Open and Closed Valleys with Shingles

Valleys form in nature where hills slope towards each other and merge. Two flows  of water gather at this point, and find their way further as a stream or a river. Drainage at roof valleys follows the same principles.

Waterproofing solutions at this point fall into two categories, namely open and closed valleys. These are one of the major causes of water leaks if done incorrectly. That’s because rain always follows the shortest available route to the ground.


However, Open and Closed Valleys Are More than Just Drains


A stream follows the contours unless it finds a way into a cavern below. We should not have this problem with an open stormwater channel in the garden if we waterproof it properly.

Much the same applies with open and closed valleys if we seal them correctly. However, if this defence fails, the water will find its way into the roof space below, and could potentially damage the contents of the house.


Two Ways to Waterproof Valleys on a Roof


In both instances we begin by installing an underlayment over the entire roof including the valleys. This is our first defence against water. Then we add a heavier-gauge shield across the valley to make it even more secure. After that the method of finishing off open and closed valleys diverges:

# In the case of a closed valley, the roofer installs the asphalt shingles across the secondary underlayment to form the closed valley

# However, in the case of an open valley we first install a specially shaped metal lining. Then we lay the shingles slightly over the edge to leave the valley open.

We prefer the latter method because shingle edges are sharp when they snap off, and could tear the underlayment damaging the waterproofing. We attended to a case recently where a contractor stood on top of a closed valley. You can guess what happened the next time it rained.


Two More Considerations Before You Decide


There is a saving on materials with a closed valley, because it does not require the specially shaped metal lining. However many clients prefer the cleaner line between roof sections an open valley provides. That said, an open valley may trap more debris than a closed one.

The right choice is the one that works for you according to your particular circumstances, although we prefer the added confidence an open valley provides. Call Valiant Exterior Solutions on 403-829-1661 in Calgary for more advice on waterproofing your home.

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