How to Build a Lean To Roof

A lean to roof is one of the simplest ways to add covered space to a home. It suits side returns, garden stores, bike sheds, utility rooms, small workshops, and rear canopies. The structure is straightforward, the footprint is compact, and with careful detailing a lean to will give years of reliable service. This guide explains how to plan, design, and build a lean to roof in the UK, with clear advice on permissions, materials, insulation options, step by step construction, and typical costs.

What a lean to roof is and where it works best

A lean to is a single sloping roof that falls away from a higher wall. One side of the roof bears on a wall plate or ledger fixed to the main building. The opposite side bears on posts or a new wall. Because the span is usually modest, the timber sizes can be kept reasonable and the roof coverings are simple to fit. Lean tos are ideal where you want weather protection without a full extension, such as over patio doors, beside a kitchen, or along a garage.

Before you begin, decide whether the space will be unheated and open sided, like a canopy, or enclosed and insulated as a habitable room. That choice affects planning rules, structure, insulation, ventilation, and costs.

Planning permission, permitted development, and local rules

Many lean to roofs for canopies and outbuildings can be built under permitted development. The detail depends on whether the roof is attached to the house and whether the space below is enclosed.

For most houses, a modest canopy or porch style lean to at the rear may be allowed if it does not project too far, sits within height limits, and does not front a highway. If you are creating an outbuilding in the garden, the usual limits apply to height, area, and distance from boundaries. Eaves heights are restricted and the overall roof height must stay within set maxima. Flats and maisonettes do not have the same permitted development rights and will usually need consent. Listed buildings and homes in conservation areas are more tightly controlled.

As a rule of thumb, if your proposal changes the roofline facing the street, increases height close to a boundary, or creates a new habitable room, speak to your local planning department first. A short pre application enquiry can save time and cost later.

Building regulations you must consider

Even if planning permission is not required, most lean to roofs must comply with building regulations. The key parts are:

Structure. The roof needs to carry its own weight plus wind and snow loads. Timber sizes, fixings, and connections to the wall must be adequate. Posts and foundations must support the line loads from rafters.

Thermal performance. If the space below is enclosed and heated, insulation standards apply. A warm roof build up above the deck is usually the easiest way to meet current targets.

Ventilation and condensation control. Roofs over heated rooms need a proper moisture strategy. Either ventilate the void above insulation, or use a warm roof with continuous insulation and an internal vapour control layer.

Fire safety and boundaries. If the lean to is close to a boundary, external fire spread rules apply. Finishes and eaves detailing must meet the relevant fire rating.

Drainage. The roof needs adequate falls, gutters, and downpipes discharging to a suitable soakaway or surface water system.

A competent builder can self certify under a recognised scheme, or you can apply to your local authority for inspection.

Choosing the roof covering

Lean to roofs accept a wide range of coverings. The choice is driven by pitch, budget, and appearance.

Flat to low pitch roofs can use single ply membranes such as EPDM, GRP fibreglass, or a multi layer bituminous felt system. These give a neat, modern finish with minimal maintenance when installed to good practice. Slightly higher pitches open the door to lightweight corrugated sheets and some interlocking tiles or slates designed for low pitch use. Always check the manufacturer’s minimum pitch for the product you intend to use.

For a quiet, domestic look next to the house, a single ply membrane or GRP with a hidden gutter often blends well. For a garden shed or workshop, corrugated bitumen or steel sheets are economical and quick to fit.

Working out falls, spans, and timber sizes

A lean to succeeds or fails on simple geometry. The roof must fall enough to shed water, and the timber must be sized for the span and loads.

Falls. For flat coverings, aim to build the structure to a fall of about 1 in 40 so that after normal deflection you achieve no less than 1 in 80. For sheeted coverings, follow the product guidance.

Span and rafter depth. For small spans up to around 2.4 metres, 47 x 100 mm rafters at 400 mm centres often suffice where loads are modest. Spans around 3.0 metres tend to need 47 x 150 mm at similar centres. Exposed sites, heavy coverings, snow loading, and wider centres push sizes up. Always check span tables or take advice from a structural designer for your exact case.

Wall plate and posts. A wall plate of 100 x 50 mm or 150 x 50 mm treated timber fixed securely to the main wall provides a reliable bearing for joist hangers or rafter seats. At the low side, either a timber beam supported on posts or a masonry dwarf wall can carry the rafters. Typical posts are 100 x 100 mm or 150 x 150 mm depending on loads. Posts should bear on concrete pads or a continuous strip foundation.

Materials and tools

A typical specification for a small domestic lean to might include:

Pressure treated structural timber for wall plate, rafters, and posts.
Joist hangers or rafter brackets, stainless or galvanised fixings, and structural screws.
OSB3 or plywood deck if using membrane coverings.
Breathable underlay and counter battens if using tiles or slates suited to the pitch.
Chosen roof covering and trims.
Lead or compatible flashing to dress into the wall above the wall plate.
Fascia, gutter, brackets, downpipe, and a suitable outlet to soakaway or surface water drain.
Sealants and tapes approved for the covering system.
Concrete for post foundations and fixings such as post bases or shoes.

Step by step: building a lean to roof

Set out and foundations. Mark the footprint and confirm levels. For a post supported edge, dig pads below frost depth and cast concrete bases sized for the loads. Install post bases or shoes to keep timber clear of ground moisture. If you are building a low masonry upstand, set it out now and allow for the roof fall in the top course.

Fix the wall plate. Mark the finished roof height on the main wall. Check that doors and windows below will still open under the new eaves. Offer up the treated wall plate. Use suitable fixings for the wall type, such as resin anchors in masonry. Pack behind the plate where the brick face is uneven so the timber bears fully and remains straight. Keep the plate level along its length.

Install posts and beam or ledger. Stand posts plumb in their bases, brace temporarily, and fix a front beam across their tops at the correct height to create the designed fall from wall plate to beam. Check the beam is straight and level along its length.

Fit rafters. Cut rafters to length with simple birdsmouth seats at the beam, or use joist hangers at the wall plate. Set centres at 400 mm or 600 mm to suit your deck or covering. Fix each rafter securely at both ends and check for a consistent fall. Add solid noggins between rafters near mid span if required to keep them upright and prevent twisting.

Add decking or battens. For membranes, fix OSB3 tongue and groove across rafters, leaving small expansion gaps as recommended by the board manufacturer. For sheeted coverings, follow the system guidance for purlin or batten spacing. For low pitch interlocking tiles approved for the angle, fix counter battens over a breathable underlay and then tile battens at the gauge specified.

Flashings and upstands. Chase a neat slot into the masonry above the wall plate and dress a lead flashing or compatible proprietary flashing over the roof covering and into the chase. Seal and point the chase to keep water out. Form upstands at side abutments so water cannot creep behind the covering.

Fit the roof covering. Lay the chosen covering in line with the manufacturer’s instructions. For EPDM, adhere the sheet to the deck, roll out air, and finish with perimeter trims. For GRP, laminate mat and resin on a dry deck, then apply the topcoat when cured. For felt, follow the specified layers with sealed laps. For sheets, fix at the correct centres with appropriate washers and seal overlaps as detailed.

Gutters and downpipes. Fix fascia boards if required, then set out gutter brackets to fall towards the outlet. Clip the gutter, fit the outlet, and connect a downpipe to a soakaway or rainwater system. Ensure the discharge does not create nuisance for neighbours and does not enter a combined sewer without consent.

Insulation options for enclosed lean tos

If the lean to will be enclosed and heated, build it as a warm roof. Above the rafters, lay a continuous vapour control layer, rigid insulation to the required thickness, and a deck on top before applying the roof covering. This keeps the structure warm and reduces condensation risk. Alternatively, insulate between and below rafters with a ventilated void above and continuous cross ventilation at eaves and high level. A warm roof is usually simpler on small lean tos because ventilation paths can be hard to maintain in shallow spaces.

Detailing for durability

Small details make a big difference to performance.

Keep water away from the wall. A good flashing and a slight overhang with a drip edge prevent rain tracking back to the masonry.

Protect cut timber. Treat all cut ends and notches with preservative. Keep posts off the ground with proper bases.

Allow for movement. When fixing trims and flashings, leave the recommended gaps and use slotted holes where required so materials can expand and contract without warping.

Choose the right fixings. Use galvanised or stainless fixings outdoors. Mixed metals can corrode when wet.

Typical costs in the UK

Prices vary with size, access, and finish. As a guide for a small timber lean to canopy of around 3 metres by 2 metres:

Structure and posts. £400 to £800 for timber, fixings, and concrete.
Deck and covering. £350 to £900 for OSB and EPDM, felt, or GRP on a simple build.
Gutters and trims. £100 to £250.
Lead flashing and sundries. £100 to £250.

If you hire a contractor, labour for a straightforward canopy often adds £800 to £1,800 depending on location. An enclosed, insulated lean to with doors and glazing becomes a small extension and costs rise accordingly. Warm roof insulation and a higher grade covering can take the total to £2,500 to £5,000 for a modest structure, and more where glazing, electrics, and finishes are included. Always compare written quotations that itemise structure, covering, insulation, flashings, gutters, waste removal, and building control fees.

Common mistakes and how to avoid them

Too little fall. A shallow roof without adequate falls will pond water and age quickly. Build in falls with firrings or tapered insulation.

Undersized timber. Guesswork on rafters leads to bounce and cracking finishes. Use span tables or ask a designer to size members.

Poor wall fixings. A loose wall plate invites leaks and movement. Use proper anchors, pack evenly, and flash carefully.

No moisture strategy. Enclosing a lean to without insulation and vapour control invites condensation. Choose warm roof or ventilated cold roof and stick to it.

Weak edge fixings. Roof edges feel the wind most. Fix trims and sheets at the correct centres into sound timber.

Safety, access, and neighbours

Work at height safely. Use a stable platform or scaffold. Secure ladders and keep working areas tidy. Protect doors, windows, and paving below from falling debris and resin spills. If the lean to sits near a boundary, discuss the plan with your neighbour early, especially if access is required for fixing flashings on a party wall. Keep noise within reasonable hours.

Maintenance for a long life

Inspect the roof in spring and autumn. Clear the gutter, check flashings for gaps, and remove leaves that hold water. Membranes and GRP need little more than an occasional wash. Sheeted roofs benefit from checking fixings after the first winter and then every few years. Keep vegetation trimmed back so branches do not rub or drop sap.

Final checklist before you start

Confirm whether planning permission is required.
Agree the building regulations route and insulation strategy.
Choose a covering suitable for the pitch and exposure.
Size rafters and posts from reliable tables or professional advice.
Detail flashings and drainage clearly.
Obtain written quotations that cover the whole job.

With those points in place, a lean to roof is a satisfying project that adds useful space and shelter, looks tidy alongside a home, and stands up well to the UK weather when built with care.