What is a valley rafter?
A valley rafter is a diagonal roof timber that runs from the ridge to an internal corner of a roof, where two roof planes meet at a re-entrant angle. In the valley between two roof planes all the rainwater collects — the valley rafter is therefore a critical point for watertightness.
Valley rafter vs. hip rafter
| Valley rafter | Hip rafter | |
|---|---|---|
| Position | Re-entrant corner (internal corner, valley) | Projecting corner (external corner, ridge) |
| Water | Water flows towards it | Water flows away |
| Risk | High (leakage point) | Lower |
The valley rafter is the “valley” of the roof — comparable to a river collecting rainwater from two hillsides.
When do you have a valley rafter?
- L-shaped house — where two roof sections meet
- T-shaped house — multiple valley rafters possible
- Extension onto an existing roof — the junction forms a valley rafter point
Watertightness — the most important concern
Water concentrates at the valley rafter. Defective detailing here will always lead to leaks. Solutions:
- Valley flashing (EPDM or lead) — a wide strip of roofing material in the valley overlapping both roof planes
- Zinc valley gutter — a gutter profile of zinc in the valley
- Lead soaker — for slate or tile roofs, the traditional method
- Cutting roof tiles — trimmed on both sides so they do not block the valley gutter
Calculation
The valley rafter runs diagonally across two roof planes. At a right angle (90°) the valley rafter makes a 45° angle to the eaves. The exact length and angle depend on the roof pitch.
Dimensions
| Roof area | Valley rafter (indication) |
|---|---|
| Small roof (up to 30 m²) | 75 × 150 mm |
| Medium roof (30-80 m²) | 100 × 200 mm |
| Large roof (>80 m²) | 120 × 220 mm |
Common mistakes
- Valley gutter too narrow — water overflows in heavy rain
- Roof tiles overlapping too far — water backs up behind the tiles
- Valley flashing not wide enough — minimum 600 mm total width (300 mm per plane)
Related terms
- Hip rafter
- Roof structure
- Ridge purlin
- EPDM
- Roof gutter
- Hip roof