Mitigation of Snow on Roofs Part 2

1. Sloped roofs are slippery ... way more slippery than dry roofs. If snow must be removed from a sloped roof, consider employing people who do stuff like that for a living, say roofers. Or maybe rock climbers, with all the safety ropes. You might be able to get the edges from the side (ladders, or whatever).

2. Don't pile snow; get it off the roof.

3. Consider if all or just some of the snow must be removed. For example, on a roof where the snow has drifted and the depth varies between 12 inches and 48 inches, and the snow at 48 inches is determined to weigh 45 psf (see my articles on measuring ... here and here), and the design snow load is 30 psf, then for sure remove the snow at depths greater than ... 30/45^ths of 48 = 32 inches (assuming the snow is constant density). If rain is coming, then remove more. (Or just remove it all and be done with it. If the roof is large, you may need to remove the excess amounts first, and as you have time remove more, or all.)

4. If you have the luxury of knowing how much rain is coming ... you can predict the weight it will add to the snow, as the snow will likely absorb it faster than it melts it.

Water weighs 62.4 lb per cubic foot (water one foot deep over a 1 ft by 1 ft area). One inch of water weights 1/12^th of that, or 5.2 lb (per square foot). So, if the weather person says that up to an inch of rain is coming, that will add to the weight of the snow an amount of 5 psf. Half an inch of rain would add 2.6 psf, and so on.

So, you might take away more snow, if rain is coming. In the above example if you removed snow to a depth of 24 inches, you'd supposedly bring the load down to about 23 psf, and if an inch of rain came, it could bump it up to 28 psf ... if the roof is designed for 30 ... you're good!

But note: snow that has been thawing and freezing, absorbing rain, etc., for a while, gets denser, and then if you get a foot more of snow, the snow on top is probably lighter, and you won't have a uniform density. "Take it down to 18 in., for chrissake!"

5. If warm weather is coming, or warm and wet, make sure the roof drains are working. (Make sure they are working anyway, but if rain and melt are coming, really make sure they are working.)

6. Melt the snow! Yeah! Turn up the heat and open up the attic/ceiling space.

7 . On one project well-meaning personnel volunteered to try and melt the snow with torches. You can imagine that the fire marshal had serious reservations. But it didn't really work. The idea was to melt it on boards elevated from the roof, and let the melt drain down an already cleared part of the roof. As soon as the air temp dropped to 32 deg F, once the water left the torch area it froze. Frozen water weighs just as much as the snow it came from, so the weight was still up there. (Just turn up the heat in the building, and maybe remove insulation if practical.)

8. In my experience the best way to physically get the snow off the roof is to shovel and/or sled it off. Use tarps or plastic sleds. Don't make and move big piles. Remember, the roof is already heavily loaded. Do small loads you can move fast. Obviously, consider where you are dumping the snow. Ideally it would be nice to dump it in the back of a dump truck and haul it off, but you may have to dump it on lawns, shrubs, sidewalks, etc. Make sure there are no people below or anything else you don't want to damage.

9. Snow removal equipment ... the idea bothers me. Maybe if it is real light. If you haul a snow blower up there, where will you blow the snow? ... just to another part of the roof? Maybe you can blow it off the sides. I just don't like the idea of something heavy on a roof that is already heavily loaded.

10. Finally, what about the added weight of people. The question is a worthy one. Let me answer here that I think you'll find that if you `dig' your way across deep snow, you will probably be removing enough snow to absorb your weight. Keep everyone at least arms' distance apart, which is a good idea anyway, with shovels.

Oh, on cool thing! ... remember that sag, deflection, etc. that I talked about in Part 1? ... Well, as you remove the snow the beams, joists, whatever, should spring back into more straighter configuration. IF THEY DON'T, then something has `yielded' ... NOT GOOD! Hire an engineer and find out what is going on up there.

I welcome your comments.

References

Thoughts on Mitigation of Snow on Roofs (Part 1), Jeff Filler, Associated Content.

Measurement of Snow Loads on Roofs, Jeff Filler, Associated Content.

Measuring Snow Density, Jeff Filler, Associated Content.