only for a starter course.
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