Valley rafter
The diagonal rafter at an internal roof corner where two slopes meet, such as a wing or dormer joining the main roof. It collects water from both slopes.
A valley rafter is the diagonal rafter that runs along an internal corner of a roof, the line where two roof slopes meet and form a úžľabie, or valley. It is the inward-facing counterpart of the hip rafter, which sits at an external corner. Typical cases are a side wing that joins a main roof, a dormer, or a garage roof built against the house. Like any rafter, it is a sloping structural member, and it carries the shorter rafters that meet it from both slopes.
What does a valley rafter carry, and where does it sit?
The valley rafter rises from the inside corner of the wall plates at the eaves toward the point where the two ridges meet. Its job is structural: the short rafters from each slope bear on it, and it passes their load down to the walls. It is therefore a more heavily loaded member than a common rafter, and the engineer checks it as such.
Keep the two jobs apart in your head. The timber carries the load, and the lining above it carries the water. They fail in different ways, and a sound valley rafter does not make a valley watertight.
Why is the valley the most leak-prone line of a pitched roof?
A valley collects water from two slopes and concentrates it into one channel. A ridge or an eave sheds water away from the building, but a valley receives everything that runs off both roof planes, along with leaves, grit and, in winter, snow. Any gap in the covering, the lining or the underlay along that line lets water run down the rafter and into the structure, usually out of sight until the damage is serious.
The geometry works against it as well. The covering has to be cut to fit the angle, and every cut is an edge that water can find. The line is also hard to reach once the roof is finished, so faults often show up only after a wet spell, when staining has already reached the ceiling.
How does a valley rafter differ from a hip rafter?
Both are diagonal rafters at a corner of the roof, which is why people confuse them. They often appear together on a hip roof with wings. The difference is which corner they sit on. A hip rafter sits at an external corner and sends water away from the building. A valley rafter sits at an internal corner and gathers water into one channel.
| Feature | Valley rafter | Hip rafter |
|---|---|---|
| Corner type | Internal (concave) | External (convex) |
| Water flow | Collects from both slopes | Sheds away from the building |
| Snow behaviour | Drifts and builds up | Tends to slide off |
| Covering detail | Lining or channel laid under the covering | Overlapping covering, finished with hip tiles or a hip cap |
| Leak risk | Highest of the two | Lower, but still a joint to detail |
How is a valley lined, and which materials are used?
The valley is protected by a lining that runs the full length of the corner and overlaps the covering on both sides. There are two common approaches. An open valley shows a metal channel between the two slopes. A closed valley cuts the covering to meet at the line and carries the water in a concealed channel or membrane beneath it. The choice depends on the covering, the pitch, and how the roof will look and be maintained.
| Option | How it works | Strengths | Weak points |
|---|---|---|---|
| Metal open valley | Formed sheet channel, fixed over the underlay and under the covering edges | Durable, shows the water path clearly, suits complex roofs | Needs correct forming, joints and allowance for movement; cost rises with length |
| Concealed metal channel | Lining laid under a closed valley, with the covering cut to meet it | Clean appearance | Hard to inspect; cutting along the line adds risk |
| Self-adhesive membrane strip | Bonded strip laid over the underlay along the valley | Continuous seal with few joints | Needs clean, dry surfaces and suitable temperature at fitting; must be protected from sunlight where exposed |
Zinc, copper, aluminium and coated steel are the usual metals for the channel. Metals must suit the covering and the fixings, because contact between dissimilar metals in wet conditions can corrode the less noble one. The designer and roofer should agree the lining before work starts, not when the covering is already on.
What role does the roof underlay play in a valley?
The underlay is the second line of defence, and it must run continuously through the valley. The lining or membrane goes over it so that water runs down the channel rather than beneath the layers. Overlaps must point the right way, because an underlay lapped the wrong way round directs water into the structure, however good the metal above it. The underlay also manages vapour in the roof build-up, so its type should match the roof design. See the article on roof underlay membrane for the wider choice.
Why does snow collect in a valley?
Snow slides down both slopes toward the same line, and wind carries more of it across the roof. The result is drifting: snow piles up in the valley and creates a local load well above the even load on the rest of the roof. Melt and refreeze at the edge of that build-up can also push water back under the covering. The general method is described under snow load, but the valley case needs its own check by the engineer, because a rule of thumb for an even roof does not capture drifting.
Why do roofs with many valleys cost more and leak more?
Every valley is a concentration of work. It needs a rafter cut into the structure, a lining run, extra flashing, and two covering edges sealed against it. A roof with several wings and dormers has many of these points, so it needs more labour, more material and more inspection. Each extra junction is one more place where a small mistake can turn into a leak.
The cost is not only in materials. Complex geometry means more detailed drawings, more site measurement and more time on the roof. Simpler plans are easier to keep dry, and a designer who understands this can reduce the number of valleys before the roof is built.
Where do valleys fail in practice?
Most valley leaks come from a few repeated faults:
- Lining laid over the covering in the wrong order, so water runs underneath it.
- Leaves, moss or grit blocking the channel, so water backs up under the tiles.
- Underlay lapped the wrong way, or torn where the lining is fixed.
- Lengths of metal left unsupported or bent sharply, which open at the joints as the roof moves.
- Fixings or joints placed where water flows, giving it a path under the sheet.
Most of these faults can be prevented at the design stage and checked on site before the covering goes on.
Frequently asked questions
- Is a valley rafter the same as a valley gutter?
- No. The valley rafter is the structural timber member under the internal corner, while the valley gutter, or lining, is the metal or membrane channel that carries the water. The two work together but are separate parts of the roof, and each can fail on its own.
- Why does a tiled roof still need a lining in the valley?
- Tiles overlap well on flat planes, but where two slopes meet at an angle they cannot form a watertight line on their own. The lining takes the water that runs down the valley and keeps it away from the timber. Skipping it is a design risk that the designer should weigh explicitly.
- Can a valley be built without an exposed metal lining?
- Yes, a closed valley hides the lining beneath a cut covering. A continuous waterproof layer is still needed under the covering, and the choice depends on the covering material and the pitch. It is a design decision to settle with the designer and roofer before the roof is laid.
- Does a valley add much to the cost of a roof?
- It adds labour and material, because each valley needs cutting, lining and flashing. The increase depends on the number of valleys, the roof geometry and the metal or membrane chosen, so it should be priced from a detailed design rather than estimated per metre.
- Who checks a valley rafter for snow load?
- The structural engineer does, using the local ground snow load and the roof geometry. Drifting in a valley can exceed the even load on the rest of the roof, so the calculation should include the valley case explicitly.
- How can I tell if a valley is leaking?
- Look for staining on the rafter or the ceiling below the valley line after heavy rain, and inspect the lining for lifted seams, corrosion or debris. A leak that shows only in wet weather often starts at a joint in the lining or at a cut in the covering.