[Download Entire Issue #6 as a PDF]
Fall/ Winter 2007/2008: Issue
#6: [Download this article as a PDF]
Rounded Slate Valleys
The first time I saw a rounded valley I was in awe. The beauty of the slate appearing to bend around the valley was to me a work of art. Then I had the opportunity to see some rounded valleys being installed at the International Federation for the Roofing Trades World Championship for Young Roofers in Edinburgh, Scotland. Since then I have installed six rounded valleys, four on my own house. Here is what I've learned so far.
The first step to a rounded valley installation is installing a valley board. A valley board is 1 inch in thickness and from 9-12 inches wide and runs up the center of the valley. The purpose of this board is to convert one deep angle into two shallow angles. If the rounded valley is to be installed on a dormer, the boards must be cut to meet each other at the dormer top. This will cause the top of the boards to angle up the roof (see illustration at left). After the valley board is installed, there will be a void under the valley, creating a hole at the bottom. This can be covered by forming a piece of copper or cutting a board to fit.
The next step is to chalk vertical lines running the length of the valley board. The first line needs to be chalked down the center. The other two lines depend on the width of valley slates to be used. I found using slates six to eight inches wide works the best. For example, if you’re using six inch wide valley slates, the other two lines need to be chalked parallel to the center line three inches out on both sides.
After the lines are chalked, the valley is ready to be slated. If you’re slating a valley on a dormer that is part way up the roof you may need to adjust the dormer field slates so the bottom of the first valley slates meet up with the bottom of the main roof field slates. The valley slates should be longer than the field slates to get maximum headlap* and the nail holes need to be punched higher than normal.
Next run the dormer starter slates up to the valley and install a starter slate on the valley board. [See Traditional Roofing #5 at traditionalroofing.com for an article on starter slates.] Continue by placing a valley slate (i.e. slate that lies in the valley) in the center of the valley board with the bottom edge flush with the bottom of the starter slate. Install two more valley slates, one on each side of the center slate. Some of the outer valley slates may need their top corners trimmed to help them lie flat. It may also be necessary to use longer nails on the valley slates.
After that course of valley slates is installed, you can run the field slate up to them. Cut the field slates as needed to meet the valley slates. Install a strip of 16 ounce copper or 20 ounce copper or terne-coated stainless steel step flashing before the next course of slates are nailed down. The copper should cover the top, unexposed portion of the valley slates, extending the width of the valley slates, and extend up the valley slates the distance of the exposure measurement plus 4” (see illustration below). The bottom edge of the copper strip will just be covered by the bottom edge of the overlying valley slates. No copper will be visible after the overlying slates are installed.
The next course of valley slates will need four slates. Start by placing a slate on each side of the center line. Then place one more slate on each side of these slates. Make sure the bottom outer corners of the valley slates meet up with the bottom of that course of field slates. Continue up the valley using these steps till you reach the top. At the top it will be necessary to cut the valley slates to meet one another causing them to sweep up the main roof. Use flashing as needed.
The slope of a slated valley should be no lower than (7:12). The steeper the slope the better. It is not recommended to slate a rounded valley over twenty feet long.
* [Editor’s Note: Traditionally, the valley slates were long enough to have a double headlap so that the head of the valley slate was overlapped by the second course above, as is normally done, but also by the third course above by an inch or two. (See the article on headlap in this issue for more information about overlap.) To calculate double headlap, triple the field slate exposure measurement, then add the 2nd headlap measurement. For example, if you’re using an 18” long field slate, the exposure with a standard 3” headlap in the roof field would be 7.5”. Therefore the valley slates would be 3 X 7.5” or 22.5” plus 1.5” headlap on the third course above, requiring a 24” long valley slate. Valley slates longer that 24” are often impractical, so the field slates must be limited in length to 18” or else flashing must be installed between each course. If flashing is used, the length of the valley slates is not critical.
Brent Ulisky is slate roof mechanic foreman at Joseph Jenkins, Inc.
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