Valley

Rain that lands on a sloped roof runs down the plane it lands on. Where two planes meet at an inside corner, both send their water toward the same joint. ARMA's Residential Asphalt Roofing Manual puts it this way: “The sloping planes direct water toward the valley, concentrating the drainage along the joint and making it especially vulnerable to leakage.” It is the reason ARMA gives for singling the valley out.

The glossaries define the joint by its angle. The FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual calls a valley “The internal angle formed by the intersection of two sloping roof planes,” and ARMA's glossary entry uses the same words. A hip is the external version of the same meeting, where the two planes fall away from each other and shed water to either side. At a valley the planes lean toward each other and gather it.

How much water a valley carries depends on the size of the two slopes feeding it, and how quickly it arrives depends on how steep they are. That incline is measured as roof pitch or slope, and the shingle and tile manuals do not write it in the same units.

The FRSA-TRI manual names one way a valley is built on a tile roof. Its open valley entry describes “A method of installing roof tiles where tiles are cut mitered and installed so that a gap or trough is created to assist in the flow” of water down the joint.

Two things follow from that design. The tiles along each side of the trough are cut to the angle, and TRI Alliance's Roof Maintenance Guide tells anyone on a tile roof to avoid walking on hips or valleys because those specially cut tiles are more difficult to replace. And a trough built to carry water is also a place where anything else moving down the roof can settle.

Whether the valleys on a particular tile roof are open, what lies under them and what condition they are in are not questions a glossary or a look from the ground can settle. TRI Alliance sends broken tile repair and inspection to a professional roofing contractor, and valley condition belongs there too.

TRI Alliance puts gutters and valleys in the same maintenance sentence: “To ensure the proper flow of water, excessive debris such as leaves and pine needles should be removed from gutters and valleys.” A valley collects debris for the same reason it collects water. Both slopes feed it, and whatever slides or blows down either one can stop in the crease.

Overhanging trees are one source of that debris, and their branches also change how long a roof stays damp. The growth side of that story is told in why a roof gets dirty under the trees.

Debris in a valley is a drainage matter in TRI's sentence, not a stain. Whether a cleaning can itself send water somewhere it should not go is a separate question, and the leak paths the roofing sources actually name, from broken tiles to exposed nails, are laid out under whether roof cleaning can make a roof leak. A leak near a valley, like the valley itself, is a roofing contractor's to trace.