Install a flat roof.

Whether you are replacing a failing roof or installing one on a new build, choosing the right material and ensuring correct insulation is vital for both weatherproofing and meeting UK building standards. This guide breaks down the costs, timelines, and technical requirements for a successful installation.

What Does the Work Involve?

• Site Inspection & Stripping: Assessing the existing deck for rot and removing old materials, including chippings and felt.
• Deck Preparation: Repairing or replacing timber OSB boards to ensure a stable, flat surface for the new membrane.
• Insulation Installation: Fitting high-performance PIR insulation boards (usually 120mm+) to comply with current "Warm Roof" regulations.
• Vapour Control: Laying a vapour control layer (VCL) to prevent internal condensation from reaching the roof structure.
• Membrane Application: Laying the primary waterproofing layer (EPDM, GRP, or high-performance felt) and securing it with adhesives or heat.
• Detailing & Flashing: Installing edge trims, wall flashings, and sealing penetrations like skylights or vent pipes.
• Guttering: Reinstalling or replacing rainwater goods to ensure efficient drainage.

Typical Costs

Prices for flat roofing vary based on the material chosen and the complexity of the roof. The following estimates include labour, materials, and VAT for a standard 20m² garage or extension roof.

Factors affecting the price include the height of the building (scaffolding costs), the condition of the underlying timber joists, and the number of "details" like roof lanterns or chimneys that require complex sealing.

How Long Does It Take?

• Small Porch or Bay Window: 1 day.
• Standard Single Garage (approx. 15-20m²): 1 to 2 days.
• Large Kitchen Extension with Skylights: 2 to 4 days.
• Full Strip and Deck Replacement: Adds 1 day to the total duration.

DIY or Professional?

While DIY kits for EPDM are available, professional installation is highly recommended for flat roofs. Errors in adhesive application or flashing detailing lead to leaks that are difficult to trace and can rot your home's structural timbers.

Professional roofers provide insurance-backed guarantees and can self-certify their work for Building Regulations, which is essential when you come to sell your property.

Choosing the Right Tradesperson

• Check Accreditations: Look for NFRC (National Federation of Roofing Contractors) or CompetentRoofer scheme members.
• Verify Insurance: Ensure they have Public Liability insurance and specifically "Heat Insurance" if they are using torch-on felt.
• Request References: Ask to see photos of completed projects that are at least 2-3 years old to see how the roofs are weathering.
• Questions to Ask:
  • Will you be replacing the decking boards or just overlaying them?
  • How do you intend to create the "fall" to ensure water drains away?
  • Does the quote include the cost of a skip and all waste disposal?
  • What is the length of the manufacturer's warranty and your labour guarantee?

UK Regulations

• Part L (Conservation of Fuel and Power): If you replace more than 50% of a roof, you must upgrade the insulation to meet current thermal standards.
• Building Control: Most roof replacements require notification to Local Authority Building Control unless the contractor is registered with a Competent Person Scheme.
• Fire Safety: Materials must meet BS 476-3 or EN 13501-5 standards regarding external fire penetration.

Common Problems

• Ponding Water: If the roof is too flat, water sits in puddles. While modern membranes can handle this, it adds weight and encourages silt buildup.
• Poor Flashing: Most leaks occur where the roof meets a wall. Incorrectly installed lead or GRP trims are common failure points.
• Trapped Moisture: Installing a new roof over wet insulation or damp timber will cause "blistering" and eventual rot.
• Inadequate Expansion Gaps: GRP roofs can "crack" or "pop" if they aren't installed with sufficient room to expand and contract in the sun.

Frequently Asked Questions