The Ultimate Guide to Roof Types for UK Home Extensions — and How Warm vs Cold Roof Build-Ups Differ
Planning an extension is exciting… until you hit the roof details. Choice of roof shape, covering, drainage and insulation build-up will affect planning permission, daylight, headroom, running costs and even how easy it is to build. Here’s a clear, UK-focused guide that walks you through the common roof types for extensions, the pros/cons of each, and a practical explanation of warm vs cold roof build-ups (with flat and pitched examples).
1) Common roof types for UK extensions
A) Flat roofs (low-slope)
• What it is: A roof with a subtle fall (typically 1:40 to 1:80) to shed water. “Flat” is a builder’s shorthand; it should never be truly level.
• Why choose it: Clean modern look, easy to tuck beneath first-floor windows, simpler junctions with the existing house, and a great platform for rooflights, lanterns, PV or green roofs.
• Typical coverings:
• Single-ply membranes (e.g., PVC, TPO): quick, light, minimal joints.
• EPDM: durable rubber sheet with few seams.
• GRP/Fibreglass: fully bonded, seamless; great around complex shapes.
• Built-up bituminous felt: tried-and-tested, versatile, cost-effective.
• Inverted warm roofs (insulation above the membrane with ballast/pavers) where heavy use or roof terraces are planned.
• Details to watch:
• Continuous fall to a gutter or internal rainwater outlet.
• Upstands at edges and around rooflights (commonly ≥150 mm above finished roof).
• Parapets can hide falls and improve fire compartmentation at boundaries but add cost/thickness.
• Choose coverings achieving appropriate Broof(t4) performance near boundaries for Part B fire compliance.
B) Mono-pitch (lean-to)
• What it is: A single sloping plane, usually leaning off the existing wall. Very common on kitchen/dining “side return” and rear extensions.
• Why choose it: Simple structure, generous ceiling heights, can integrate contemporary glazing at the high end.
• Coverings: Tiles or slates at standard pitch; low-pitch tiles or metal sheet for shallower slopes; or you can treat it as a “flat” roof with a pronounced fall.
C) Dual-pitch (gable) or hip
• What it is: Classic pitched roof forms scaled to the extension.
• Why choose it: Matches traditional houses, sheds water fast, long life with clay/slate tiles, and gives the option for a vaulted ceiling if insulated at rafter level.
• Considerations: Tie-in under existing eaves, ridge height control for planning, and roof-window positions.
D) Butterfly (inverted dual-pitch)
• What it is: Two pitches sloping inward to a central valley.
• Why choose it: Architectural statement; allows higher parapet edges near boundaries, and can bring daylight via clerestory glazing.
• Gotchas: Valley detailing and drainage must be impeccable.
E) Roof terraces over the extension
• What it is: A flat roof designed for regular foot traffic.
• Why choose it: Valuable outdoor space in dense plots.
• Build-up: Inverted warm roof (membrane protected beneath insulation and paving) or a robust warm deck with paving supports. Pay particular attention to guarding, door thresholds, acoustic privacy and planning conditions.
F) Green roofs (extensive or intensive)
• What it is: Vegetated build-up over a waterproofing layer.
• Why choose it: Softens outlook, slows rainwater run-off (SuDS), boosts biodiversity and thermal moderation.
• Build-up: Always a warm or inverted roof; never a ventilated cold flat roof. Add root-resistant membrane, drainage layer, substrate and planting.
2) Roof coverings at a glance
• Clay/concrete tiles: Long life, traditional look; need adequate pitch (often ≥30° for many profiles, though low-pitch systems exist).
• Natural slate: High-end, lightweight, low porosity; requires competent fixing and specific pitches.
• Metal standing seam (zinc, aluminium, steel): Works at low pitch (~5–7° depending on system), crisp contemporary lines; needs a compatible underlay and detailing.
• Single-ply/EPDM/GRP: Best for flat/low-slope; fewer penetrations = longer life.
• Green roof: Works over membranes designed for root resistance; adds dead load—engineer the structure accordingly.
3) Warm vs Cold Roof build-ups — what’s the difference?
Think of the insulation as a cosy coat. Where you put that coat—above or below the structure—changes performance, risk and detailing.
Warm roof (the UK go-to for new extensions)
• Where the insulation sits: Above the structural deck (flat roofs) or between/over rafters (pitched roofs), keeping the structure warm.
• Vapour control: A VCL (vapour control layer) sits beneath the insulation, on the warm side, to limit moist indoor air reaching cold layers.
• Ventilation: Generally not required within the build-up because there’s no cold void (always follow the chosen system’s guidance).
• Pros:
• Lower condensation risk and simpler moisture control.
• Better thermal continuity (fewer cold bridges).
• Airtightness is easier to achieve.
• Works brilliantly with tapered insulation to form falls on flat roofs.
• Cons: Slightly deeper build-up; may affect thresholds, window cills, or parapet heights.
Cold roof (use sparingly on modern projects)
• Where the insulation sits: At ceiling level (flat roofs: insulation between joists; pitched roofs: at ceiling with a ventilated loft/void). The structure above becomes cold.
• Ventilation: You must ventilate the void (e.g., 50 mm continuous cross-ventilation for flat roofs; eaves/ridge vents for pitched) to carry away moisture.
• Vapour control: A good VCL above the ceiling finish is essential to reduce interstitial condensation risk.
• Pros:
• Keeps roof build-up thinner above joists/rafters.
• Convenient when you want an accessible, ventilated loft in pitched roofs.
• Cons:
• High condensation risk if ventilation is blocked by insulation/stored items or in complex roof shapes.
• More thermal bridging (cold joists/rafters).
• Harder to make airtight.
• Frequently not recommended for flat roofs on dwellings unless detailing is exemplary.
Hybrid roofs
• Combine a ventilated void and some insulation at rafter level. Used in “room-in-roof” situations or tricky refurbishments. Needs careful dew-point analysis and airtightness strategy.
4) Flat roof build-ups (section details in words)
Warm deck (typical modern choice)
1. Internal finish (plasterboard).
2. Airtight layer/VCL on top of deck structure.
3. Structural deck (e.g., plywood/OSB over joists or concrete slab).
4. Rigid insulation above the deck (often PIR or mineral wool; tapered to form falls if needed).
5. Waterproof membrane.
6. Optional: protection layer, ballast or paving (for inverted/terrace designs).
Cold deck (use with caution)
1. Internal finish + VCL at ceiling level.
2. Insulation between joists.
3. Ventilated void (usually ≥50 mm continuous).
4. Roof deck.
5. Waterproofing.
6. Through-ventilation via soffits or proprietary vents.
Risky in practice because ventilation paths are often interrupted by noggins, rooflights and complex junctions.
Inverted roof (great for terraces)
1. Structural deck + membrane (fully warm and protected).
2. Filter layer.
3. Rigid XPS insulation (above membrane).
4. Ballast/pavers on supports.
Requires rainwater correction factor in U-value calculations; excellent durability and maintainability.
5) Pitched roof build-ups (section details in words)
Warm pitched roof (insulate at rafter level)
• Between-rafters insulation + continuous over-rafter layer (or a high-performance between-rafter system) to cut thermal bridges.
• Ventilation may be unnecessary if using an airtight, vapour-open sarking system per manufacturer’s guidance; otherwise maintain a ventilated 50 mm airspace above insulation.
• Ideal for vaulted ceilings.
Cold pitched roof (loft insulation at ceiling)
• Insulation at ceiling plane; loft void ventilated at eaves and (ideally) high-level.
• Simple and cost-effective if you are not using the loft for habitable space.
Room-in-roof (loft conversions)
• Often a hybrid: rigid insulation between/over rafters on the slopes, plus insulation at dwarf walls and flat ceilings. Needs continuity at steel beams, dormers and around roof windows.
6) Choosing the right roof for your extension
• Planning & streetscape
• Many UK extensions use flat or mono-pitch roofs to sit below first-floor windows and keep the ridge/eaves low for permitted development.
• In conservation areas or on traditional homes, a dual-pitch with tiles/slate may blend better.
• Daylight strategy
• Flat roofs excel with large rooflights/lanterns or long slot rooflights along party-wall lines.
• Pitched roofs take roof windows well and can give dramatic vaulted interiors.
• Thermal performance & regs
• Aim for a roof U-value around 0.15 W/m²K or better for extensions (designers often target ≤0.13 to be future-proof). The exact requirement depends on your route to compliance under Approved Document L—your designer or energy assessor will set the target for your project.
• Warm roofs typically make hitting the target easier.
• Fire & boundaries
• Near boundaries, choose coverings tested to Broof(t4) and consider parapets; check Part B details for rooflights and separation distances.
• Acoustics
• Close to rail or busy roads? Consider mineral wool layers and solid deck build-ups; terraces/green roofs help damp footfall and airborne noise.
• Buildability & cost
• Flat warm decks are fast and forgiving; pitched tile/slate roofs use common trades but require more carpentry if tying into awkward existing forms.
• Complex ventilation paths make cold flat roofs slower and riskier; most contractors prefer warm or inverted solutions now.
7) Warm vs Cold: quick comparison
Aspect Warm Roof Cold Roof
Insulation location Above deck / at rafters At ceiling (joists)
Structure temp Warm Cold
Ventilation Usually not needed in build-up Essential (void ventilation)
Condensation risk Low (with VCL) Higher in practice
Thermal bridging Lower Higher (cold joists/rafters)
Airtightness Easier Harder
Depth above structure Thicker Thinner
Best use Most new flat roofs; vaulted pitched roofs; terraces; green roofs Simple pitched roofs with unused, ventilated lofts
8) Practical tips that save headaches
1. Decide early: Roof choice affects wall heights, window heads and steel sizes.
2. Draw the section: Even a simple sketch showing finishes–VCL–structure–insulation–membrane will flush out clashes.
3. Mind the dew point: Use the manufacturer’s build-up (or a WUFI/Glaser check) if deviating from standard details.
4. Continuity at junctions: Carry insulation and airtight layers over steel beams, at parapets, and along abutments to avoid cold spots and mould.
5. Falls first: On flat roofs, design the falls/tapered insulation before fixing door thresholds and parapet heights.
6. Rooflights: Keep frames proud of ponding; maintain upstands and integrate pre-formed kerbs with the chosen membrane.
7. Future-proofing: If you might add solar later, size joists and parapets for wind loads and set out cable routes now.
8. Green roof loading: Get structural sign-off—saturated loads add up fast.
9. Drainage access: Always include leaf guards and accessible rodding/inspection points.
10. Documentation: Ask for the system warranty and the as-built build-up in writing.
9) Typical build-ups to copy-paste into a spec (edit to suit)
Flat warm deck (single-ply)
• 12.5 mm plasterboard & skim;
• Airtight layer/VCL sealed to perimeter;
• 18 mm OSB3 deck on C24 timber joists;
• Tapered PIR insulation to achieve target U-value and falls;
• Self-adhesive carrier layer (if required);
• Single-ply membrane with manufacturer-approved trims;
• PPC metal parapet capping;
• External gutter or internal RWO with overflow.
Mono-pitch warm roof (standing-seam metal)
• Plasterboard & skim;
• VCL/air barrier;
• Rafters with between-rafter mineral wool;
• 50 mm continuous over-rafter PIR;
• Breathable sarking/underlay per metal system;
• Counter-battens + seams on deck;
• Eaves and verge flashings;
• Half-round gutter to low eaves.
Cold pitched roof (ventilated loft)
• Plasterboard & skim with well-sealed VCL;
• 270–400 mm quilt insulation at ceiling (build to required U-value);
• 50 mm clear ventilated loft void;
• Rafters, underlay, battens;
• Tiles/slates;
• Continuous eaves ventilation + high-level ridge/roof vents.
10) So… which should you pick?
• Most UK extensions benefit from a warm flat roof or warm mono-pitch, because they:
• fit under existing first-floor windows,
• are easy to detail airtight,
• avoid condensation pitfalls, and
• make rooflights and future PV simple.
• Choose a cold pitched roof only when you truly want a ventilated loft and a conventional, unused roof void.
• For roof terraces or green roofs, go inverted (or a robust warm deck with paving) and design the structure for load and edge restraint.