How to repair a flat roof

Flat roofs are common across the UK in garages, porches, extensions, garden buildings, and many commercial structures. Although modern membranes are far more durable than older felt systems every flat roof can eventually develop faults such as cracks, blisters, ponding water, or loose edges. When these issues are repaired early the roof can last many more years. When they are ignored small defects can become costly leaks that damage insulation, ceilings, and structural timber. Repairing a flat roof correctly is essential for protecting your property and preventing avoidable deterioration.

This guide explains how to repair a flat roof in the UK including how to diagnose faults, identify the roofing material, choose the correct repair method, and complete repairs safely. It covers the most common roofing materials used in UK homes including felt, EPDM rubber, fibreglass, asphalt, and single ply systems. Real world examples, safety advice, tools, costs, and long term maintenance tips are also included. By the end you will know exactly how to approach a flat roof repair and how to restore the waterproof layer effectively.

Understanding what repairing a flat roof involves

Repairing a flat roof means fixing damage to the waterproof membrane so that water cannot penetrate the surface. Repairs vary in scale. Some repairs involve patching a small tear in the membrane or sealing a loose joint. Others involve applying a new surface coating across the entire roof. More serious structural damage may require replacing sections of the roof deck or installing a new membrane.

A flat roof is made up of the following components:

A roof deck
A waterproof membrane
Flashings and trims
Insulation either above or below the deck
Drainage outlets and gutters

To repair a flat roof effectively you must ensure the membrane is sound and that no water is reaching the deck beneath it. If the deck becomes saturated it can rot or weaken which requires more extensive repairs.

Who needs to know how to repair a flat roof

This information is useful for homeowners managing a garage, extension, or porch that has developed leaks. It also helps landlords who must maintain rental properties. Builders tackling small roof repairs benefit from understanding correct methods for different materials. Homeowners considering DIY repairs need to know which jobs they can do safely and which should be handled by a roofing professional.

People purchasing a home with a flat roof also benefit from this knowledge because it helps them identify which issues are minor and which indicate deeper problems.

Identifying the type of flat roof before repairing it

The method of repair depends heavily on the roofing material. You should identify the roof type before choosing how to repair it.

Felt roof

Felt is a traditional bitumen based material. It often has a granular surface. Older felt roofs may have multiple layers and visible seams.

EPDM rubber roof

EPDM is a smooth rubber sheet with almost no joints. It feels like thick rubber and often comes in a single continuous piece.

Fibreglass roof (GRP)

Fibreglass has a rigid, smooth, shiny surface. It is made from resin and matting with a topcoat.

Single ply membrane

These are flexible PVC or TPO sheets with heat welded seams. They often look smooth and uniform.

Asphalt roof

Asphalt roofs are heavy and solid. They have a smooth dark finish often on older flat roofs.

Each type of roof needs a different repair approach. Incorrect repair materials can cause further damage.

Common problems found in UK flat roofs

Before carrying out repairs you must diagnose the cause of the problem. Typical issues include:

Cracks
Often found in felt membranes or older asphalt roofs.

Splits
EPDM can split if stretched around sharp edges or if punctured by debris.

Blisters
Air or moisture trapped under felt membranes cause raised bubbles.

Loose edges or flashing
Wind and weather can lift edges over time allowing water ingress.

Ponding water
Areas where water does not drain away indicate insufficient fall or sagging.

Pinholes
Tiny holes in fibreglass or felt allow slow leaks.

Loose or cracked joints
Seams and joins between layers can deteriorate.

Leaks around rooflights or vents
Poor flashing or wrong sealing materials cause issues around penetrations.

Understanding the defect helps select the correct repair.

Safety considerations before repairing a flat roof

Working on a flat roof requires careful preparation. Although flatter and easier to stand on than pitched roofs they still present risks. Follow these safety steps:

Ensure the roof deck is stable
If it feels soft or spongy stop immediately because the deck may be unsafe.

Avoid working in rain or frost
Membranes become slippery and adhesives will not bond in wet conditions.

Use stable access equipment
Use a secure ladder or scaffolding tower with someone available to assist.

Protect yourself
Wear gloves, non slip footwear, and eye protection when using tools or adhesives.

Do not walk on weak areas
If blisters or cracks are present step only where the deck is solid.

Avoid working alone
Have someone nearby in case assistance is required.

Once the roof is safe you can begin the repair process.

Tools and materials needed for flat roof repairs

The exact tools vary depending on the material but typically you will need:

Brushes and rollers
Roofing adhesive or primer
Patches of matching membrane
Scissors or knife for EPDM
Resin and topcoat for fibreglass
Bitumen paint for felt
Sealant for flashing
Heat gun for some single ply repairs
Buckets and mixing tools

Professional roofers have specialist equipment for heat welding and resin mixing. DIYers should stick to repair products designed for their skill level.

How to repair a felt flat roof

Felt roofs are the most commonly repaired because they are the most vulnerable to wear. Here is a step by step approach to repairing typical felt roof issues.

Step 1: Clean the area

Brush away debris and remove any moss or dirt. Clean surfaces give better adhesion.

Step 2: Locate the damaged area

Look for cracks, blisters, splits, or loose joints.

Step 3: Cut open blisters

If you find a blister cut across the raised area to release trapped moisture. Dry the area completely.

Step 4: Apply bitumen

Use bitumen mastic to seal the cut blister or crack. Spread it evenly.

Step 5: Add a patch

Cut a piece of felt slightly larger than the damaged area. Apply bitumen adhesive and press the patch down firmly.

Step 6: Seal the edges

Coat the patch edges with bitumen to create a watertight seal.

For larger repairs multiple layers may be required.

How to repair an EPDM rubber roof

EPDM is one of the easiest flat roofs to repair because patches bond well to the membrane.

Step 1: Clean the damaged area

Wash with a suitable EPDM cleaner. Remove dirt and dust.

Step 2: Cut a patch

Use EPDM repair material. The patch should extend beyond the damaged area.

Step 3: Apply primer

EPDM repairs require a primer to help the patch bond.

Step 4: Apply the patch

Peel the backing and press the patch firmly over the damaged section.

Step 5: Roll the patch

Use a roller to remove any air pockets and strengthen the bond.

EPDM repairs are long lasting when done correctly.

How to repair a fibreglass flat roof (GRP)

Fibreglass repairs require resin and new matting.

Step 1: Sand the damaged area

Use sandpaper to roughen the surface so resin can bond.

Step 2: Clean the surface

Remove dust and debris.

Step 3: Apply resin

Lay fibreglass matting over the area then apply resin with a brush or roller.

Step 4: Add topcoat

Once cured apply a topcoat to match the existing roof.

Fibreglass must be repaired in dry weather because moisture affects curing.

How to repair a single ply flat roof

Single ply membranes such as PVC or TPO require specialist repair tools so DIY repairs must use manufacturer approved patch kits.

Step 1: Clean the surface

Remove dirt using approved cleaning solution.

Step 2: Cut a patch

Use matching membrane where possible.

Step 3: Heat weld or use adhesive

Some kits require hot air welding others use solvent based adhesives.

Step 4: Seal edges

Press the patched area with a roller.

Single ply repairs last well when completed correctly.

How to repair an asphalt flat roof

Asphalt roofs are strong but cracks appear over time.

Step 1: Clean the area

Remove loose material.

Step 2: Fill cracks

Use asphalt repair compound.

Step 3: Recoat

Some repairs require a liquid asphalt coating to waterproof the area.

Large cracks may require a professional.

Repairing leaks around flashings and roof penetrations

Leaks often occur around rooflights vents and walls rather than the main surface.

Step 1: Inspect flashing

Look for cracks or loose edges.

Step 2: Remove old sealant

Old sealant must be scraped away.

Step 3: Apply new sealant

Use a roof grade sealant suitable for your roof type.

Step 4: Add reinforcement tape

Some systems use tape to strengthen the joint.

Good flashing repair can prevent repeated leaks.

Repairing ponding water issues

Standing water can cause leaks even if the membrane is intact. Solutions include:

Adding new outlets
Creating new drainage routes
Using levelling compounds
Adding tapered insulation to create fall

These repairs are best handled by professionals because they affect the structure of the roof.

Real world UK flat roof repair examples

Small felt crack repair on a garage

A homeowner notices a small leak along a seam. A simple felt patch applied with bitumen prevents further water ingress and extends life by several years.

EPDM tear from dropped tool

A small tear from garden equipment is repaired with an EPDM patch kit in under an hour.

Fibreglass crack on a dormer

A hairline crack is sanded and laminated with new fibreglass matting then topcoated. The repair blends with the roof.

Ponding issue on an extension

A roofer installs tapered insulation to create proper drainage which prevents future deterioration.

Flashing repair on a kitchen extension

Cracked lead flashing is replaced preventing water from running behind the membrane.

When a flat roof can be repaired and when it must be replaced

Repairs are suitable when:

The roof is relatively new
Damage is isolated
The membrane is still flexible
The deck is dry and intact

Replacement is usually required when:

The membrane is brittle or cracked across large areas
The roof is older than its expected lifespan
The deck has rot
Ponding water cannot be resolved easily
Multiple leaks have formed

Knowing when repair is no longer cost effective prevents money being wasted on short term fixes.

Costs of repairing a flat roof

Costs depend on the roof type and scale of repair.

Felt patch
£80 to £250 depending on size

EPDM repair patch
£90 to £180

Fibreglass repair
£150 to £300 depending on resin work

Flashing repair
£120 to £350

Ponding correction
£300 to £900 depending on drainage work

Full replacement
£40 to £150 per square metre depending on material

DIY repairs reduce labour costs but must be done properly.

Planning permission considerations

Repairing or replacing a flat roof like for like does not require planning permission unless:

You increase the height significantly
You add a balcony or terrace
You alter roof drainage affecting neighbours
You live in a listed building or conservation area

Repairs that maintain the existing structure do not usually trigger planning issues.

Maintenance tips to prevent future repairs

Clean debris regularly
Trim trees that drop leaves
Check roof annually after winter
Sweep away standing water
Repair small cracks promptly
Avoid walking on the roof
Keep gutters working correctly

Minor care significantly extends lifespan.

When to call a professional roofer

You should seek professional help when:

Leaks are persistent
The deck may be compromised
You need warm roof upgrades
The membrane requires heat welding
The roof will be accessed frequently
You want a long warranty

Professionals ensure correct materials and installation methods which protect your investment.

Final thoughts

Flat roof repairs are straightforward when defects are identified early and the correct repair materials are used. Each roofing material requires a different approach and a careful assessment ensures the repair lasts. Whether patching a small felt split, fixing a rubber tear, repairing fibreglass cracks, or improving drainage, repairing a flat roof promptly prevents leaks from becoming structural problems. With regular maintenance and correct repair techniques your flat roof can remain reliable for many years.