Snow Melt System for Roof

[McKraut]

I am re-roofing our house this summer. One section of our roof has very little slope and every winter when it snows, I have to shovel the snow off of it. There are two reasons why this won't work any more. First, I am putting on a metal roof, and the shovel will scratch it up. The second reason is me. I am not as young as I used to be and I doubt I'll be getting any younger. The thought of shoveling the roof when I'm in my 80's is not pleasant.
I have seen some discussion of using a zone off the boiler to heat sidewalks and driveways, but is it reasonable to apply this to a roof? I would like to run 1/2" PEX under the new metal roof, have it run through pex aluminum heat transfer plates, and running AF through a HE set up as a zone (with a manual switch). To me this seems like a reasonable possibility, but that is just me. Can anyone give me insight as to why this might be a good/bad idea?

As always I appreciate your help!

Bob

 

[ABMax24]

Why does the roof need to be shoveled off in the first place?

 

[McKraut]

Large surface area with insufficient support. We had a heavy wet snow years many ago and it caused the drywall on the ceiling to crack. Since then I have shoveled the roof whenever we get over 1 foot of snow or if we get over 6 inches of snow and then rain.

 

[mark cline]

I would think that melting that much snow would cause problems as far as ice buildup in the gutters or huge icicles. How large of a roof are you talking and what is the pitch?

 

[mark cline]

How high off the ground is the drip edge for this roof? What amount of insulation is in the ceiling of this roof?

 

[ABMax24]

Unfortunately in my opinion the only true fix is to fix the structure itself. Either by replacing with stronger trusses or adding another portion on top to increase the slope. Both of which require consultation with a qualified professional, likely a structural engineer.

I believe a snow melting system to be a band-aid solution, and as noted already could cause issues with water infiltration and ice damming.

Another consideration is in a storm where lots of snow falls and you loose power will the system continue to work? Metal roofs are extremely slippery when covered in snow, making manual removal as a backup difficult.

 

[E Yoder]

Seems like melting off the roof would be fairly simple, the gutter drains would be the problem as was mentioned.

 

[peakbagger]

It’s a matter of “could you versus should you”. Going to a metal roof is going to reduce the amount of snow that will build up on roof, snow will slide off it easier and ice damming is less likely (but still possible). Typically in ice damming situations self-regulating heat tape is used to melt channels for water to flow under the snow piled on the roof at the edges of the roof. This doesn’t typically burn up a lot of wattage as heat escaping the roof of the main house typically is doing most of the melting and the heat tape is just directing the water out over the edge of the roof.

If the space under this shallow roof has no heat source or is well insulated and there isn’t a lot of sun it takes a lot of warm glycol to melt snow. The design BTUs required to melt snow varies widely dependent on outdoor conditions but grabbing a value from Wikipedia I see 70 to 170 BTUs/square foot. This is lot of heat when it’s needed. The standard recommendation for snow removal is the cost to remove it mechanically is usually far less than to remove it by melting. When you melt snow you are melting ice crystals and there is a hidden amount of energy called the latent heat of fusion to convert the ice to water which is 144 BTUs/lb. of snow or ice. You then need to add the difference in temperature between the snow and the 33 degrees and add 1 BTU/lb. of water per degree. That’s for perfect system where all the heat goes into the snow so the result will be higher number. Snow is a pretty good insulator so as long as you turn off the system when the roof is melted you only have to heat it when there is snow on the roof.

Mechanically removing the snow is far easier task. Contrary to your statement, folks do rake metal roofs, they just use a plastic bladed rake and take it easy. As long as there is not an ice dam situation, once the snow gets moving on a Kynar coated metal roof it keeps moving.

I have not seen or had any experience with commercial hydronic roof melting systems as I expect they are quite rare. I don’t think it’s particularly difficult but definitely not inexpensive. I would suggest the approach used with radiant walls be used where the roof structure would be insulated then strips of plywood with standard radiant tubing reflectors would be put between the strips of plywood and then tubing put in place. I would set it up so the tubing runs went vertical to make sure that drainage channels were kept open. Then the metal roof would be installed on top of the tubing making sure to tie any hardware into the plywood. The system would need to be glycol blend. A control system would be needed to detect when the roof surface is less than 32 degrees, and something to detect snow. Sure you can leave it running all the time but consider that when there isn’t snow on the roof you are heating the outdoors. No doubt that the heat required to melt the snow is going to mean a much bigger heating system.

The only time I see ice melt systems is when the heat is free or the alternative to removing snow is too expensive. In most cities, private property owners are required to keep the sidewalks clear but cannot shovel snow into the street. They can melt it and let the water run down a sewer or they have to pay to have it hauled to snow dump. Most of these properties have central utility plant so they have steam or hot water to spare or they just get it off the local district heating system.

I used to have lot of old flat roofed buildings at the plant I worked for in northern NH. The roofs were designed for 5 or 10 pounds per square foot snow load. They didn’t have insulation in the roof and were heated so as long as the roof drains were clear it wasn’t a problem. Unfortunately over the years some of these buildings became surplus and were used for unheated storage. They would lose a building on occasion when we had a heavy snowstorm as the roofs were not designed for the load, It happens all over whenever there is unexpectedly heavy snow storm. The code folks finally decided in most areas that the roofs need to be designed to handle the maximum expected snow load without melting or shoveling. The current snow load at the old mill site is now 90 pounds per square foot and in the town next to me its 110 lbs. per square foot with an elevation modifier that could bump it to 120 psf. If there is low roof adjacent to higher roof there usually is 30% multiplier on top of that for some distance away from the adjoining higher structure. This usually means deeper joist set at closer distances. It’s about the same amount of labor to go with deeper joists so most go with deeper joists.
It pretty well comes down to spend the money to avoid getting into the situation where you need to melt snow!

 

[McKraut]

Thank you for the information and insight. Let me digest this and do some thinking!