Roof shapes compared: which roof suits an extension or outbuilding?

Flat, mono-pitch, gable or hip roof – the roof shape determines the look, cost and build effort of your project more than any other component. This comparison sets out the strengths and weaknesses of the four common shapes, realistic cost ranges and the minimum pitches for the main coverings.

Why the roof shape decides cost and effort

On an extension or outbuilding, the roof quickly swallows a quarter to a third of the total build cost – as a rough guide. The roof shape sets how much timber goes into the frame, which covering is even permitted, where the rainwater runs and whether you can later use the surface for solar PV or as a roof terrace. Then there is the look: an extension feels coherent when its roof picks up the lines of the main building – either through the same shape or through a deliberate contrast, such as an understated flat roof on a classic gable-roofed house. So before you work out timber sizes or order any wood, it pays to compare the four common shapes systematically.

The four roof shapes in detail

Flat roof: modern and usable, but sealing-critical

A flat roof is never truly flat: it needs at least 2 per cent of fall (around 1.1°), and preferably 5 per cent, so that water drains reliably and no puddles are left standing. The build-up is simple: a run of joists (e.g. 8 × 20 cm at 60–70 cm centres over a 4 m span), a deck of OSB or boards on top, and above that the waterproofing in EPDM membrane or two-layer bitumen felt. Advantages: the lowest build-up height, a clean modern form, and the option to use it as a terrace or green roof. Disadvantages: the waterproofing is the only thing between timber and weather – workmanship errors come back to bite you, and every few years the junctions need checking. Snow stays put completely and has to be allowed for structurally.

Mono-pitch roof: the pragmatic choice

The mono-pitch roof (also called a lean-to or pent roof) has a single sloping surface, typically 5–20°. Structurally it is the simplest "proper" roof: the rafters sit on two walls of different height, or on two purlins, and that is it. Advantages: little material, quick to assemble, the high side leaves room for a run of windows, and a south-facing surface is ideal for solar PV. Disadvantages: all the drainage runs off a single eaves (size the gutter generously), and at very shallow pitches the choice of covering shrinks to metal and sheet materials.

Gable roof: the proven classic

Two equally pitched surfaces meet at the ridge – 25–45° is usual. Advantages: a construction proven over centuries, virtually any covering possible, rain and snow shed reliably, and storage space or even a small loft appears under the ridge. Disadvantages: more height than a mono-pitch or flat roof – with boundary building and limited wall height this can become a problem – and a frame with a ridge, two eaves and gable triangles is more involved than a mono-pitch roof.

Hip roof: prestigious and wind-stable

On a hip roof all four sides are pitched and the gables disappear. Usual pitches are 20–35°. Advantages: a very small surface for the wind to attack (worth having in exposed locations), the roof protects all four elevations, and the look is regarded as high-quality. Disadvantages: the frame is the most expensive of the group – hip and valley rafters call for compound (jack-rafter) cuts, the covering produces noticeably more offcut waste along the hips, and barely any usable roof space is left. For small outbuildings a hip roof is rarely economical; for a prestigious extension it can be the right choice.

Cost and effort compared

The ranges below are guide values for the roof frame, sheathing and a simple covering, assuming you do most of the work yourself, and excluding insulation and interior fit-out. Regional prices and tradespeople's rates can run well above these.

*Guide values incl. material for the structure and a simple covering, at typical builders' merchant and timber-yard prices; excluding drainage, insulation and scaffolding.

Minimum roof pitch by covering

Every covering has a recommended minimum pitch below which it is no longer considered rainproof. Go below it and you need extra measures such as a waterproof secondary roof – or simply a different covering. The values below are common guide figures; the manufacturer's data and the roofing trade's technical rules are always what count.

Which roof shape suits which project?

• Extension to the house: a flat roof or a shallow mono-pitch at 5–10°, because the roof junction usually has to stay below the upstairs windows. Measure the connection height at the house wall precisely beforehand.
• Carport: a flat roof at 3–5° fall or a mono-pitch with trapezoidal sheeting – light, cheap and quick to erect.
• Garden room and tool shed: a gable roof at 25–30° looks classic and creates storage under the ridge; a mono-pitch, by contrast, saves material and working time.
• Garage: a flat roof (a green one is nice) or a gable roof that picks up the pitch of the house.
• Workshop or studio: a mono-pitch with a tall north wall and a run of windows – even light without direct sun.
• Exposed, windy location: a hip roof or a shallow-pitched gable roof; both give the wind little to attack.

Structure and timber sizes: guide values for small buildings

For outbuildings up to around a 4 m rafter span, sections in structural solid timber (KVH, roughly C24) have proven themselves: rafters 8 × 16 to 8 × 20 cm at 60–80 cm centres, purlins 12 × 20 cm, posts 12 × 12 cm. On a flat roof the joists do both jobs – here a rough rule of thumb is a joist depth of the span in centimetres divided by 20, i.e. around 20 cm over 4 m. On a hip roof the hip and valley rafters are chosen at least one size heavier than the common rafters. Don't forget the bracing: diagonal wind-bracing straps across the rafters, or a bracing layer of sheathing, keep the roof in shape in a storm.

Don't underestimate snow load

The shallower the roof, the longer snow stays on it. In a region with moderate snowfall you can quickly need to allow for 0.85 kN/m² or more – on a 20 m² flat roof that equals the weight of a small car. Flat and mono-pitch roofs in snowy regions therefore need noticeably heavier sections than the same area with a 40° gable roof.

Permission and structural design: what to clarify first

Whether your extension or outbuilding needs approval depends on your local building regulations and council – the limits for enclosed volume, boundary setbacks and maximum wall height at the plot boundary vary considerably from place to place, so check with your local building authority before you start. Many local development plans also prescribe the roof shape, pitch or covering colour directly; a look at the plan saves expensive redesigns. And crucially: all the timber sizes in this article are values from experience. Binding sizing with the snow and wind loads for your specific site belongs in the hands of a structural engineer or master carpenter – all the more so if you are connecting to an existing building or transferring loads into an existing wall.

Conclusion: compare first, then build

For most extensions and outbuildings the mono-pitch roof is the value-for-money winner, the gable roof is the right choice for detached, classically proportioned buildings, and the flat roof is the answer for modern massing and usable roof surfaces – while the hip roof pays off where looks and wind stability justify the extra cost. Tip: in a 3D planner like HolzBau 3D you can run the same footprint through mono-pitch, gable and flat-roof versions one after another, and the cutting list shows you instantly how the timber quantity and cost differ between the options.