Updated · 3 min read
In short
- The ridge sits off-centre, so one slope is longer than the other.
- Chosen for a taller loft on one side, a large south-facing slope for solar panels, or looks.
- The long slope needs longer rafters, intermediate purlins and a pitch above the covering's minimum.
Two variants
- Equal pitches, offset ridge: one slope runs further down, often over a terrace or extension.
- Different pitches: one steep slope (35–45°) and one shallow (15–25°) — typical for a loft with a tall wall and vertical windows on one side.
Structure
Both variants use the same elements as a normal gable roof: wall plate, rafters on each slope, ridge and, depending on span, intermediate purlins on posts or internal walls. Differences: the long slope's rafters span further and are sized separately; the horizontal thrust differs on each wall; with different pitches the ridge is not centred and posts are positioned by calculation. Sizing is done by a structural engineer using local snow and wind loads — especially on the shallow slope, where snow lies longer.
Pros and cons
- + more usable loft space under the steep side;
- + a long south-facing slope gives a large area for solar panels;
- + can extend over a terrace, entrance or garage without a separate structure;
- − the shallow slope limits the choice of covering;
- − the long slope sends more water to its gutter — size downpipes on its area.
Area and materials
Calculate each slope separately: house length × slope length (plan projection ÷ cosine of the pitch), plus overhang. A 6 m plan projection at 20° is about 6.4 m on the slope; 4 m at 40° is about 5.2 m.
Frequently asked questions
Can I use tiles on the shallow slope?
Only above the minimum pitch the manufacturer declares for the tile model — often around 20–22° for standard clay tiles without extra measures. Below that, choose another model, metal, or an additional underlay as the data sheet specifies.