I think this has been discussed before, either on this forum, or one of the narrow gauge modeling sites.
The D&RGW used a rolled roofing on some of the outfit cars which used elaterite, a mineral something like gilsonite, to make it weatherproof. Looks a lot like composition roofing in today's building supply stores. The actual 'tar paper' is roofing felt, which is normally placed between the roof and the outer roofing material such as rolled roofing or composition shingles.
Charlie Mutschler
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