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 5-10 years for cheapest felt and 20-25 years for better felts.
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 # 2 layers of roofing felt
=þlt=Roof felt is simply fibres in [[bitumen]]. Sometimes stone flakes are added to the top surface too.
===Fibre==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
===Bitumen==Workability depends on the bitumen: * [[bitumen]]: goes hard when cold ** can be softened with a blow torch, but hassle to lay in winter * modified bitumen: stays supple, costs more
===Surface stone==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.
==History====Tarred roof==The precursor of roofing felt is the tarred roof. Tar and sand were applied to a boarded pitched roof.
===Paper 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.
===Wet roofs==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.
==Removal=Old felt can be removed with a [[knife]] and [[scraper]], and the wood deck removed with crowbar and [[hammer]]. Sometimes its easier to remove the two together.
=Þck=3 types of deck are in common use: chipboard, OSB and ply.
Chipboard is far cheaper than the other two. 12mm sags badly and can't be recommended, 18mm works ok. 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.
OSB and ply increase roof life expectancy as they survive wetting, but not by a lot, and the cost is higher. * Choose OSB3, not OSB1 or 2 * Shuttering or WBP ply are [[water]] tolerant. Shuttering can have a very rough surface though, and voids are best [[fill]]ed to avoid felt damage.
=þlt layers=2 layers are usual. Underlay and top capsheet layers are very similar but not identical, underlay lacks a surface covering such as stone chips to block sun, and can be less nicely textured. * Capsheet can be used as underlay without problem * Underlay used as capsheet is likely to look inferior and not last as well.
[[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, reducing tendency to split along decking joins.
Where 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.
==Fixing=Clout [[nails]] can be used on the underlay, and on folded over edges where any [[water]] penetration runs out rapidly. Its not best 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.)
Roof structures too light to have clout nails [[hammer]]ed in, ie some sheds, can use waferhead [[screw]]s instead of clout nails, though they're seldom used.
[[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.
Felt is sometimes applied with a blowtorch. This melts the felt surface, making it stick to the deck.
Finally there are self [[adhesive]] felts now available, at a price.
==Toppings=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.
Stone waste pre-adhered to the 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 [[paint]]s don't reflect any more sunlight than white paint. [[Sand]] surfaced roofing can use [[Limewash|lime paint]].
[[Water]] topping, done by adding a raised lip round the edge, is obsolete.
==Repairs====More felt==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 [[rot]]ten, and the roof needs stripping.
===Gloop==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.
==Insulation====New roof==Warm deck v cold deck: * [[Insulation]] fitted under the deck creates a cold deck. ** [[Insulation]] between the joists leaves the joists uninsulated ** can be done with cheap fibreglass * Rigid insulation fitted atop the deck creates a warm deck, and can create a higher level of insulation.
===Retrofit==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.
==Alternatives=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.
==See also=* [[Bitumen]] * [[Insulation]]
[[Category:Basics]] [[Category:Construction]] [[Category:Roofing]] [[Category:Sheds]]