Roof styles, drawn and compared

Two planes, one ridge, a triangular wall at each end. It is the most common roof in North America because it is the cheapest way to get good drainage and usable attic volume out of simple, repeatable cuts.

How it sheds water. Straight off two faces, no valleys, nothing to dam. This is the best-draining roof shape there is.

What it costs to frame. The baseline everything else is measured against. Every common rafter on a symmetrical gable is the same length and the same pair of cuts, so one pattern rafter produces the whole roof. Trusses make it cheaper still.

Where it fails. The gable end wall. In high wind the end wall takes the pressure as a flat sail, and older houses often have inadequate bracing behind it — gable end failure is one of the classic hurricane loss patterns, which is why retrofit programs pay to brace them. The other weak point is the rake edge, where wind gets under the overhang and peels.

Put a gable on a house in a low-to-moderate wind region and stop thinking about it. In a coastal wind zone, brace the ends properly or choose a hip.

All four sides slope, meeting at hips and often a short central ridge. No vertical end walls.

How it sheds water. Well, but with hips and — on any L-shaped plan — valleys, which are the parts that leak when flashed badly.

What it costs to frame. Meaningfully more than a gable. Hip rafters run at a compound angle, jack rafters step down in length along each hip, and the cutting is skilled work rather than repetition. Add 10–20% to the framing labor and 10–15% to the roofing waste, since every jack cut leaves an offcut.

Where it fails. The valleys and the hip ridge caps. Also the attic: a hip roof gives you much less usable volume and much less room for ventilation, and hip roofs with no ridge to vent need a different exhaust strategy.

The trade you are making is money for wind performance. A hip roof presents a sloped face to wind from every direction and has no gable wall to blow in, and insurers in Florida give a specific wind mitigation credit for it. On the coast that credit is worth real money every year, and I'd pay the framing premium. Inland, a gable is the better value.

Two pitches per side — shallow above, steep below — meeting at a break line. The barn roof, and the Dutch Colonial roof.

How it sheds water. The steep lower slope drains beautifully. The shallow upper does not, and it is where snow sits and where leaks start.

What it costs to frame. More than a gable, less than a mansard. The structure needs a purlin plate or a collar at the break where the two slopes meet, because the change of angle wants to push the walls out. Site-built gambrels frequently need engineering; the manufactured gambrel truss solved this and is why you see so many on garages.

Where it fails. The break line itself. Two roof planes meeting at a shallow angle change is a detail that has to be flashed and lapped correctly, and it is often just shingled over. Watch for it on any gambrel over thirty years old.

The reason to build one is floor area. You get most of a second storey inside the roof, at roof prices rather than wall prices, which on a detached garage or a small house is a genuinely good deal.

A gambrel taken all the way round the building: steep on all four elevations, near-flat on top, dormers punched into the steep face.

How it sheds water. The steep faces are fine. The flat top needs a membrane roof and its own drainage, so a mansard is really two roofs on one building, with two different failure modes and two different trades.

What it costs to frame. The most expensive shape here after a genuinely complex custom roof. Framing, dormer construction, two roofing systems, and often a structural change of direction at the top.

Where it fails. The junction between the steep slope and the flat deck, and every dormer. Each dormer is four flashings and a small roof of its own. Mansards leak at dormers more than anywhere else.

The reason people build them is a full extra storey with full headroom, historically because of how floors were taxed and now because of how zoning height limits are measured. If you do not need the storey, this shape costs a great deal for its looks.

One plane, sloping one direction. High wall on one side, low on the other.

How it sheds water. Everything goes one way, to one gutter, which you should size generously because it is taking the entire roof.

What it costs to frame. Cheapest of the lot. Every rafter is identical, there is no ridge, no valleys, no hips. It is the roof on nearly every addition, porch and outbuilding for exactly that reason, and it has become fashionable on modern houses because it looks deliberate.

Where it fails. The high wall. Where a shed roof abuts a taller wall you need proper step flashing and counter flashing let into the cladding, and the head-wall detail at the top of the slope is the single most-botched detail in residential roofing. Second problem: the tall side of a shed roof catches wind like a sail, so uplift connections matter.

For an addition or a garage, I'd choose a shed roof nearly every time — assuming the pitch clears the covering's minimum, which for asphalt shingles means 2/12 absolute minimum with doubled underlayment and 4/12 to do it without special measures.

Not flat. Code requires a minimum of ¼ inch of fall per foot toward drains, and roofs built dead level pond, and ponding is what kills membranes.

How it sheds water. Slowly and reluctantly, through internal drains or scuppers, which means the drains must be kept clear or the roof becomes a pool. A blocked drain on a parapeted flat roof is a structural load problem, not just a leak: water weighs about 5.2 lbs per square foot per inch of depth.

What it costs to frame. Cheap to frame, more expensive to cover. Joists rather than rafters, no complicated angles, but the covering is a single-ply membrane (TPO, EPDM, PVC) or modified bitumen installed by a commercial-style roofer, and tapered insulation to create the fall adds cost quickly.

Where it fails. Penetrations, seams and the parapet. Every HVAC curb, vent and drain is a detail. Seams age. The parapet cap is a favourite entry point for water because nobody looks at it.

It suits a modern house, a rooftop deck, or rooftop mechanical equipment. It is not a shape to choose in heavy snow country without engineering the load properly, and it is not a shape for anybody who does not intend to inspect a roof twice a year.

Two planes sloping inward to a central valley — a V rather than a peak. Mid-century modern's signature, and back in fashion.

How it sheds water. Into the middle of the building, and then out through an internal drain or a single scupper. That sentence contains the entire risk. Everything the roof collects converges on one point, inside the envelope, above your living space.

What it costs to frame. Comparable to a shed roof doubled, plus the structure to carry the central beam, plus an engineered drainage design. Not cheap, and not a shape to improvise.

Where it fails. The valley and its drain. A butterfly roof with a leaf-blocked central drain in a heavy rain is the worst-case version of every flat roof problem, because there is no overflow path unless one was designed. Any butterfly roof should have a secondary overflow scupper, and if yours doesn't, add one.

It is a genuinely good shape for two things: catching rainwater deliberately, and pushing tall glass up on the outside walls for light. It is a bad shape in snow country and a bad shape on a house nobody will maintain.

Most of the time the decision is already made — you are re-roofing what is there, and the shape is not up for discussion. Where it is, the reasoning is short.

Building in a hurricane or high-wind region: hip, or a gable with properly braced ends. The insurance credit alone often justifies it.

Building an addition, garage or porch: shed. Simplest, cheapest, fewest details to get wrong.

Need habitable space inside the roof: gambrel if it is a simple rectangle, mansard if you need the full storey on all four sides and can afford the dormers.

Modern architecture with rooftop use: flat, done properly, with tapered insulation and two drainage paths.

Everything else: gable. It is the most common shape because it is the correct answer more often than any of the others.

Whatever the shape, the roof's real cost is driven by how many planes intersect. Every valley, hip and dormer is hand work, hand flashing, and a candidate for the leak you get in year eight. Complexity is what you are buying when you move away from a gable, and it is worth being deliberate about it.