Quick checklist – what a correct flat roof includes

If you only have a minute, run through this list. A durable flat roof should have:

• Falls that shed water – design at roughly 1:40 to deliver a finished fall of at least 1:80.
• A solid deck – OSB3 tongue and groove or exterior plywood fixed to sound joists.
• A continuous vapour control layer (VCL) – sealed laps and perimeters to control condensation.
• Insulation sized to current targets – typically achieving around 0.16 to 0.18 W/m2K.
• A quality waterproofing system – EPDM, GRP, liquid or multi-layer felt from proven manufacturers.
• Upstands at least 150 mm above the finished roof level around walls, doors and rooflights.
• Proper drainage – sized outlets with leaf guards and an overflow or scupper where needed.
• Secure edges – drip trims, kerbs and perimeter restraint for wind uplift.
• Ventilation where required – cold roofs need cross-ventilation, warm roofs do not.
• Compliance and documentation – Building Control notified when required and a written warranty.

Call 07311091804 if you want us to check these items on your roof. In many MK postcodes we can attend the same or next day.

Flat roof build types and membranes

Warm, cold and inverted roofs

• Warm roof – insulation sits above the deck, with a VCL directly on the deck and the membrane on top of the insulation. Best choice for most homes because it controls condensation without cross-ventilation and delivers reliable thermal performance.
• Cold roof – insulation between the joists below the deck. It must have continuous cross-ventilation and a well-sealed internal VCL. Tends to be riskier in refurbishments if ventilation is interrupted by extensions or complex layouts.
• Inverted roof – insulation sits above the waterproofing with ballast or paving. Common on terraces and commercial roofs and requires specific loading and drainage design.

Waterproofing systems we install

• EPDM single-ply – lightweight, minimal seams, fast to install, excellent UV stability and 25 to 30+ year life when detailed correctly.
• GRP fibreglass – fully bonded, seamless finish that excels on complex shapes, parapets and rooflight kerbs. Requires dry, stable conditions during installation.
• Reinforced bitumen membrane (felt) – multi-layer systems, torch-on or torch-free, with superb puncture resistance and proven longevity. Ideal for overlays where the deck is sound.
• Liquid-applied systems – ideal for intricate details, awkward penetrations and partial refurbishments.

We specify systems with appropriate external fire performance for your site and surroundings. Where required we select coverings that achieve suitable test classifications for use on domestic properties.

Step by step – building a warm flat roof

1. Survey and design
   • Check structure and joist spans, especially if adding rooflights or heavier finishes.
   • Plan falls and outlet positions to avoid ponding. Tapered insulation is usually the cleanest way to build falls on domestic roofs.
   • Confirm fire performance, wind load and access requirements.
2. Decking
   • Install OSB3 tongue and groove or exterior-grade ply to manufacturer fixing schedules.
   • Ensure a clean, dry and level surface ready for the VCL and insulation.
3. Vapour control layer
   • Lay a continuous VCL over the deck, sealing laps and perimeters.
   • Turn the VCL up at abutments to keep the layer unbroken around the warm side of the build-up.
4. Insulation
   • Use rigid PIR or equivalent. Thickness is designed to meet current targets for thermal performance.
   • Use tapered boards to create drainage falls without packing joists.
   • Fix boards in line with the chosen system – adhered or mechanically fixed.
5. Waterproofing
   • Install EPDM, GRP, felt or liquid membrane exactly to the manufacturer method.
   • Form 150 mm minimum upstands above the finished roof surface at walls, doors and rooflights.
   • Reinforce corners, laps and penetrations and check seams for full bond.
6. Perimeters, trims and wind restraint
   • Fit drip edges, kerbs and metal trims that direct water off the roof and protect edges.
   • Install perimeter restraint to resist wind uplift, especially on exposed plots.
7. Outlets, gutters and overflows
   • Set proprietary outlets at low points. Dress the membrane into the outlet with the correct adaptor.
   • Add an overflow or scupper where required to prevent overfilling during heavy storms.
8. Interfaces and rooflights
   • Use insulated kerbs and maintain 150 mm upstand. Respect damp proof course heights at doors.
   • Flash neatly to walls and parapets with the recommended detail for the chosen system.
9. Quality checks and handover
   • Check moisture levels, lap integrity and drainage. Clear all debris.
   • Provide documentation, warranty and any Building Control paperwork where applicable.

Want this standard on your project? Call 07311091804 to book a free survey.

Cold roofs – essential ventilation rules

Cold roofs place insulation between joists below the deck, leaving the deck cold. That design needs two lines of defence against condensation: an airtight VCL on the warm side and reliable cross-ventilation in the void. Without both, moisture can build up and damage timbers.

• Provide continuous cross-ventilation at opposite sides of the void. Pay attention to eaves details on extensions that join older roofs.
• Install and seal a proper VCL on the warm side of the insulation. Seal around downlights, extractors and cables.
• Avoid blocking air paths with insulation at eaves or by adding storage boards that close vents.

Where possible we recommend upgrading to a warm roof on refurbishments because it is simpler to keep airtight and does not rely on a ventilated void.

The small details that prevent big leaks

• Falls – standing water accelerates ageing. Tapered insulation is straightforward, precise and avoids packing joists.
• Upstands – maintain at least 150 mm above the finished roof level around walls, doors and rooflights. Cutting this down to squeeze a low threshold is a common cause of leaks.
• Thresholds – bi-folds and sliders need thought. We keep membranes out of the splash zone, respect DPCs and use compatible trims and closers.
• Penetrations – pipes, cables, posts and fixings should have proprietary collars or liquid detailing. Avoid silicone-only fixes.
• Parapets – use proper internal linings, cappings and drips. Add overflows to prevent flooding if outlets block.
• Edges and wind – secure trims with the correct fixings, spacings and strap supports. Exposed estates around MK can be windy.
• Rooflights – use insulated kerbs, compatible flashings and factory-finished units. Many leaks start at poor kerbs, not the membrane itself.
• Fire – select coverings and build-ups that deliver appropriate external fire performance for your location and boundary conditions.

Building Regulations and when to notify

For most domestic flat roof refurbishments the key parts to consider are:

• Part A – Structure: check joists, spans and deflection, especially when adding rooflights or heavy finishes.
• Part B – Fire safety: use coverings and build-ups with suitable external fire performance. Consider proximity to boundaries and adjoining roofs.
• Part C – Moisture: design out rainwater ingress and interstitial condensation with a correct VCL, insulation and ventilation strategy.
• Part F – Ventilation: cold roofs require cross-ventilation. Warm roofs rely on airtightness and VCL continuity, not vents.
• Part L – Energy: roof upgrades should target current thermal performance standards for domestic roofs.

If you replace or refurbish more than 25% of a roof, alter structure, add solar or convert a loft, Building Control must be notified. We include compliance in our planning and can liaise with the local authority. Where suitable, we can arrange self-certification through approved schemes and provide a compliance certificate on completion.

Unsure whether your project needs notification? Call 07311091804 and we will advise before you commit to anything.

Choosing materials for Milton Keynes weather

Local weather matters. Milton Keynes experiences freeze-thaw cycles, bright summer sun and some exposed plots that funnel wind across estates and open edges. We match systems to the shape of your roof, access, exposure and your maintenance preferences.

• EPDM – ideal for simple shapes and quick installs. Large sheets mean fewer seams and a tidy look. Excellent UV performance.
• GRP – a rigid, seamless shell that shines on complex edges, parapets and rooflight clusters. Installs best in settled, dry weather.
• Premium felt – multi-layer durability and excellent for overlays where the deck is sound. Great puncture resistance and a long track record.
• Liquids – unbeatable around awkward details, balcony posts, balustrades and plant. Often used as a detailing partner with other systems.

Whichever membrane you choose, success comes from the specification and detailing first – correct falls, robust upstands, smart interfaces and tidy terminations.

Typical costs, timelines and warranties

Every roof is different, but as a local guide:

• Timelines – single garages often 1 day, single storey extensions 1 to 2 days, larger or parapet roofs 2 to 4 days. Add time for rooflights, parapets and decking repairs.
• Cost drivers – access, insulation thickness, tapered schemes, number of outlets and rooflights, parapet details and your chosen membrane. We provide an itemised written quote so you can see exactly what you are getting.
• Warranties – system and workmanship warranties typically range from 10 to 25 years depending on specification and manufacturer partnership.

We will also advise on thermal upgrades so your investment reduces heat loss and running costs.

What to expect on the day

1. Set-up and protection – we protect access routes, set up safe working areas and agree how we will move materials in and out.
2. Strip and inspect – we carefully strip old coverings, check the deck and structure, and replace any damaged boards.
3. Build and detail – we install the VCL, insulation, membrane, trims and outlets, and form neat abutments and rooflight kerbs.
4. Clean-down and handover – we tidy thoroughly, remove waste, test drainage and hand over your warranty and any certificates.

Your property will be respected at every stage and we aim to leave the area cleaner than we found it.

Flat roof FAQs

What is the best flat roof for a typical Milton Keynes home?

For most homes a warm roof with tapered insulation and either EPDM, GRP or a premium felt system delivers the best balance of durability, thermal performance and low maintenance.

Do I need to tell Building Control about my flat roof replacement?

Yes if you are replacing more than 25% of the roof, changing the structure, adding solar or altering thermal performance. We can manage the process and provide the right paperwork once complete.

How much fall should a flat roof have?

Design around 1:40 to achieve a finished fall of at least 1:80. That allowance helps counter timber movement and on-site tolerances so water still drains properly.

Can you overlay my existing felt roof?

Sometimes. If the deck is dry and sound and moisture readings are within tolerance, a specification-led overlay can be safe and cost-effective. We test before recommending it.

How long will a new flat roof last?

With correct design and quality materials: EPDM typically 25 to 30+ years, GRP around 20 to 30 years, premium felt systems 15 to 25 years. Good maintenance extends life further.

Can you add a skylight during the roof upgrade?

Yes. We supply and fit flat roof skylights and lanterns with insulated kerbs and correct 150 mm upstands, fully integrated into the waterproofing.

Why choose Maxwells Roofing

• Family-owned and local – no call centres or faceless subcontracting. You deal with our team from first call to final sweep-up.
• Quality-led – we build to the details that matter: correct falls, airtight VCLs, robust upstands and tidy terminations.
• Proven reputation – 80+ 5* reviews on Checkatrade and 5* on Google Business across MK and the surrounding towns.
• Fast response – same or next day for urgent leaks where possible.
• Clear pricing – written, itemised quotations with no hidden extras.
• Tidy, respectful work – we protect access, keep neighbours informed and leave your property clean.

Ready to stop leaks or plan a long-life flat roof for your home? Call our friendly team on 07311091804 or email sales@miltonkeynesroofing.uk.

Next steps

Book a free roof survey. We will check your deck, measure falls, discuss membranes and insulation options, and provide a clear, fixed quote. While you are here, you might also find these services helpful for keeping your home watertight and efficient: Roof Repairs, Flat Roof Systems, Roof Inspections, Gutter Maintenance and Skylight Installations.