HI Dan. Sorry I didn't see this posting until now, but here goes. I don't know if you've already built your structure or not...in any case, a straw bale building doesn't require a 3'-4' overhang; most buildings that we build have between 24" and 36", depending on the design. A small shed such as you are (did?) planning on building could get away with a much smaller overhang. The plastered bale walls can be further protected by a silicate dispersion paint manufactured by a small Canadian Company, Eco-House. This paint, while still being permeable (critical to a bale wall) repels water- like gore-tex for bales!