I install snow-guards on existing slate, copper, tin and steel roofing. On copper, I like seam clamps, tied together with copper tubing to span the flat areas.
Richmond and the surrounding area can go for 2 years without any snow. Most years, we only get about 4" per event. BUT, on occasion, we get from 14" to 24" in one storm. When that amount of snow starts sliding down the roof, it takes gutters and snow guards with it, if they're only installed along the bottom edges, and people can get hurt. Sometimes, it only trashes the snow guards.
Some installers put a staggered row of guards just up the roof from the gutter. This will allow snow halfway up the roof to slide down, sometimes from near the ridge. This wad of snow and ice can tear snow guards from the roof along with the gutter.
I much prefer 2 staggered courses on the roof, about 6' or so apart. I don't use any particular measurement, I just put them on according to how I think that particular roof needs them, and with regards to how it looks on that roof. Below are pictures showing a sample of my work.
This roof has 15" rafters so we decided on two courses of guards in the top 3/4 of the roof. The first three courses are from about 12" above the gutter, to about six foot up the roof. There is only a six foot gap between the two courses.
These are some pictures of a roof I've added Snow Guards to.
And here is the same roof with snow on top. Notice how the snow is staying in place. This is 12" of snow. And here is a picture of his neighbor's house that doesn't have snow guards. Over 1/3 of the snow has come off the roof on the left side. Here is another picture of a roof without snow guards and the gutter appears to be the only thing holding the snow up there. Then, we have a picture of another one I did. The owner had 'Flip-Clean' gutters and he flipped them before the snow.
There is no snow buildup at the eave and the snow is melting evenly across the roof.
Same house after the storm. Most of the snow is gone, but still in place, as are the gutters.
Next door, the situation was different. And the house on the other side.
I couldn't get a good picture, but here is a copper roof I did that has pipe spanning the snow guards over the rear door.
As of 8-1-2011, Galvanized SG's are $16.00 and Copper SG's are $26.00 each, hooked to existing slate nails. If slate have to be pulled and more nails added, add $10.00 each.
Snow Guard Specs, courtesy of Berger Bros.
Albert's Roofing Repairs | Slate and Window Flashing Co., Rich., Va.