Closed valleys, Open Valleys, Woven Valleys, When to Use them
Never forget that water usually goes down hill. It’s those exceptions that get the novice in trouble. Perfectly good valley systems fail because the wrong one was used.Most roofs have slopes of the same pitch. Take your pick. Any valley system will work. Open, closed, laced… with the only factor being water quantity. The side with the most water is the top side of a closed valley because the water will shoot over onto the lower side rather than under it. If a California valley, and not the type that Cesar Chavez frequented, is installed, the high side must be cut up and out of the center like one side of an open valley or debris will collect under the roofing. A little know part to most humans is the tips of the high side must be cut, the points left when the valley is trimmed, because torrential water, I like that scenario, will be forced sideways by the pressure of mucho aqua trying to go down hill but being impeded by all the water in front of it. Just think of a Paris with a free wine give-a-way. Some of the frogs will not end up in the pond but will be pushed wherever the laws of fluid dynamics take them. The French don’t have any valleys that I know of anyway.
In Calgary the most common valley is still the closed valley but in the nicer communities, the open metal valley is making a big putsch.
If a pointer is used, one course of material used to make a straight line up the valley so cutting is not necessary, a bulge may occur on the top side. Add that to thicker shingle, with the lapping in the valley and you have a bulge. The Battle of the Bulge did take place in France but it was a forest and not a valley.
This lumping on the high side of the valley may be exacerbated by using a sheet like 90lb. or base sheet instead of metal because it is thicker. These sheets are not stronger or more expensive or better or better looking. They cause bulges.
Metal, metal, metal. There is a lot of water here. Firehouse quantities. Gushes of water cascading down from the valley blasting whatever is below unless a good gutter with a splash guard is present. The splash guard tells the water to walk, not run.
I was sickened when I was searching for more detailed evidence on the internets (go away red squiggly line) on how to figure how much roofing is needed to roof a house. One maniac said the best part of a closed valley is that expensive metal is not required and that saves money. Well if a small branch hits the valley a pokes it where there is a void, we’ll see how much is saved when a strong wind visits your living room. Fifty dollars or a crappy valley, the place where there is more water than at any other point on the roof by a factor of a gazillion. you know that the plywood meeting in the valley is not cabinet grade cut, leaving the famous voids. And, oh my gosh, on plank decks the boards are not mitered. The sharp edge of the 1″x whatever will cut any sheet of asphaltic roofing sheet.
That’s why I recommend an open factory painted “w” valley. It costs more, a lot more, but less shingles are used, the human mistake variable is removed from the installation process, it is more self cleaning, it can flex or expand and contract as the house moves, and it looks great. No bulges. Clean.
Rolls of galvanized tin make good underlayments for closed valley but are not suited for open valleys. Due to movement of the home and temperature changes in the metal, the nailed down sheet must buckle, then it rusts, and finally tears. The “w” valley comes in 10′ sections so it can absorb the thermal shock of sun then cloud, dry foundation then soggy earth. Up and down she goes captain.
If the top slopes causing a valley are open at the base more than 90 degrees then a closed valley is out. If an open valley is used, the metal must be wider than normal because the momentum of the water crossing the valley is not as inhibited. The pitch of the valley rafter is less so the water can be carried further by momentum from the larger rain basin or wind.
Hold your hands like you are praying but upside down and pointed down like a roof valley. If you spread your hands apart at the bottom while keeping your wrists together you’ll see the angle of the slope from your wrist to the bottom lessens. If you go the opposite direction, making the base of your palms separate while keeping your little finger ends touching, you’ll make the valley steeper.
Consequently, if the base of the valley is open less than 90 degrees the valley rafter pitch drops substantially. Laced, or woven, valleys are a bad idea here because the lacing causes more bulging reducing the pitch even more. The only safe way is to use an open valley. Again wider than normal. The water is not hot to run off from gravity because the slope is less but sheer quantity can cause push. These valleys have large but slower flows. Ice and snow are more likely to build up. Don’t forget the angry co occupant. I recommend an underlayment of StormGuard plus more StormGuard on both sides of the valley. We’re dealing with Niagara here guys and it’s no time to fool around trying to save that fifty bucks.
If the valley has a real low slope it has become a Dead Valley, like the one near open valley at the border.
If the valley changes directions because the roof slope changes then it is a new valley. The valley must be installed to the pitch change, terminated, and a new valley installed on top of the lower roof surfaces. If you don’t do this the water might infiltrate beneath the lower valley. Trust me here. When slopes change it’s like the water meets a dam and wants to roll back upon itself. The greater pressure meets the lower pressure of less gravity and a confluence of undecided water meets to convene for a moment. It can be like standing water because water doesn’t know it’s moving. It will take a moment and jostle the molecules around it looking for the easy way out. We know where we don’t that to be, don’t we honey?
Path of least resistance, like the French.
Slate roofs must use copper valleys and even though it’s a hassle to leave it exposed it is better than letting the slate butt up in the valley because debris can build up in the valley. Concrete tile roofs have a double hump w valley so the tile won’t fall into the center and let trash dam the water.
I say we shall fight the water with the best product available for a roof:metal. All valleys are fulcrums. The nexus of a valley is a point of movement, massive water collection, a place for leaves and frozen stuff to sit, and a good place to take a nap on a pretty day.