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Discussion Forum

Roof Valleys in snow country

Shaken_not_Stirred | Posted in General Discussion on February 15, 2006 10:24am

If planning to build in an area of relatively heavy snow fall, are the number and length of roof valleys a consideration for the design?  Not so heavy as to really require a tin roof to get the snow off the roof.   Clearly valleys don’t allow snow to slide off the roof well.  But if you get snow loads in the 60 to 100 lb area, are there reasons to be concerned about valleys?

They say valleys are “weak” points in a roof system.  I can see that a lot of water enters this area and therefore could possibly be an issue.  However, if you can keep people from walking up the valley a good valley with tin flashing should be stable.  NO?  And if the house is insulated correctly ice damns should not be an issue in the valleys.  So are there any real reasons to be concerned about valleys in a heavy snow area?

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  1. MikeCallahan | Feb 15, 2006 11:02pm | #1

    Valleys will hold snow longer than the rest of the roof. They can cause severe uneven loading on metal roofs. Metal roofs are NOT recommended. The weight of the snow will crush the metal ribs and bend over any other protrusions like jacks, murphy splitters and chimneys unless they are at the ridge. The conventional wisdom in the Tahoe area is 50 year comp on the whole roof. Metal is so 80s. We know better now. Do not even use W metal in the valley. Valleys are always lined with ice membrane three feet each side. Most roofs are engineered to hold the snow. You do not want snow sliding off and impacting Grandma or your dog. Gutters will slough off from ice and are generally not installed. Entries should be on gable ends. Consider where the drip line will be before planning decks and entries.

    Mike Callahan, Lake Tahoe, Ca.
    1. Shaken_not_Stirred | Feb 15, 2006 11:20pm | #2

      Excellent post Mike!  Nice to hear from someone from Tahoe.  Actually will be visiting next week!

      Anyway, I was not considering metal.  I agree, I don't need it sliding off and a valley would be mess.  But I was wondering if, at all possible, to design a home with no valleys.  You don't seem concerned about them if don't right, like everything else (LOL!)

      But you did say not to use even W metal in valleys.  Why?  And what roofing method is used in valleys?  Lap across one way and cut the valley on the ohter side?  (Is that called a cut valley?)  There is the woven valley, but that seems difficult for steep pitch roof valleys.

      1. MikeCallahan | Feb 16, 2006 06:52pm | #3

        I guess you would call it a "Cut Valley". One side is lapped up the other side and the return side is cut along the valley trough. I like the way it looks if the line is straight. I have seen W metal fail from being crushed.
        To avoid valleys you could use shed dormers. Valleys are hard to avoid if you build anything outside a box with two gable ends. I see no reason to avoid valleys altogether. As long as they are done right then they should hold up as well as the rest of the roof.Mike Callahan, Lake Tahoe, Ca.

        1. Shaken_not_Stirred | Feb 16, 2006 08:35pm | #5

          THAT'S the name I was looking for! Cut Valley. I used them on my current house. Looks nice. But do teh shingles wear faster than the rest of teh roof due to the major flow of water down the valley during rain and snow melt? I always thought tin in the valley (W or just flat) was a good idea.Secondly, do they put tin under a cut valley or just the ice shield?

          1. Shaken_not_Stirred | Feb 16, 2006 08:38pm | #7

            Great discussion guys! Thanks for the thoughts.

  2. DanH | Feb 16, 2006 07:10pm | #4

    You can get ice damming in valleys even with a properly insulated roof. The complex roof shape means that some areas are in the sun and some in the shade. Quite often the valley is shaded and the area above is in the sun. So snow will melt above and run down into the shaded area.

    With the proper conditions (temp about 20-25 and lots of sun) an ice dam will form and can dam up a lot of water, easily bypassing the typical metal flashing. Generally, ice shield run up the valley sides 2-3 feet will be sufficient to protect against leaks, but remember I said "generally".

    If ignorance is bliss why aren't more people

    happy?

    1. Shaken_not_Stirred | Feb 16, 2006 08:37pm | #6

      I suppose if you have a valley as you describe that is prone to damning you could lay one of those electric cable right down in the bottom of the valley to keep the valley open (for the flow of water).

      1. DanH | Feb 16, 2006 08:39pm | #8

        Yeah, though the cables are ugly and need to be replaced every 3 years or so.

        If ignorance is bliss why aren't more people

        happy?

        1. Shaken_not_Stirred | Feb 16, 2006 08:44pm | #9

          Probably cost you a pretty penny to keep hot!As for looking good, one cable straight up a valley might not look too bad compared to teh Zig Zag run along a eave, YUCK. Agreed.

          1. Piffin | Feb 17, 2006 01:49am | #10

            What you are calling a cut valley is what I would call a lapped cvalley. There are several ways to runa valley that requires cutting the shingles so that name might only be confusing to your roofer. An open metal valley gets cut on both sides.
            That lapped valley gets cut on one side,
            A closed metal valley gets cut on both sides.
            A woven valley gets no cuts.
            A stepped valley gets cut both sidesI ALWAYS use metal in the valley even when I have to lap or weave it.
            And I ALWAYS back it up with ice and water shield at least six feet wide in heavy snow country.I hate woven or lapped valleys and only use them when necessary.
            Reasons - The extra material always makes a shadow bulge that can be seen from the ground, more so in certain light conditions.
            _ the second reason is that archy shingles are laminated and can be easily broken when rolling through the valley at the lamination
            _ thirdly, if you are not experience at cutting the shingles, there is good chjance you will end up wioth a pokehole or two from cutting a lapped valley
            _ fourth is that the volumn of water running thru a valley can make certain that this is where the surface granules will wear off fastest and cause you to have a keak before the rook has had its full lifeThe main reason I can think of for lapping or weaving a valley is that a shingle layer being paid by the square wil get more squares of materials used on the same roof for about the same or less work than iof he were using an open metal valley.You mention tin. I have not seen a tin valley since I tore off roofs that had been laid before WWII. Use heavy coil stock AL with colour anodized, or copper. Heavy 26GA galvanized is OK in some places too. Light AL is a waste of time except for backing under a woven or lapped valley. It will corrode and wear thin or break long before the 50 year shingles are cooked. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          2. Shaken_not_Stirred | Feb 28, 2006 12:13am | #12

            Sorry Piffin, I just called it tin. Could not think of what it really was made of. However you say Aluminum? I think/ thought most of teh stuff was some form of galvanized something (sheet steel?). Now the W stuff I would guess is aluminum, but teh sheet stock is steel (no?).

          3. Piffin | Feb 28, 2006 12:28am | #13

            You can have it whatever kind of metal you want. I can buy rolled copper, rolled galvanized or rolled coil stock aL in plain or coloured.I can buy sheets of each 8' or 10' long ( or my sheetmetal guy can, depending on his brake size) and trhen rip it and bend it to the W formIn certain areas there are ceratin metals more commonly used than in others, sometimes with good reason, and sometimes only guided by economy of cost. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          4. Shaken_not_Stirred | Feb 28, 2006 07:20pm | #14

            Thanks Piffin.

          5. DanH | Feb 17, 2006 04:01am | #11

            Yeah, that's another problem. You only need the cables turned on between roughly 20 and 30F, and then only when there's snow on the roof. The time of day also factors in. There's no sort of thermostat or controller that will do a halfway decent job of working this out, so you end up having to turn them on and off manually.

            If ignorance is bliss why aren't more people

            happy?

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