How to Install Roofing Felt

Roofing felt is one of the simplest and most cost effective ways to keep weather out and extend the life of a roof. It is widely used on garden sheds and outbuildings as a primary waterproof layer, and it also appears under tiles and slates on pitched house roofs as an underlay. Knowing how to install roofing felt correctly will save money, reduce the risk of leaks, and keep maintenance straightforward. This guide explains the tools you need, the right methods for different roof types, the rules that apply, and the costs you should plan for, all in clear UK terms.

What roofing felt is and where it is used

Roofing felt is a flexible sheet that resists water and protects the structure beneath. On sheds and small outbuildings it is usually the visible top layer, fixed to timber boards and finished with trims at the edges. On a tiled or slated roof it sits below the outer covering as a secondary barrier, catching wind driven rain and helping to control draughts. On flat roofs there are specialist felt systems that create a durable waterproof membrane when installed by a competent contractor.

For most homeowners the phrase roofing felt means one of three things. Traditional shed felt made from bitumen and mineral granules for pent or apex sheds. Self adhesive or cold applied felt for small flat roofs. Torch on bitumen systems for larger flat roofs which should be fitted by trained operatives for fire safety. Underlay felt for pitched roofs which is fitted during re roofing under tiles or slates. The installation steps differ, so it is important to match the method to the product.

Tools, materials, and preparation

A successful job starts with preparation. For a shed you will need a hammer, galvanised clout nails in the recommended length for your felt, a sharp knife with spare blades, a straight edge, a tape measure, a chalk line, and a sealant suitable for bituminous products. Many sheds benefit from timber drip battens and metal or PVC edge trims to carry water cleanly into the gutter. For self adhesive felts you will also need a clean roller to press the laps. For cold applied systems you will need the correct adhesive or primer as specified by the manufacturer. Avoid mixing brands unless the instructions say they are compatible.

The deck must be sound, dry, and smooth. Replace any rotten boards, refix loose fixings, and brush away dust and grit. Moisture trapped beneath felt shortens its life, so wait for a dry spell. Plan the layout so laps shed water in the right direction. On pent roofs start at the lowest edge and work uphill. On apex roofs start at the eaves on each side and finish with a ridge piece that overlaps both slopes.

Safety and access

Working at height carries real risk. Use a stable platform rather than standing on the roof surface. For single storey sheds a sturdy pair of steps with a handhold is often adequate, but larger jobs may justify a small scaffold tower. Wear cut resistant gloves when handling felt and trims. Never use a naked flame near dry timber cladding or felt unless you are trained and insured to do so. If in doubt, choose self adhesive or mechanically fixed products that avoid heat altogether.

Installing felt on a pent shed roof

Measure the roof and cut the first length with a small allowance for trimming. Offer it up from the eaves so the felt overhangs the front edge by around 40 to 50 millimetres. This forms a drip into the gutter or beyond the fascia. Tack along the top edge at modest centres, then smooth the felt down and fix at regular intervals in staggered rows so fixings are not in a straight line. Keep the sheet flat without stretching it. Cut the next length allowing a lap of around 75 to 100 millimetres over the first sheet. Apply a thin bead of bituminous sealant in the lap before nailing so water cannot creep under. Repeat up the slope until you are within a sheet width of the back edge. Finish with a final overhang that matches the front.

Edge trims give a neat result and protect the felt from wind lift. Fix a timber drip batten at the eaves, then fit metal or PVC drip trim through the felt into the batten. At the back edge consider a raised check batten under the felt to stop water blowing under in strong winds. Neatly trim all edges, seal nail heads where exposed, and check that laps face downhill.

Installing felt on an apex shed roof

Work one side at a time. Fit the first course at the eaves with an overhang for a drip as above. Fix subsequent courses with sealed laps until you reach the ridge. Repeat on the other slope. Finish the top with a dedicated ridge strip cut from the felt roll, wide enough to overlap both slopes by at least 150 millimetres each side. Warm sunshine helps the ridge piece relax so it moulds to the crown. Where the felt meets verge boards, bring it slightly over the edge and cap with a timber barge or a purpose made trim to keep wind out of the laps.

Self adhesive felt on small flat roofs

Self adhesive felts are popular for bike stores, bin stores, and small porches because they avoid flames. Prime the deck if required by the instructions and allow it to dry. Roll out the first sheet to check alignment, then peel back the release film a little at a time while pressing the sheet down. Use a clean roller to consolidate. Laps should be firmly rolled and, where specified, chased with a compatible sealant. Many systems use a base layer and a mineral capping layer. Always follow the sequence and lap sizes set out by the manufacturer.

Torch on felt on larger flat roofs

Torch on felt gives a robust, long lasting finish, but it requires experience and strict safety controls. A typical system has a perforated vent layer, an underlay, and a mineral cap sheet. Joints are sealed by heating the bitumen until a small bead bleeds out. This is specialist work and is best left to accredited installers. If you own a timber outbuilding near fences or planting, a cold applied or self adhesive system is the safer choice.

Underlay felt for tiled and slated roofs

On a house roof the felt underlay is fitted during a re roof or a new build beneath battens and the outer covering. It controls wind uplift, sheds rain that gets past the tiles, and helps the roof breathe when used with the right ventilation. Strips are rolled horizontally from eaves to ridge with the printed side uppermost, and are fixed to rafters with staples or nails before battens are installed. Eaves support trays or tilting fillets keep the felt from sagging into the gutter. Counter battens or ventilated laps are used where required by the design to keep air moving beneath the covering. Installing or replacing underlay on an existing house roof is not a small repair. It forms part of the re roofing process and should be undertaken by a competent roofer who understands loading, fixings, and the ventilation strategy for the loft.

Correct laps, fixings, and details

Laps are the most common source of leaks when felt is fitted poorly. Keep laps generous and consistent. Lay sheets straight, avoid wrinkles, and seal end laps as well as side laps. Use galvanised or stainless fixings so they do not rust through. Where vents, flues, or roof windows penetrate the felt, cut tight, dress the felt up the sides, and use compatible flashings so water is led back onto the main surface. At eaves and verges, trims and neat overhangs matter. Water should always have a clear path to a gutter or a drip edge.

When to install, and how weather affects the job

Felt performs best when installed in dry, mild weather. Cold makes it stiff and hard to form. Heat can make it too soft to cut cleanly. Aim for a dry forecast so you are not trapping moisture under the layers. If the deck gets wet, allow it to dry before covering. Do not leave bare timber exposed overnight if heavy rain is due.

Costs you should expect

For a typical garden shed, a roll of mineral felt costs between £25 and £60 depending on thickness and brand. Many sheds take two to three rolls, so materials often sit between £50 and £150. Edge trims and fixings add another £20 to £60. If you hire a tradesperson to re felt a medium shed, expect £150 to £350 for labour depending on access and condition of the deck. Self adhesive flat roof felts for small stores cost more per square metre but save on labour time and avoid flame risks.

For flat roofs fitted by a contractor, a two or three layer felt system usually sits in the range of £40 to £70 per square metre for materials and £80 to £120 per square metre supply and fit, depending on complexity, insulation, and access. Re roofing a tiled house where new underlay is part of the work is priced across the whole job. A typical three bedroom house might sit between £5,000 and £12,000 depending on tile choice, scaffolding, and details. Within that total, the underlay and battens account for a modest share, but fitting them correctly is essential for a durable result.

Building regulations, planning, and good practice

Replacing shed felt like for like is classed as maintenance and rarely needs planning permission. Outbuildings that meet normal height and placement rules usually fall within permitted development. If the property is listed or in a conservation area, check with the council before you change external appearance.

House roofs are different. Replacing the main roof covering or changing the build up can trigger building control notification. The work must meet fire performance, loading, and energy efficiency standards. A competent roofer can self certify through a recognised scheme or you can apply to the local authority for inspection. Where a flat roof is insulated during refurbishment, the thermal performance must meet the current target, which can change the build up height and require careful detailing at edges to maintain falls and thresholds. Always consider safe access and waste management. Old felt and contaminated debris should go to a licensed facility rather than into general household waste.

Common mistakes and how to avoid them

Rushing laps, using the wrong fixings, and skimping on trims are the quickest paths to failure. Stretching felt to make it reach will cause it to shrink back and open joints later. Fixing only at edges allows wind to lift sheets. Cutting corners on preparation traps defects under the new layer. Take your time, cut cleanly, and check each course before moving to the next. If the deck flexes underfoot, replace it before you cover it. Good edges and tidy terminations are the small details that make a roof last.

Maintenance to extend service life

A well fitted felt roof gives years of service if you keep it clear. Sweep off leaves and twigs that hold water. Keep overhanging branches in check so they do not abrade the surface. Check laps and trims after winter. Promptly seal small nicks before water finds a path. On sheds, a simple re cap of the ridge strip after several seasons can keep the whole covering working well.

When to call a professional

Comfort with tools and a clear, single storey working area make shed felting a reasonable DIY job. Anything that involves hot works, complex detailing, or work at height on the main house should be left to an insured contractor. Professionals bring safe access, the right primers and sealants, and the knowledge to handle junctions around chimneys, roof windows, and parapets. The added cost buys durability and peace of mind.

Conclusion

Installing roofing felt is a practical way to protect a shed, upgrade a small flat roof, or form the underlay on a re roofed house. The core principles are simple. Start with a sound, dry deck. Lay straight courses with generous, sealed laps. Fix securely with the right nails or adhesives. Finish edges with proper trims that throw water clear. Plan for safety and compliance where the main house is concerned. With careful preparation and tidy workmanship, felt will keep water out and value in, giving you a neat finish and reliable service for years.