Originally Posted by Raine
Call around to the different hardware or lumber yards and ask for untreated cedar.
Locally, (NW Oregon Coast) Western Red Cedar is used in shakes, shingles for siding and roofing. To the best of my knowledge -- which is pretty knowledgable -- these products are untreated.
Cedar is natually water and rot resistant and doesn't typically get treated with chemicals. However, nonetheless . . . cedar roofing is highly flammable, and so cedar roofing material may have a flame retardant applied.
It would be wise to ask.
But cedar shakes and shingles should be available at most of the more "up scale" home building sites. Ask the contractor if you can sort through their scrap piles.