Roofing felt is a thin flexible waterproof layer laid on a wooden deck to create a low cost watertight roof. Life expectancy is far less than tiled roofs, and is of the order of 20-25 years for better felts, and 5-10 years for the cheapest felt.
Felt is used on both flat and pitched roofs, being more common on flat roofs.
A felt roof consists of the following layers:
- joists, normally timber
- deck, typically chipboard, ply or OSB
- 1,2 or 3 layers of roofing felt
Roof felt is simply fibres in bitumen. Sometimes sand or stone flakes are added to the top surface too. BS 8217:2005 "Reinforced bitumen membranes for roofing", describes best practice for laying most types. This document gives an overview.
Life expectancy of felt depends primarily on the type of fibre used:
- mixed rag: shortest life expectancy, sold as low cost shed felt
- polyester & other plastic fibres: longer lived
- glass fibre: longest lived
Workability depends on the bitumen:
- bitumen: goes hard when cold, and softens in the sun
- can be softened with a blow torch, but hassle to lay in winter
- modified bitumen: stays supple, costs more
- polymer modified backings like SBS and APP make the backing handle and perform more like a rubberised layer
Some felts have various types of stone waste on the top, giving a much nicer appearance than bare black felt. It also helps keep some of the sun's heat off, prolonging service life.
3 types of deck are in common use: chipboard, OSB and ply.
Chipboard is far cheaper than the other two, and is used on sheds. Its not a good choice for habitable use. 18mm is good, 12mm sags badly. Chip has more or less no water resistance, and attention to detailing is necessary to ensure a completely watertight roof. When the roof felt perishes, the chip gets wet and fails rapidly. A water resistant grade of chip is recommended, sometimes green in colour, but experience with this on uk.d-i-y so far seems to show it has quite limited water survivability.
OSB and ply increase roof life expectancy as they survive wetting, but not by a lot, and the cost is higher.
- Choose OSB3, not 1 or 2
- Shuttering or WBP ply are water tolerant, other diy grades aren't. Surface voids on shuttering are filled with bitumen as the felt is laid.
Flat roof decks should be laid at a slight slope rather than totally level. The slope is usually created with timber furring strips on top of the joists, but other options can also be used, such as laying the joists at a slight angle.
If there is metalwork on the deck it should be primed before bitumen is applied.
1,2 or 3 layers are usual. Habitable work should use 3 layers, sheds more often use 2, and occasionally 1.
Underlay and top capsheet layers aren't identical. Underlay lacks a surface covering such as stone chips to block sun, its often thinner, and can be less nicely textured.
Each layer should be used for its intended purpose, but there are occasions when a shed needs to be finished quickly and on minimum budget.
- Capsheet can be used as underlay. If its mineral coated it doesn't stick as well.
- Underlay used as capsheet doesn't last as well or look as good. But its normally sufficient till the next visit to suppliers for capsheet.
1 Layer work
Pitched roof shed felting is occasionally done with one layer, resulting in a weak covering with shorter life expectancy. Given the low cost of budget felts, omitting underlay seems a false economy. Even a cheap underlay can give a smoother support for the top layer and more toughness, reducing any tendency to split.
When economy is necessary, different grades of top and underlay can be used. Since felt is degraded by sun, and in the case of the cheapest felts, water as well, the underlay being an economy type has less effect on roofing life than the choice of the capping felt.
2 Layer work
Underlay felt is glued all over with bitumen in solvent, and nailed in place round the edges and along joins. This protects the capsheet from the cracks/gaps in the deck, and acts as a secondary waterproof layer in case of minor capsheet damage.
Capsheet is then laid, using bitumen adhesive to glue the 2 layers into one stronger sheet.
3 Layer work
Houses should have 3 layer work.
- Layer 1 is a "nail prep" layer in tilers felt - this is the tough woven but slightly open stuff used in older houses. It has good pull through resistance to clout nails. The nails are used all over the surface in a random pattern.
- Layer 2 is underlay bonded with bitumen. No nails are used. The bitumen penetrates the bottom layer, fixing it to the deck too.
- Finally a capsheet is bonded on with bitumen
Direction of Layers
Flat roofs should have the capsheet laid at 90 degrees to the underlay.
There are a few different ways to stick roofing felt down. Clout nails and bitumen in solvent are good options for DIY.
Clout nails can be used on the first layer, and on folded over edges where any water penetration runs out rapidly. Its not good to use them on the top surface where they allow water to enter. (Slight felt movement can break the seal between felt and nail head.)
Bitumen in solvent is widely used to glue roofing felt down. This gives a much better fix than nails, and is used to glue both felt layers into one stronger layer. Nails are used too to hold the felt while it sets.
Hot bitumen is the most popular method used by professional roofers. Bitumen is melted in a metal pot on a gas ring at over 200C, the bitumen is spread onto the deck and the felt applied, pressing it down with a brush. Burners and pots can be hired. The hot sticky bitumen is a serious burn risk, suitable clothing is important.
There are also self adhesive felts now available, at a price.
Torch on felts are becoming increasingly popular. The felt is bonded using heat from a large blowtorch. The felt usually has a heat dispersible membrane on the back, which melts when heated, and the modified bitumen under the membrane liquefies on the surface. The torch is used to heat both the deck and the underside of the roll of felt as it is slowly unrolled. As the felt and deck cool, the bitumen solidifies and makes a very strong bond to the deck. One can only use only torch on felts designed for this - ordinary felt will simply burn or melt and fall apart!
The basic idea is to sweep the torch across the space where the underside of the rolled felt and the deck meet. As the felt surface melts, the felt is unrolled onto the roof. the weight of the felt roll alone is usually ample to ensure good adhesion. If heating is too uneven the felt can be holed.
Mislaid torch on felt can't be relaid, so its laid out first, then rolled up in situ and relaid with the torch.
The cheapest shed felts have relatively short life, aren't adequate for habitable structures, and are better avoided on sheds too. Good quality felts are much longer lasting.
3 layers last longer than 2, which last longer than one.
Ensure the deck is well fixed along all edges and joins. Differential movement at joins is bad news for roofing felt, and tends to result in cracking in time.
On pitched shed roofs, where the 2 deck sheets meet along the top ridge there's a small gap, due to the straight board edges. Filling the gap eliminates another cause of felt cracking. Hot bitumen will do it, or bitumen adhesive plus a little sand.
Toppings can increase felt life by keeping the sun off the felt.
Stone keeps summer sun off the felt, and their open spaced structure allows heat dissipation. Stone covered felt must not be trodden on, or the stones hole the felt. Stone is a useful life prolongong strategy, but doesn't protect the outer edges of the felt. White stone chips reflect more of the heat off.
Stone waste coated felt is another option. This works on pitched roof as well as flat, and avoids the work of lugging stone home and onto a flat roof. It has less heat blocking effect than separate stones.
Solar reflective paints reflect sunlight, but no more than white paint. Grey paint doesn't look that great.
Water topping, done by adding a raised lip round the edge, is obsolete.
Where the deck is still sound, but felt broken, adding a new layer of felt over the top can give the roof a fair bit more life. Glue it to the existing felt. More often the deck is rotten, and the roof needs stripping.
Repairing split felt doesn't last. Once the felt is breaking, it will soon split more. Its possible to seal splits with a patch or gloop, and some people do it.
Gloop: clean the torn felt, apply gloop. As well as roof repair gloops its possible to use bitumen in solvent plus synthetic fibre.
Patch: clean around the tear, apply a patch of new felt with plenty of bitumen in solvent. Or torch-on felt can be applied with a blowtorch.
- Insulation fitted under the deck creates a cold deck.
- Insulation between the joists leaves the joists uninsulated
- can be done with cheap fibreglass
- Ventilation of the cold timber is advisable to avoid condensation & rot.
- Rigid insulation fitted atop the joists creates a warm deck
- sheet timber sits on top of the insulation
- bonded sheet timber plus insulation in one is available
- can create a higher level of insulation
- Rigid insulation costs more
- Ventilation of timber not needed
Where a plaster ceiling is fitted, loosefill insulation can be blown into the gap through a small hole or holes. No disassembly or rebuild required, and loose fill insulations are relatively cheap. The space above the loosefill should have a little ventilation to the exterior to avoid condensation & rot - but not enough to blow the insulation about.
The precursor of roofing felt is the tarred roof. Tar and sand were applied to a boarded pitched roof.
Another precursor of roofing felt was the paper roof. This was once used in Scoltand & some other areas, and was simply layers of tarred paper. Small repairs could be done with a household iron.
When flat felt roofs were first introduced, a raised lip was sometimes used round the edges, designed to hold a thin layer of rainwater. This water cooled the bitumen in hot weather, protecting it against the sun's heat to prolong its life.
There are several more expensive alternatives to felt, such as GRP, butyl rubber, lead, aluminium, and copper. There are also various rigid roofing sheets, such as corrugated sheets in steel, plastic, composition and fibre cement, and hollow polycarbonate roof sheet. And finally roof tiles in concrete, slate or terracotta.