Installation and construction
What is a roof valley, and what keeps it watertight?
In brief
The watertightness of a roof valley is determined above all by its geometry, correct laps and an unobstructed drainage path, not by the amount of additional sealant. It is worth checking whether the substrate gives the valley channel stable support and whether the underlay remains continuous all the way to the eaves. Watch the width of the visible channel: sheets pushed too close restrict outflow and make cleaning harder. Dimensions depend on roof pitch, valley length and the volume of incoming water, so the choice of solution should be left to the designer.
Where on the roof does a valley form?
The roof valley sits in the internal break of the roof slopes. It often occurs with a complex building form, a dormer or the junction of two parts of the building. Water flows into it from both sides and onwards towards the eaves. Leaves, small branches and sliding snow can also accumulate in this zone.
The roof valley must not be confused with a convex corner, which joins the slopes on the outer side of the roof. These places behave differently. The roof valley must be treated as a channel draining water away, not merely a decorative joint between sheets. Its actual length and the catchment area affect the required detail.
What should be underneath the flashing?
The substrate must provide stable support for the valley gutter. How it is constructed follows from the structure and the roofing instructions. Depending on the solution, additional timber elements or boarding may be required. The flashing should not deflect freely between randomly placed supports.
The underlay must remain continuous and drain water towards the eaves. This is important even when the outer flashing appears tight. A torn membrane or a poorly executed lap beneath the roof valley can remain invisible until the first heavy rainfall. This stage is worth photographing before it is covered.
How to choose the width and shape?
There is no single width suitable for all roofs. The pitch of the slopes, the length of the roof valley and the volume of incoming water all matter. It is also important whether both slopes have a similar pitch. With uneven inflow, the flow from one side can act on the opposite edge.
The width of the visible channel also depends on the distance between the cut roofing and the axis of the roof valley. Sheets pushed too close restrict drainage and make cleaning difficult. This space should not be reduced merely to conceal the flashing. The dimension should follow from the instructions and the specific roof geometry.
How are section joints made?
Successive sections of the roof valley must form laps running in the direction of water flow. The lower section receives water from the higher one. The joint must not create a lip on which debris will collect. The size of the lap and any sealing should follow the system used.
The fixing should hold the flashing without unnecessarily piercing the flow zone. At the same time, the thermal movement of the sheet metal must be taken into account. A long element fixed too rigidly may deform. A random screw in the middle of the roof valley channel creates a point requiring sealing in a location of intense water contact.
How is the roofing cut at the roof valley?
The cutting line is determined after checking the geometry of the roof slope. With tile-effect metal roofing, the profiles and the fixing point of the cut piece must be taken into account. A small piece of sheet must not remain without proper support. With trapezoidal steel sheet, the direction of the profile and the finishing of its side edges are also important.
Tools suitable for coated sheet metal are used for cutting. Swarf must be removed from the surface, including from inside the roof valley. Metal particles left in the wet zone can corrode and stain the coating. Seals at the edges are fitted according to the detail, without closing off the required drainage path.
What most often obstructs drainage?
Leaves, moss and cuttings can form a small barrier. The water then rises above the level at which the joint normally works. In winter, ice can cause a similar effect. The roof valley therefore requires inspection even when no damage or soiling is visible on the plain roof slope.
Do not remove debris with a sharp metal tool. The coating can easily be scratched or the edge bent. Cleaning should be carried out from a safe position. If debris returns quickly, it is worth checking whether there is a local depression in the roof valley or a badly shaped joint retaining sediment.
How should a completed roof valley be inspected and accepted?
Check the continuity of the fall, the termination at the eaves and the absence of loose covering fragments. A specialist should assess the support and the laps before they are covered. The appearance of the top surface alone does not confirm that all layers are correctly laid. Photographs of the stages are particularly useful here. The roof valley outlet must not direct a concentrated stream beyond the gutter or under the covering edge. This point is also worth inspecting during a controlled flow test.
In the event of a leak, do not limit the inspection to the visible joint. The problem may start higher up, at a dormer or the flashing of an adjacent roof slope. The repair should restore a clear channel and tight joints. If the roof valley has an incorrect cross-section, local sealing may only help briefly.
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