how to felt a flat roof
Share
Felting a flat roof is a long established method of waterproofing garages, extensions, sheds, and outbuildings across the UK. A properly installed felt roof can last many years provided it is laid on a sound deck with correct layering, adhesive application, and heat sealing. Although homeowners often consider felting a DIY possibility the work must be done carefully because flat roofs are vulnerable to pooling water, wind uplift, and leaks if installed incorrectly. This guide explains how to felt a flat roof safely and effectively following modern UK roofing standards, best trade practice, and practical experience.
You will learn what materials are needed, how to prepare the roof deck, how to apply each layer of felt, how to use a torch on system safely, and how to detail edges, corners, and upstands. The guide also covers cold adhesive systems for homeowners who prefer not to use a gas torch. You will also see common mistakes, real world examples, costs, lifespan expectations, and when it is better to call a professional roofer.
If you want to understand how to felt a flat roof the right way this detailed walkthrough will give you the knowledge to complete the work confidently.
Understanding what roofing felt is
Roofing felt is a multi layer bitumen product reinforced with polyester or glass fibre. It provides a waterproof layer that forms a hard wearing surface when bonded together in sheets. Traditional felt roofs used three layers though modern torch on systems often use two specialised layers that offer improved performance.
Roofing felt works by creating a continuous bonded membrane. Overlaps, upstands, and edges are sealed with heat or adhesive so water cannot penetrate beneath the surface. Most UK garages and extensions still rely on felt because it is affordable, durable, and straightforward for trained installers.
Who benefits from a felt roof
• Homeowners repairing old garage roofs
• Those replacing worn asphalt or mineral felt
• DIY renovators needing a cost effective waterproofing solution
• Landlords maintaining outbuildings
• Tradespeople repairing small flat roofs on porches or utility rooms
Felt is suitable for most timber and concrete deck roofs as long as the structure is sound and has enough fall to shed water.
Safety before starting
Felting a roof involves working at height. You must take proper precautions.
• Use stable ladders and secure access
• Keep the working area dry
• Do not work in high winds
• If using a torch ensure full fire safety awareness
• Have a fire extinguisher within reach
• Avoid working alone
Torch on systems require experience and care. If unsure use a cold applied adhesive system instead.
What tools and materials you need
• Torch on felt rolls or cold applied felt rolls
• Underlay felt and cap sheet
• Gas torch and cylinder if using hot bonding
• Bitumen primer
• Cold adhesive if not torch bonding
• Stanley knife
• Straight edge
• Hammer and clout nails
• Roofing deck boards if replacement is required
• Mineral felt shingles for detailing edges if needed
• Brush or roller for primer
• Aluminium drip trims
• Gloves and safety glasses
Choosing high quality materials makes a big difference to the lifespan of the roof.
Inspecting and preparing the roof deck
Before any felt can be applied the deck must be sound, clean, dry, and free from rot.
Step 1: Remove old felt
Strip the old felt entirely. Leaving old layers in place makes bonding difficult and hides damage.
Cut the old felt into manageable sections. Lift it using a pry bar and remove rusted fixings. If the old roof has pooling water marks or sagging areas you must investigate the deck beneath.
Step 2: Inspect decking boards
Most garages use OSB or plywood sheets. Replace any that are swollen, soft, rotten, or delaminating. A poor deck will cause the felt to fail quickly.
Boards must be fixed firmly to the joists with no movement.
Step 3: Check roof fall
Flat roofs are never completely flat. They require a slight fall typically around 1 in 80 to allow rain to drain. If the fall is incorrect water will pool and shorten the roof lifespan.
Step 4: Clean and prime
Sweep the deck then apply a bitumen primer. This helps the felt bond properly especially on older decking.
How to felt a flat roof: step by step
This method explains both torch on and cold applied systems. Choose the method that suits your skill level.
Step 1: Install drip trims and edge details
Fit metal drip trims to the lower edge so water runs off cleanly. Fix them with clout nails. Any upstand areas such as walls or parapets must be prepared with a timber fillet that allows the felt to curve smoothly without cracking.
Step 2: Lay the first underlay layer
Roll out the first layer of underlay felt across the roof to relax the material. Then roll it back halfway.
For torch on felt:
Heat the underside lightly with the torch until the bitumen starts to melt then roll it forward. Press firmly to bond.
For cold applied felt:
Spread adhesive onto the deck then roll the felt into it while pressing out air.
Ensure each roll overlaps the previous by at least 75 mm on side laps and 150 mm on end laps.
Step 3: Nail perimeter edges
Use clout nails only around the perimeter or where specified. Never nail through the centre of the roof. Nails should be covered by later layers.
Step 4: Lay the second underlay or intermediate layer
Depending on the system you may install a second underlay to improve strength. Stagger the joints so overlaps do not line up with the first layer.
Torch or glue this layer the same way as the first.
Step 5: Install the cap sheet
The cap sheet is the visible top layer usually finished with mineral granules that protect the felt from UV light.
Roll out the sheet to relax it then apply it in the same direction as previous layers.
Torch on application requires controlled heat to avoid burning the felt. The edges should ooze a small bead of molten bitumen when bonded correctly.
Cold applied adhesives must be spread evenly on the underlay before bonding.
All overlaps must be sealed carefully because this is the main waterproof layer.
Step 6: Upstands and corners
Upstands must go at least 150 mm up adjacent walls according to good practice. The felt should curve over the timber fillet and be fixed with a secure termination bar or covered with lead flashing slightly above the membrane.
Corners require small pre cut patches to ensure full weatherproofing.
Step 7: Check for bubbles and imperfections
Walk the roof and look closely for:
• Air bubbles
• Gaps in overlaps
• Areas where bitumen did not flow
• Loose edges
Use the torch or adhesive to correct them immediately.
Step 8: Finish edges
Fix metal edge trims where required. These protect the felt from wind lift and provide a clean finish.
How to felt a flat roof with cold adhesive only
Some homeowners prefer cold applied systems because they avoid using a gas torch.
Steps:
- Apply bitumen primer
- Spread adhesive with a serrated trowel or roller
- Lay first underlay sheet
- Leave to cure
- Apply adhesive for second layer
- Fit cap sheet with final adhesive layer
- Seal all overlaps thoroughly
Although cold applied systems are safer the curing times are longer and work must be done in dry weather.
Common mistakes when felting a roof
Poor preparation
Trying to felt over rotten wood or damp surfaces leads to bubbling and early failure.
Incorrect fall
If water cannot run off the roof it will form puddles that stress the felt.
Insufficient overlap
Overlaps smaller than recommended lead to leaks.
Overheating the felt
Torches should be used lightly. Burning the felt weakens the membrane.
Leaving gaps at edges
Wind uplift is a major cause of failure. Edges must always be fully bonded or mechanically fixed.
Building regulations and planning considerations
Replacing or felting a roof usually does not require planning permission unless:
• The building is listed
• The roof height is being altered
• The structure is within a conservation area with special conditions
Building regulations apply when:
• The roof decking is being replaced
• The structure is being insulated
• The garage forms part of a habitable space
Flat roofs over heated areas must meet insulation standards under Part L. Garages usually do not require insulation unless being converted.
How long a felt roof lasts
A well installed modern felt roof typically lasts:
• 10 to 15 years for basic felt
• 15 to 20 years for high performance felt
• 20 to 30 years for premium reinforced systems
Lifespan depends on installation quality, exposure, maintenance, and drainage.
Maintenance tips to keep a felt roof lasting longer
• Clear moss and debris regularly
• Keep gutters clean
• Inspect for blisters each spring
• Reseal edges if required
• Maintain nearby trees to reduce leaf fall
Regular checks help avoid water ingress that could damage the decking.
Real world UK scenarios
Scenario 1: Garage roof replacement
A homeowner with a 1970s garage finds that the original felt has cracked. After stripping it they discover rotten OSB. Replacing the deck and installing a modern two layer torch on system resolves the leaks and extends the lifespan significantly.
Scenario 2: Garden outbuilding used as workshop
A detached wooden shed with a felt roof suffers from minor leaks at the edges. The owner replaces the single felt layer with a multi layer cold applied system which improves durability without torch use.
Scenario 3: Extension roof alongside house wall
A flat roof on a rear extension shows water marks near the wall junction. The issue is traced to poorly installed upstand felt. Replacing the section and fixing the flashing eliminates the leak.
Cost of felting a flat roof in the UK
Costs vary depending on size, condition of the deck, and the chosen felt system.
Roofing felt costs
• Underlay felt: £20 to £40 per roll
• Cap sheet: £30 to £60 per roll
• Torch on high performance felt: £40 to £80 per roll
Labour and installation costs
• Small garage roof (15 to 20 m²): £600 to £1,200
• Medium garage or outbuilding (20 to 30 m²): £1,000 to £1,800
• Large flat roof (30 to 50 m²): £1,800 to £3,000
Deck replacement costs
• New OSB or plywood deck: £500 to £1,500 depending on size
DIY costs are lower but torch on roofing should only be attempted if you are confident and safe.
Alternatives to traditional felt roofing
Although felt is popular several alternatives exist.
EPDM rubber
A single sheet membrane with no seams on small roofs. It lasts 20 years or more but must be installed correctly.
GRP fibreglass
Durable and attractive but can crack if not laid well.
Liquid roofing systems
Applied with rollers providing a seamless finish.
Metal roofing sheets
Useful for sheds or workshops but noisier in rain.
These alternatives vary in cost and durability but felt remains one of the most cost effective options.
When to hire a professional instead of doing it yourself
You should hire a professional roofer if:
• The roof is large
• The deck is extensively damaged
• The garage is attached to the house
• You are uncomfortable using a torch
• The roof has complex detailing or multiple upstands
Professionals follow safe working practices and can provide warranties.
Conclusion
Learning how to felt a flat roof is valuable for homeowners maintaining garages and outbuildings across the UK. A correctly installed felt roof offers strong waterproofing, good durability, and a budget friendly solution for most flat roof structures. The key is proper preparation, correct layering, careful bonding, and attention to details such as edges and upstands. Whether using a torch on system or cold applied adhesive the principles remain the same. With the right approach you can create a long lasting weatherproof roof that protects your building for many years.