Planning a Patio Roof – Lean-To, Roof Covering, Timber Sizing
A patio roof is more than four posts and a lid: whether you build a lean-to against the house or a freestanding canopy determines the structure, the wall connection and the amount of work involved. This guide walks you through every planning step with concrete guide values – from the roof covering to the drainage.
Attached to the house or freestanding?
This first fundamental decision shapes almost everything that follows. A lean-to canopy uses the house wall as its fourth support: you save two posts and their footings, the structure is stiffer, and the step out of the patio door stays dry. In return you take on the trickiest detail of the whole project – the wall connection. The wall also has to be capable of carrying the loads in the first place. With externally insulated facades (EWI) or a roller shutter box directly above the patio door, the fixing quickly becomes involved.
A freestanding canopy stands on four or six posts and does not touch the house. In return it needs its own bracing against wind in both directions – usually knee braces or diagonal struts – and two additional pad foundations. It makes sense when the facade cannot carry loads, when the patio is not directly against the house, or when you want to avoid flashing details and thermal bridges on the building altogether.
- Lean-to: uses less material (two fewer posts and footings), stiffer, dry transition into the house – but the wall connection and load transfer must be detailed properly.
- Freestanding: independent of the facade, no flashing into the house wall, flexible placement – but more foundations and rigorous bracing are required.
The wall connection: the most critical detail
Fixing the wall plate
On a lean-to, a horizontal wall plate (guide value 10 × 12 to 12 × 16 cm) carries the house-side rafter ends. It is anchored into sound masonry or concrete with M12 heavy-duty or resin anchors at roughly 50–80 cm centres. In perforated clay bricks or hollow blocks you need injection resin with a mesh sleeve, otherwise the anchor will not hold reliably. On externally insulated (EWI) facades there is rarely a way around a stand-off fixing with thermally separated brackets – the insulation itself must not take any compressive loads.
Sealing against driving rain
Above the roof covering sits an aluminium wall flashing profile. The most durable solution is a flashing let into a chase cut in the render: cut a groove about 2 cm deep, insert the profile, back it with compressible foam sealing tape and close it with a permanently elastic sealant. A silicone bead applied straight onto the render typically lasts only a few years and is the most common cause of moisture damage on lean-to patio roofs.
Roof covering: laminated glass or polycarbonate sheets?
Two materials dominate for the covering, and they differ markedly in weight, price and feel – which in turn drives the sizing of the entire supporting structure.
Laminated safety glass
For overhead glazing, laminated safety glass is the standard; 8–10 mm total thickness is typical, for example two 4 mm panes with a tear-resistant interlayer. Glass looks high-end, does not yellow, is quieter in the rain and is easy to clean. The price you pay: around 20–25 kg/m² of weight that the whole supporting structure has to carry, and material costs of roughly EUR 80–150 per m² (guide value, depending on build-up and pane size).
Multiwall polycarbonate sheets
Multiwall polycarbonate sheets – usually 16 mm thick – weigh only about 2.5–3.5 kg/m² and cost around EUR 20–45 per m². They diffuse the light, which can be pleasant in summer, but they creak audibly with temperature changes and look more basic. Important: only install the sheets with the manufacturer's matching glazing bar system, and allow for thermal expansion of about 2.5–3 mm per metre of sheet length using slotted fixing holes, otherwise the sheets will crack at the screw points.
| Criterion | Laminated glass (8–10 mm) | Multiwall polycarbonate (16 mm) |
|---|---|---|
| Weight | approx. 20–25 kg/m² | approx. 2.5–3.5 kg/m² |
| Material cost (guide value) | approx. EUR 80–150/m² | approx. EUR 20–45/m² |
| Rafter spacing (centres) | approx. 60–90 cm | approx. 98–120 cm (= sheet width) |
| Looks and ageing | clear, scratch-resistant, does not yellow | diffuses light, may yellow after 15–20 years |
| Breakage behaviour | interlayer holds fragments together, expensive to replace | very impact-resistant, cheap to swap |
Sizing: timber sections as guide values
The timber sections depend on the projection (depth), post spacing, roof covering and above all on the snow load at your location – depending on region and altitude it can range from around 0.65 to well over 1.1 kN/m². The following figures are therefore deliberately given as guide values for structural graded softwood or glulam:
| Member | Guide section | Typical spacing / span |
|---|---|---|
| Posts | 12 × 12 cm; from about 4 m projection, better 14 × 14 cm | post spacing approx. 3.0–4.0 m |
| Front purlin (main beam) | 12 × 20 to 12 × 24 cm | spans between the posts, 3.0–4.0 m |
| Rafters for polycarbonate | 8 × 16 to 8 × 18 cm | projection 3.5–4.0 m, approx. 100 cm centres |
| Rafters for laminated glass | 10 × 18 to 10 × 20 cm | projection 4.0–5.0 m, approx. 70 cm centres |
| Wall plate (lean-to) | 10 × 12 to 12 × 16 cm | M12 anchors every 50–80 cm |
| Knee braces | 10 × 10 cm, at 45° | at every post, in line with the main beam |
This table is no substitute for a structural calculation: as soon as snow load, spans or glass weight depart from the usual, the sizing belongs in the hands of a structural engineer. Many building authorities require proof of structural adequacy anyway.
Roof pitch and drainage
Both laminated glass and polycarbonate sheets need a minimum fall of about 5° (roughly 9%) so that rain runs off and leaves do not settle – 7–10° is better. Work through the heights early: at 4 m projection and an 8° pitch you lose around 56 cm of height. With an eaves height of at least 2.10–2.20 m at the front edge, the wall connection then ends up at about 2.70–2.90 m – check whether a roller shutter box or an upstairs window is in the way there.
At the eaves, a 125 mm half-round gutter collects the water, laid with a fall of 2–3 mm per metre towards the downpipe. As a rule of thumb, one 75 mm downpipe drains around 50–70 m² of roof area, which is enough for most patio roofs. In most places the water must not simply run onto your neighbour's land or the pavement: a water butt, a soakaway or a connection to the drains – your local authority decides what is permitted.
Permission: ask first, build second
Whether a patio roof needs a permit depends entirely on where you live. In many places, canopies up to around 30 m² in area and 3 m deep can be built without a formal application – elsewhere completely different limits apply, and boundary setbacks of usually 2.5–3 m to the property line have to be respected almost everywhere. Even a permit-free build is not rule-free: local development plans, neighbour rights and structural requirements still apply. A quick call to your local building authority before you buy the timber can spare you having to take the whole thing down again.
Costs at a glance
For a self-built canopy of 4 × 3 m, a rough budget looks like this: about EUR 1,500–2,500 in materials with multiwall polycarbonate sheets, more like EUR 3,000–5,000 with laminated glass. That covers the timber (EUR 400–900), the roof covering including glazing bars, post anchors and pad foundations, gutter and downpipe, plus connectors and screws. Ready-made kits or installation by a contractor cost considerably more – often two to three times as much.
How to approach the planning
- Fix the main dimensions: typical depths are 3.0–4.0 m, widths 4.0–6.0 m – the depth determines the rafter section and the height profile.
- Decide between lean-to and freestanding, and for a lean-to check the wall (material, insulation, roller shutter boxes).
- Choose the roof covering, because glass or polycarbonate dictates the rafter spacing and sections.
- Work out heights and falls, and plan the drainage all the way to the discharge point.
- Contact your local building authority and, if in doubt, have the structure checked by an engineer.
- Plan the structure in full before ordering any material. Tip: in a 3D planner such as HolzBau 3D you can see the height profile and rafter grid to scale and get a parts list for your shopping straight away.