We built the roof before assembling the bale walls; this allowed us to keep the bales dry during the building process (straw bales will rot if they get wet).
Initially we had hoped to use slate shingles, since slate is a local, low-tech material that will last virtually forever. Unfortunately, at the time the cost of the slate was prohibitive for us. Instead we used shingles made from Eastern White Cedar, a sustainably-harvested, highly durable local wood. We expected those shingles would last 20-30 years; by then we hoped to have found enough free slate to put on a slate roof. (For more information on slate roofs, see The Slate Roof Bible by Joseph Jenkins- link?) Unfortunately acid rain caused our roof to degrade even more quickly than expected and we found that after 13 years we needed to replace the cedar shingles. Since we planned to put a slate roof on the new building, we decided to put a slate roof on our old house as well.
We purchased “salvaged” slate from the owner of a slate quarry in Poultney Vermont; slightly outside of our ideal 30mile radius but certainly close enough to be called “local”. Steve Taran of Taran Brothers slate company, a slate quarry that has been operating for four generations, buys old slate and sorts and sells it to people wanting a more “weathered” look. We were not so interested in the look of the slate, as in the fact that “salvaged” slate lasts about 75-120 years (as opposed to the 100-150 for new slate) and is less expensive than new slate. We also liked the fact that salvaged slate has already been used and in using it again we are not contributing to the environmental effects of slate quarrying.
We chose a hip-roof (4-sided roof) mainly for aesthetic reasons but practically it is a great roof for a straw bale house mainly because it’s tough to cut bales into the triangular wedge required for other roof styles, such as a gabled roof. Also, a hip roof enabled us to have a generous overhang on all four side of the house which is critical for keeping moisture off the exterior walls. Our roof has a moderate slope, enough to shed snow slowly, over time, but not steep enough to cause dangerous avalanches. (Although we usually have one or two by the end of the winter.)