Roof shape
What is a gambrel roof?
The barn roof: a gable that bends partway down each side to make room underneath, and the joint in the middle of each slope that needs watching.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionShapeAlso called: barn roof, Dutch colonial roof
Gambrel roof — a roof of two sides, each side having two pitches: a steep lower slope and a shallower upper slope meeting at a ridge, the change of pitch running the length of the roof; the NRCA glossary records the term, and the shape is the gable-ended counterpart of the four-sided mansard.
What it is
Picture a gable roof whose two slopes have each been broken into two, the lower half tilted nearly vertical and the upper half laid back toward the ridge. The result encloses much more of the space under the roof than a gable of the same height — a hay loft in a barn, a full second storey in a Dutch colonial house — while keeping the gable ends and the simple two-sided plan. The line where the pitch changes is the break or purlin line, and in traditional framing a purlin or a row of struts carries the rafters there.
The two slopes are two different roofs in the code's eyes. The steep lower slope sheds water fast and can be covered with almost anything; the upper slope is often shallow enough that the IRC's slope rules in Section R905 require double underlayment under shingles (between two and four units vertical in twelve) or a low-slope system altogether if it is shallower than that.
Why it matters
The break is the gambrel's weak point. Water and snow sliding down the shallow upper slope hit the change of angle; if the shingles are not lapped and the flashing not run across that line properly, water is driven under the lower course, and snow tends to bank there. Dormers, which gambrel roofs usually have on the steep lower slope, add valleys and sidewalls to the same area. A well-built gambrel carries a strip of self-adhering membrane along the break and a metal transition flashing under the upper course.
Structurally the shape has no gable-end weakness a gable roof does not have, and the broad, steep lower slopes are strong against uplift; the IBHS wind research on gable ends applies to a gambrel's ends in the same way. The upper slope, being shallow, is the surface to check after hail.
What it costs and who does it
Re-roofing a gambrel is priced by the square with two labour rates in effect: the lower slope is steep, and steep work is slower and needs more staging, while the upper slope is easy to walk but may need a different underlayment or system. The break flashing and the dormer details are line items. Roofing contractors do the work; on a barn-style outbuilding with metal roofing, a metal-roof specialist may be the better fit. The directory's roofing squares calculator handles the two-pitch geometry, and its guide to a new roof installation covers the sequence.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
What is the difference between a gambrel and a mansard roof?
Why do gambrel roofs leak?
Can the upper slope of a gambrel have shingles?
Next step
Find a roofer who does this
A gambrel is two roofs in one, and the estimate should show it. The directory lists roofing contractors by state and city.
Sources
- Roofing glossary — National Roofing Contractors Association (NRCA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)
- Wind research — Insurance Institute for Business & Home Safety (IBHS)