Tom Rank, president of the Eastern Wisconsin Purple Martin Association (see their page on this site) has ideas to help martins, and other species, too.
I'll let Tom tell his own story:
"Last year out of necessity we made a decision to have our 100 year old barn wrapped in steel, but making that decision meant that we would lose our cliff swallow colony. We waited until all nestlings had fledged before beginning the remodel. The thought of having them arrive this spring only to find their nest gone really bothered me so I came up with the idea of making a replica overhang. The 16' section of rough sawn cedar is the same pitch as the original eave. I'm happy to report the birds were nest building within a few hours of arriving here last week Wednesday. As of right now there are 18 nests under construction with some near completion.
For those who have steel buildings and would like to try to start a Cliff Swallow colony, this might be a good option. I purchased (2) 1"x12" x16' long rough sawn cedar boards along with (1) 8' cedar 2"x4" The contractor who remodeled our barn cut one the 16' boards to match the 10/12 pitch of the old overhang. This was pre built on the ground as one complete piece. We then lifted the replica overhang up until it made contact with existing overhang and attached it directly to the wall of the barn using long hex-head screws.
My early observation makes me think a fascia board might offer a little more protection from a driving rain, I'll know more as time goes on."