If you're building an extension, replacing a garage roof, or converting an outbuilding, one of the first decisions you'll face is whether to go with a flat roof or a pitched roof. Each has genuine advantages and limitations, and the right choice depends on your budget, the planning constraints of your property, and what you want the finished structure to look and feel like.
This guide gives you an honest comparison of both options — including realistic UK costs, material choices, lifespan expectations, and the factors that should drive your decision.
What Counts as a "Flat" Roof?
First, a common misconception: flat roofs aren't actually flat. A properly constructed flat roof has a slight pitch of between 1 and 10 degrees — just enough to direct rainwater toward a gutter or outlet. Without this fall, water would pool on the surface (a problem known as "ponding"), which dramatically shortens the roof's lifespan.
If you've seen old flat roofs with standing water on them, that's a sign of either poor original construction or structural movement over time — and it's one of the main reasons flat roofs got a bad reputation in the UK. Modern flat roof systems, properly installed with the correct fall, are far more reliable than the felt-and-bitumen roofs that were standard in the 1960s–1980s.
Pitched Roof: Advantages
Better Drainage
A pitched roof sheds water by gravity. Rain runs straight off tiles or slates into gutters, with no opportunity to pool or sit on the surface. This fundamental advantage is why pitched roofs consistently outlast flat roofs in the UK's wet climate.
Longer Lifespan
A well-built pitched roof with quality materials will last 30–100+ years depending on the covering. Concrete tiles typically last 30–50 years, clay tiles 60–100 years, and natural slate 80–150 years. By comparison, even the best flat roof systems top out at around 30–50 years.
Better Insulation Potential
The void created by a pitched roof provides natural insulation and ventilation. It's also easier to add insulation at ceiling level or between the rafters, helping meet current Building Regulations for thermal performance (Part L). Flat roofs can be well-insulated too, but the options are more limited and typically more expensive per square metre.
Planning and Aesthetic Preference
Local planning authorities in the UK generally prefer pitched roofs on extensions, particularly on houses in conservation areas or traditional streetscapes. A pitched roof that matches the main house in material and style is more likely to get planning approval — and it adds more kerb appeal and resale value to your property.
Flat Roof: Advantages
Lower Upfront Cost
Flat roofs are typically cheaper to construct. For a standard domestic extension or garage, you can expect to pay £50–£120 per square metre for a flat roof, compared to £80–£200 per square metre for a pitched roof. On a 20m² garage roof, that difference could be £1,000–£2,000.
Height Restrictions
Flat roofs are lower profile. This makes them the natural choice for single-storey rear extensions where a pitched roof would overshadow neighbouring properties or block light. Many permitted development extensions have maximum height limits that make a flat roof the only viable option without applying for full planning permission.
Usable Roof Space
A flat roof can potentially be used as a terrace, green roof, or for solar panel installation (though this requires structural engineering to ensure the deck can handle the additional load). Pitched roofs don't offer this option.
Easier Access for Maintenance
Flat roofs are safer and easier to inspect and maintain. You can walk on them (carefully, on most modern systems), which makes cleaning gutters, checking for damage, and carrying out repairs more straightforward than working on a pitched roof at height.
Flat Roof Materials Compared
The material you choose for a flat roof has a bigger impact on its lifespan and performance than almost any other factor. Here are the four main options available in the UK:
Traditional Felt (3-Layer Built-Up)
- Cost: £40–£60 per m²
- Lifespan: 10–15 years
- Verdict: The cheapest option, but not recommended for new construction. Traditional felt roofs use layers of bituminous material bonded with hot or cold adhesive. They're prone to cracking, blistering, and splitting — particularly in the UK climate where freeze-thaw cycles stress the material. If you're replacing an old felt flat roof, it's worth upgrading to EPDM or GRP rather than like-for-like replacement.
EPDM Rubber (Ethylene Propylene Diene Monomer)
- Cost: £60–£90 per m²
- Lifespan: 25–30 years (some systems warranted for 50 years)
- Verdict: An excellent mid-range option. EPDM is a synthetic rubber membrane supplied in large sheets, which means fewer joints and fewer potential leak points. It's flexible (handles building movement without cracking), resistant to UV degradation, and copes well with standing water. Firestone RubberGard is the most widely used brand in the UK. Installation is relatively straightforward for experienced roofers — the membrane is adhesive-bonded to the deck.
GRP Fibreglass
- Cost: £70–£100 per m²
- Lifespan: 30–40 years
- Verdict: GRP (glass-reinforced polyester) creates a seamless, rigid, waterproof shell over the roof deck. It's particularly good for complex shapes — around upstands, corners, and outlets — because the fibreglass is laid wet and moulds to any profile. The finished surface can be walked on (making it suitable for balcony-style flat roofs) and it has an attractive, smooth finish. The downsides: it's rigid (so it doesn't flex with building movement, making it less suitable for large unsupported spans) and installation is weather-dependent (GRP can't be laid in cold or damp conditions).
TPO/PVC Single-Ply Membrane
- Cost: £80–£120 per m²
- Lifespan: 20–30 years
- Verdict: Single-ply membranes like TPO (thermoplastic polyolefin) and PVC are heat-welded at the seams, creating very strong waterproof joints. They're lightweight, energy-efficient (white membranes reflect solar heat), and increasingly popular on commercial buildings. For domestic use, they're an excellent choice for larger flat roofs. However, they're the most expensive domestic flat roof option and require specialist installation — not every roofer can work with them.
Pitched Roof Materials Compared
For pitched roofs on extensions and new builds, the three main options are:
Concrete Tiles
- Cost: £80–£150 per m² (fully installed)
- Lifespan: 30–50 years
- Best for: Budget-conscious projects where you want the look of a traditional tiled roof. Brands like Marley and Redland offer interlocking tiles (faster to lay, slightly cheaper) and plain tiles (more traditional appearance, more labour-intensive). Available in a wide range of colours and profiles to match existing properties.
Natural Slate
- Cost: £120–£200 per m² (fully installed)
- Lifespan: 80–150 years
- Best for: Period properties, conservation areas, and high-end projects where long-term value and aesthetics justify the higher upfront cost. Welsh slate (Penrhyn, Cwt-y-Bugail) is the gold standard, but quality Spanish and Brazilian slates are also widely used and more affordable. Slate roofs require skilled installation — not every roofer can lay slate properly.
Clay Tiles
- Cost: £100–£180 per m² (fully installed)
- Lifespan: 60–100 years
- Best for: Traditional and heritage properties. Clay tiles have a distinctive warm colour that weathers beautifully over time. They're heavier than concrete tiles, so the roof structure needs to be designed accordingly. Machine-made clay tiles are more affordable than handmade, but both offer excellent longevity.
Planning Permission: What You Need to Know
For most single-storey rear extensions, you won't need planning permission regardless of whether you choose flat or pitched — provided you stay within permitted development limits. However, there are important nuances:
- Permitted development height limits for single-storey rear extensions are 4 metres maximum. A flat roof typically sits at 2.5–3 metres, leaving plenty of headroom. A pitched roof can easily exceed 4 metres if the pitch is steep, which would require a full planning application.
- Conservation areas and Article 4 directions often require planning permission for any extension, and planners will typically insist on a pitched roof that matches the main house in material and style.
- Side extensions in many local authorities must have a pitched roof to be considered permitted development.
- Planners generally prefer pitched roofs on front-facing or street-visible extensions, even when planning permission isn't technically required. If there's any doubt, a pitched roof is the safer option.
Your local planning authority's pre-application advice service (typically £50–£100 for a householder enquiry) can tell you what's likely to be acceptable before you commit to a design.
Which Should You Choose? A Decision Framework
Use these questions to guide your decision:
Choose a Flat Roof If:
- You're building a rear extension with height restrictions
- Budget is a primary concern and you want to keep costs down
- You want the option of using the roof as a terrace or green roof
- The extension isn't visible from the street and aesthetics aren't critical
- You're replacing an existing flat roof on a garage or outbuilding
Choose a Pitched Roof If:
- The extension is visible from the front of the property
- You're in a conservation area or traditional streetscape
- You want maximum lifespan and minimum long-term maintenance
- You're planning to use the roof void for storage or future conversion
- The main house has a pitched roof and you want a cohesive appearance
- You value long-term property value over upfront savings
Consider a Hybrid Approach
Many modern extensions use a flat roof with a parapet wall, creating a clean contemporary look while still benefiting from modern flat-roof systems. Some designs incorporate a partial pitched roof (a lean-to or mono-pitch) that offers better drainage than a flat roof but with less height than a full gabled pitch. Discuss the options with your architect or roofer — there's often a design solution that gives you the best of both approaches.
Get Expert Advice for Your Project
The flat vs pitched decision affects your build cost, planning application, long-term maintenance, and the look of your finished home. It's worth getting advice from both your architect and your roofer before committing — and always get itemised quotes that let you compare the cost difference directly.
At Pinnacle Roofing, we install both flat and pitched roofs across the UK. We're happy to survey your property, discuss the options, and provide a detailed, no-obligation quote for whichever approach suits your project best.