Roof Thoughts

| 10 Comments

Here in Vermont we really see it all when it comes to what rains down from the sky and onto our roofs. Rain, snow, sleet, freezing rain, frogs, tree limbs. It seems that the severity of what our roofs deal with has increased in recent years. This winter I have observed lots of snow rakes in action.

This is when the homeowner is concerned about too much snow on the roof. The concern may be the weight of the snow or ice dams. Alex Wilson wrote an article in EBN as well as in the local paper titled “Drive by Energy Audits” the title says it all.

Traditionally metal or slate roofs with a steep enough pitch shed snow. Often all at once with a seismic whump. In the past few years we have seen more freezing rain which turns the metal or slate into sandpaper which then holds the snow, allowing it to build up to dangerous levels. Valleys and dormers exacurbate the problem by allowing more snow to build up in an uneven loading situation and creating areas where heat can migrate out from inside, melting snow which then flows downhill until it refreezes again - Ice dams.

This brings me to the discussion of what constitutes a problem free roof. A large shallow pitch roof (1 to 5 in 12) with an EPDM membrane or slippery metal can be problematic. below a certain pitch, shingles are no longer an option and slippery materials really do not have enough pitch to shed consistently allowing snow to build up to dangerous levels. Dangerous in terms of what happens when the snow does let go.

Steeper pitched metal roofs tend to let snow slide more often, preventing a serious buildup – unless a valley or dormer prevents this from happening.

With modern EPDM or PVC roofing I am starting to think that flat (1/4″ per foot to 1″ per foot pitch) roofs with little or no over hang represent the best long term low maintenance option in our climate. I expect to get jumped on here) Let me explain: Any new roof is required to be structurally designed to hold a large amount of snow (snow load) as specified in local codes even if it is a steeply pitched metal roof. So no new roof should have to be shoveled or raked because of loading issues. The other issue is ice. A flat roof allows for a simplified structure with less opportunity for weak areas of insulation due to thermal bridging or difficult to insulate areas where heat can melt snow. There is also less opportunity for snow to build up unevenly. There is also less likelihood for snow to slide – It can just stay up there until spring when it melts fairly evenly. I also like the idea that a membrane is one large piece of material with a long life span if well treated (this means don’t go up there and walk around especially with a shovel) Whereas all the other roofing materials are made up of hundreds of seams representing hundreds of opportunities for water to get in. If a flat roof is not an option just try to keep it simple – design a fairly steep pitch, avoid valleys and dormers, pay close attention to air sealing details and insulate to r60 consistently across the whole roof. A technique I use on the sidewall of dormers is to treat it as roof not wall in terms of the amount of insulation. I add rigid foam to the sidewalls of dormers to lessen snowmelt there. Lower shallow pitched roofs such as a porch roof need to be designed to hold much more snow than code. This is addressed in engineering manuals and code books but are often ignored by the average builder.

10 Comments

  1. Hi,

    I read this post, and I am about to buy an old building with a flat roof and center drain. What keeps the drain from freezing in the winter?
    I read that it is heat from the center chimney-but I am thinking of changing from oil to direct vent propane-so there won’t be chimney heat anymore.

    Thanks!

    Lori

  2. I’m not a big fan of interior drains. If there is excess heat it certainly can keep things from freezing. Heat tape around the center drain for a cheap solution?

  3. Hello,
    I would be interested in reading / hearing more about this topic, specifically flat (EPDM) cold roof design in Vermont. I am researching the wee house design by Alchemy Architects and the roof design is currently what keeps me up at night. Do you think having this type of roof on a modular design would be ok? Im thinking about the joint between the two pieces… Any thoughts would be great!
    Thanks – and enjoy the snow!
    Corey

  4. My concern is usually more about penetrations such as plumbing vents. If the roof is engineered properly for the snow loads, has r60 or so insulation to minimize heat loss causing melting then icing it should be good. Also, plan for how things drain off – keep it away from the siding. It all boils down to simplicity. you can also look at a modular green roof design at some point. or if there is a low parapet, stone ballast.

  5. Hi Bob,

    Any thoughts on Solar Panel placement on a flat roof? On or Off solar trackers. Would one assume that the solar panels melting the snow would be bad. Would extra drainage/gutters help?

    Also,

    How does a parapet/cornice help hurt the situation?

    Thanks,

    Cathy

  6. For solar panels, the usual is to mount non-tracking. anchoring is an issue (how to minimally attach?) melting snow shouldn’t be too much of an issue if the roof is done well. A parapet allows for ballast (small stones) or protection for a green roof or even protection for the panels themselves. The biggest design issue would be to provide enough pitch – no flat roof should be truly flat – and a clear drainage path for water. It would be ideal if the water can drain via a scupper away from the siding of the building and in a manner to minimize splashing.

  7. Thank you for answering so quickly!

    And I would also assume that skylights are not recommended, as both points of weakness and leakage?

  8. I’m not always so quick! for skylights, I would try to elevate them as much as possible above the plane of the roof – think boxes. Better flashing details that way.

  9. “With modern EPDM or PVC roofing I am starting to think that flat (1/4? per foot to 1? per foot pitch) roofs with little or no over hang represent the best long term low maintenance option in our climate. ”

    Why minimize the overhang?

  10. @ Bob D – I guess it would depend on what you are trying to achieve with the overhang. I would probably say that in most cases an overhang is good. – shade, protection for siding and windows etc. Recently, did a house with siding materials and windows that really needed no protection and shading wasn’t an issue so I went for the simplicity of no overhangs. (actually it was the client the convinced me to do it)

Leave a Reply

Required fields are marked *.


*