Single-Pitch (Mono-Pitch) Roof: What It Is and How to Build One

A single-pitch roof is defined by its one slope, and that slope is the pitch you set. The Roof Pitch Calculator turns your chosen pitch and span into the rise, the rafter length, and the cutting angles, so you can frame a mono-pitch roof accurately from the numbers before you cut any timber.

What a single-pitch roof is

A single-pitch roof, also called a mono-pitch or lean-to roof, is a roof with just one sloping surface, running from a higher side down to a lower side. Unlike a gable roof with two slopes meeting at a ridge, a mono-pitch roof has a single plane, so rainwater runs off one edge. It is the simplest pitched roof form, needing only two wall plates at different heights and a set of rafters spanning between them, which is why it is popular for sheds, extensions, and modern homes.

The single slope gives a clean, contemporary line and is easy to build, insulate, and cover. Because it has no ridge and all its rafters are the same, it is the most approachable roof to frame, and it suits everything from a small shed to a whole modern house. A mono-pitch roof can be shallow for a low, discreet look or steep for a dramatic one, and it sheds water to one side, so it works well against a wall or where drainage to one edge is wanted. This guide covers what it is and how to build one.

Mono-pitch versus other roofs

Compared with a gable roof, a mono-pitch roof is simpler and cheaper to build, with one slope instead of two, no ridge, and identical rafters, but it gives less symmetrical headroom and sheds water to only one side. Compared with a flat roof, a mono-pitch roof drains far better and can take a wider range of coverings, while being nearly as simple to frame. So the mono-pitch sits between the flat roof and the gable: more drainage than flat, less complexity than a gable.

This makes the single pitch a strong choice where you want a proper draining roof without the work of a gable, such as on a shed, a lean-to extension, or a modern house wing. It is also the natural roof for a lean-to against an existing wall, sloping down away from the wall. The lean-to pitch guide covers that use, and the flat versus pitched comparison shows where a mono-pitch beats a flat roof. For many small builds, the mono-pitch is the practical middle ground between the two, giving proper drainage for little more effort than a flat roof and far less than a gable.

Where mono-pitch roofs are used

Mono-pitch roofs are used on sheds, garages, lean-to extensions, porches, modern houses, and outbuildings of all kinds. On a shed or garage they are the simplest way to give a draining roof; on a lean-to extension they slope down from the house wall; on a modern house they give a striking single-slope or split-level look that architects favour. Their simplicity and clean line make them popular wherever a simple, contemporary roof is wanted.

They are also used in pairs or groups for effect, such as two mono-pitches at different heights giving a split-level roof with clerestory windows between, a common modern design. And they suit green roofs and solar arrays well, since the single plane is easy to work with. So the mono-pitch ranges from the humblest shed to architect-designed homes, always on the strength of its simple single slope. The shed roof guide covers the mono-pitch on sheds, where it is perhaps most common.

What pitch for a mono roof

A single-pitch roof can be built at almost any pitch, but common choices run from about 1/12 for a near-flat modern mono roof with a membrane, through 3/12 to 4/12 for a shed or extension with shingles, up to 6/12 or more for a steeper, more dramatic slope. The pitch you choose sets how much higher the high wall is than the low wall, so a steeper pitch means a bigger height difference across the span, which matters on a wide roof.

Choose the pitch for your covering (membranes suit low, shingles need about 3/12+), your climate (steeper for snow), and the look you want, remembering that a steep pitch over a wide span raises the high wall a lot. For a shed or extension, 3/12 to 4/12 is common and practical; for a modern low roof, a shallow membrane pitch; for a striking slope, steeper. The best shed pitch guide and lean-to pitch guide cover choosing a mono pitch. Set it before framing, since it drives everything.

How to frame a single-pitch roof

Framing a single-pitch roof is the simplest of all pitched roofs. You fix a wall plate along the top of the higher wall and another along the top of the lower wall, then cut rafters that span from the high plate down to the low plate at the pitch you chose, each with a birdsmouth notch where it sits on a plate. Fix the rafters evenly across the span, add an overhang and fascia, deck the rafters, and cover. There is no ridge and all the rafters are identical.

Because every rafter is the same and there is nothing to align at a ridge, the mono-pitch is quick and forgiving to frame, ideal for a first roofing project. The height difference between the two wall plates sets the pitch, so if you are building the walls too, make one higher than the other by the rise you want. The steps that follow break the framing down, but the essence is: two plates at different heights, matching rafters between them, fixed evenly. The calculator gives the rafter length and angles.

The high and low wall plates

A mono-pitch roof rests on two wall plates: one along the higher wall and one along the lower wall. The difference in their heights, over the span, sets the pitch: a bigger height difference gives a steeper slope. So the first framing step is to establish these two plates level along their walls, at heights that give the pitch you want across the span. Fix each plate securely to the top of its wall, bedded level, since the rafters sit on them and the whole roof builds off them.

If the walls are already built, the plates sit on top of them, and the existing height difference sets the pitch, so check it gives the slope you want. If you are building the walls, set the high wall above the low wall by the rise (pitch/12 x span) so the plates give the intended pitch. The plates must be level along their length and parallel, so the rafters all sit true. Getting the two plates right, level and at the correct height difference, is the foundation of a good mono-pitch roof.

Cutting the rafters

The rafters of a mono-pitch roof span from the high plate to the low plate, all cut the same. Each rafter needs a plumb cut at the top where it meets the high wall (or sits on the high plate), a birdsmouth where it crosses each wall plate, and a plumb cut at the low eaves. The angles come from the pitch: a speed square set to the pitch marks the plumb and level lines directly, as the speed square guide shows. Cut one rafter as a pattern, check it fits, then mark the rest from it.

Because all the rafters are identical, the pattern-rafter method makes framing quick: get one right, and the rest follow. The calculator gives the rafter length from the span and pitch, and the rise, run and rafter length guide covers the maths. Buy the rafters a little longer than the calculated length for the overhang and cuts. Accurate, identical rafters are what keep the single slope true and flat for the deck, so take care with the pattern rafter and mark the others carefully from it.

Birdsmouth and plumb cuts

As on any pitched roof, two cuts define how a mono-pitch rafter sits: the plumb cut and the birdsmouth. The plumb cut is the vertical-facing cut, angled to the pitch, at the top of the rafter (and at the low eaves), so it sits flush and vertical. The birdsmouth is the notch cut into the underside of the rafter where it crosses a wall plate, with a level seat that rests on the plate and a plumb heel against it, so the rafter beds down securely and transfers its load to the wall.

A mono-pitch rafter has a birdsmouth at each plate, high and low, since it sits on both. Both cuts are set by the pitch, and a speed square set to the pitch marks them directly, which is why the square is so useful for rafter work. Keep each birdsmouth to no more than a third of the rafter depth so you do not weaken it. Getting these cuts right on the pattern rafter, so it sits flush and firm on both plates, is what makes the whole set of rafters bed down true across the single slope.

Fixing the rafters

With the rafters cut, fix them across the span onto the two wall plates, spaced evenly (typically 16 to 24 inches apart). Nail or screw through each rafter into the plates at the birdsmouths, and use framing anchors or truss clips for a stronger fixing that resists wind uplift. Mark the rafter positions along both plates first, so they line up, then fix each rafter, checking the slope stays true. Add any noggins or bracing between the rafters that the design calls for to stiffen the roof.

Even spacing and secure fixing give a roof that carries its load and holds the covering well. Because there is no ridge, fixing a mono-pitch roof is simply setting the matching rafters evenly between the two plates, which is quick once the pattern is right. When all the rafters are fixed and the roof feels solid, you have the single-slope frame ready for the overhang, deck, and covering. This simplicity, no ridge and identical rafters fixed evenly, is exactly why the mono-pitch is such an approachable roof to build.

Mono-pitch trusses

For wider spans, a mono-pitch roof can be built from mono-pitch trusses rather than simple rafters. A mono-pitch truss is a triangular or right-angled frame, made up of a rafter, a ceiling tie, and internal webs, engineered to span farther without support than a plain rafter can. The trusses are made to the span and pitch, delivered or built to a design, and fixed evenly across the walls like rafters, giving a strong single-slope roof over a wide opening.

Trusses suit a wider mono-pitch roof, such as a large garage or a house, where plain rafters would need to be very deep or supported partway. They are usually designed and often factory-made to the span, pitch, and load, so building mono-pitch roof trusses is more a matter of ordering or making them to a design than free-cutting rafters. For a small shed or extension, plain rafters are simpler; for a wide span, trusses carry the load efficiently. The pitch still drives the truss geometry, so set it first.

Wider spans

The span a mono-pitch roof covers affects how it is framed. A narrow span, such as a small shed or a lean-to, is easily crossed by plain rafters, which is the simplest build. A wider span needs deeper rafters, or mono-pitch trusses, or intermediate support such as a beam partway across, because a rafter spanning too far without support will sag or need to be impractically deep. So size the rafters to the span, or move to trusses or a supporting beam for a wide mono-pitch roof.

Because a mono-pitch rafter spans the full width in one piece (unlike a gable rafter that spans only half), spans matter more for a mono roof than a gable of the same width. This is one reason wide mono-pitch roofs often use trusses or a mid-span beam. For a shed or modest extension, plain rafters sized to the span are fine; for a wide building, plan the support. The rafter or truss depth needed for a span is a structural matter, so for a large mono-pitch roof, size the members properly or use engineered trusses.

Overhang and fascia

As on any pitched roof, an overhang throws rainwater clear of the walls and finishes the roof neatly. Cut the rafters long enough to project beyond the low wall (and optionally the high wall) at the eaves, then fix a fascia board across the rafter ends to carry the gutter and close the eaves. On a mono-pitch roof the main gutter goes on the low side, where all the water sheds, so the low eaves overhang and fascia are where the drainage is finished.

A modest overhang of a few inches protects the walls and mounts the gutter, and a soffit can close the underside if you want it fully finished. The high side may have a small overhang or meet a wall, depending on the design. Plan the overhang into the rafter length from the start, so the rafters are cut long enough to project by the amount you want. The fascia and gutter on the low side are what catch the single slope’s runoff, so finish that edge well to carry the water away.

Decking and covering

With the rafters and fascia in place, deck the roof with plywood or OSB fixed across the rafters, staggering the joints and nailing into every rafter, to give a solid base for the covering. Then cover the single slope with felt, EPDM, roll roofing, shingles, or another material suited to the pitch, working from the low eaves up so overlaps shed water down the slope. The single plane makes decking and covering a mono-pitch roof quick, with no ridge or valleys to detail.

Choose the covering to suit the pitch: membranes such as EPDM for a low mono roof, shingles for about 3/12 and up, as the coverings section of the shed guide covers. Lay any underlay the covering needs, and finish the edges so water is thrown into the low-side gutter. The roof area with pitch guide tells you how much decking and covering the single slope needs, which is a little more than the footprint because of the pitch. The clean single plane is one of the mono-pitch roof’s advantages when it comes to covering.

Shingling a single-pitch roof

If the mono-pitch is steep enough, usually about 3/12 to 4/12 or more, you can shingle it for a smart, lasting finish. Lay an underlay over the deck, then nail asphalt shingles from the low eaves up, each course overlapping the one below so water sheds down the single slope. Shingling a mono-pitch roof is simple because there is only one plane and no ridge to detail, though the top edge against the high wall or eaves is finished with a suitable trim or flashing.

Start with a starter course at the low eaves, work up in overlapping courses, and finish the top edge neatly. Because a mono-pitch roof has one continuous slope, shingling it is quicker than a gable, with no ridge capping needed, just a clean top edge. Keep the pitch at or above the shingle minimum, since below it water can drive under the shingles, as the shingle pitch guide covers. For a shed or extension mono roof at 3/12 to 4/12 or more, shingles give a durable, tidy finish to the single slope.

Drainage and gutters

A mono-pitch roof sheds all its water to one side, the low eaves, so the gutter and drainage go there. This is actually an advantage, since all the runoff goes to one predictable edge, easy to gutter and drain to a downpipe or water butt. Fit the gutter along the low eaves with a fall toward the outlet, sized to the roof area and rainfall, and the single slope’s water is collected in one place. There is no need to gutter both sides as on a gable.

Because all the water goes one way, position the low side and its gutter where you want the runoff to go, away from doors, paths, and boundaries. A mono-pitch roof against a wall sheds away from the wall to the low front edge. The roof area guide helps size the gutter for the single slope’s runoff, which all arrives at the one low gutter. This one-sided drainage is part of the mono-pitch roof’s simplicity: one slope, one gutter, water shed cleanly to a single, chosen edge, with nothing to detail on the other three sides of the roof.

Insulating a mono roof

A single-pitch roof is insulated much like any pitched or flat roof, with the insulation either above the rafters (a warm roof) or between and under them (a cold roof), and a warm-roof build-up is often preferred to avoid condensation. On a habitable mono-pitch roof, such as a modern house or a heated extension, the insulation is designed into the build-up to meet the required standard, with a vapour control layer and any needed ventilation. On a shed, insulation may be minimal or absent.

The single slope makes a mono-pitch roof simple to insulate, with one plane to build up, and it suits a warm-roof approach where insulation sits above the rafters and the covering over that. For a room under a mono-pitch roof, this gives a clean sloping ceiling and good insulation. So insulating a mono roof is simple: choose a warm or cold roof build-up to suit, and design it to the required standard for a habitable space. The clean single plane, as with covering, makes the insulation build-up simple to detail, with no ridge, valleys, or hips to work around.

Getting the look right

Because a single-pitch roof is so visible as one clean plane, its appearance rewards a little thought. The direction of the slope, the depth of the overhang, and the covering all read strongly on a mono roof, since there is no ridge or second slope to break up the line. A shallow mono roof with a slim fascia and a hidden gutter looks crisp and modern, while a steeper mono roof with a generous overhang reads as more traditional and sheltering. Deciding which effect you want helps you set the pitch and detail the eaves to match.

On a house or garden room, aligning the high side with the taller part of the building, and sloping down toward the view or the garden, often looks most natural and lets light in through tall windows on the high wall. On a shed or lean-to, keeping the slope direction consistent with nearby buildings ties the group together. None of this changes the structure, but it turns a purely functional single slope into a deliberate architectural feature, which is a large part of why architects reach for the mono-pitch on modern designs.

Pros and cons

The single-pitch roof’s advantages are simplicity, low cost, easy building, a clean modern look, one-sided drainage that is easy to gutter, and suitability for green roofs and solar. Its single plane and identical rafters make it the easiest pitched roof to frame, insulate, and cover, and its contemporary line suits modern designs. It also works perfectly as a lean-to against a wall, sloping down away from it, which a gable cannot do as neatly.

Its drawbacks are less symmetrical headroom than a gable, a high wall that rises with the pitch and span (which can look tall or use material on a wide, steep roof), and drainage to only one side. So a mono-pitch roof is ideal where simplicity, a modern look, and one-sided drainage suit, and less ideal where you want the symmetry and two-sided drainage of a gable or need to keep a wide roof low. Weighing these, the mono-pitch is a strong choice for sheds, extensions, lean-tos, and modern homes, which is where it is most used.

Mistakes to avoid

The common mistakes when building a single-pitch roof are these.

Avoiding these means keeping the pitch moderate on wide spans, cutting matching rafters off a pattern, fixing level parallel plates, sizing the rafters or trusses to the span, and matching the covering to the pitch. Do that and the single slope will be true, strong, and watertight.

Step by step

The build in order: first, choose the pitch and work out the rise and rafter length from the span in the calculator. Second, fix level, parallel wall plates on the high and low walls at the height difference that gives the pitch. Third, cut a pattern rafter with its plumb cuts and birdsmouths, check it, and mark the rest from it. Fourth, fix the rafters evenly across the two plates. Fifth, extend for an overhang and fix the fascia on the low side.

Sixth, deck the rafters with plywood or OSB. Seventh, cover the single slope from the low eaves up with a covering suited to the pitch. Eighth, fit the gutter along the low eaves. For a wide span, use mono-pitch trusses or a supporting beam instead of plain rafters. Throughout, the pitch chosen at the start sets the height difference, rafter length, and cuts, so use the calculator for the numbers. Follow this and a mono-pitch roof is the most approachable pitched roof to build.

Working out the pitch

Everything in a single-pitch roof flows from the pitch, and the roof pitch calculator turns your chosen pitch and span into the rise (the height difference between the two walls), the rafter length, and the cutting angles. Enter the span and the pitch, and it gives the numbers you need to set the wall plate heights, order the rafters, and mark the cuts. That makes framing a mono-pitch roof a matter of following the numbers, which is why it is such an approachable build.

The connected guides help with each part: building a pitched shed roof for a mono shed, lean-to pitch for a lean-to mono roof, rise, run and rafter length for the rafter maths, and best shed pitch to choose the slope. The roof pitch tool sits with the other estimators in the construction calculators category, near site tools like the dirt calculator, and the full roofing library is in the roof pitch blog off the homepage. Set the pitch, and the single slope follows.

How to frame a single pitch roof: frequently asked questions