Wiki: Roofing felt

Another one for feedback...
NT
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
=lt=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.
=ck=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.
=lt 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]]
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On 22/12/2010 14:15, Tabby wrote:

Two or three. A traditional felt roof would have a layer of underlay nailed to the deck, to allow for movement between the two, a layer of underlay bonded to the first layer and a top layer bonded to the second.
Colin Bignell
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Nightjar wrote:

The modern torch on 3 layer stuff has a perforated underlay loose laid on the deck. The second layer has a bitumen layer to which a burn off chemical has been added, the torch melts this and the roll is skilfully unrolled surfing a wave of molten bitumen which adheres to the deck through the perforations. Wear welders gauntlets. Cap layer is done in the same manner but at right angles. The chap I watched rolled out the felt first for correct alignment and then re rolled prior to torching.
I tried a bit myself but ended up burning through on one edge and failing to melt the far edge ;-)
I learnt later that any lead flashing under the felt should be suitably primed first.
AJH
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On 22/12/2010 14:15, Tabby wrote:

1 to 3 layers really...

Might want to mention bit more info on the polymer modified backings like SBS and APP - they make the backing handle and perform more like a rubberised layer...

Another option is the silver reflective paints you can apply to unfinished felt. Better sun reflection, although not as nice to look at.

I think I would stick this at the end, it seems to interrupt the flow here...

Flooring grade (green finish) chipboard is water tolerant...

The number of layers is influenced by the fixing method as well.
Underlays are often thinner than the capsheet - 2 and 4mm respectively is not uncommon.

Its harder to get a good bond to if its mineral clad.

Making sure you deck is well fixed at the edges helps as well.

Traditional nailed felt is done in three layers, with three different types of felt. The first would use 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. Then you have a bonded underlay - this will have no through holes from nails at all, and finally a bonded capsheet.

The primary professional fixing method is probably still hot tar mopped all over and the felt laid onto that. (although torch on is overtaking it). You can hire tar pots.

Should highlight that you must use purpose designed torch on felts for this. There is also a BS for torch on felt application: BS 8217: 2005

Torch on patches work well here as well.

Under the deck sounds like warm deck to me...

This probably wants some expansion.
Traditional flat roof construction places insulation on the inside of the roof timberwork and/or between the joists and firrings. This creates a cold deck, where the roof tembers are not within the heated envelope of the building. The roof void should be vented at the soffits to eliminate interstitial condensation.
Warm deck places the insulation layer over the roof structural timbers so they are included in the heated envelope of the building. This is a warm deck construction. Here no ventilation of the roof void is needed. Typically the deck material sits directly on top of a rigid roof insulation panel (or in fact may be purchased pre bonded to it)

Ventilation ought to be added to the void above the insulation though or you may accelerate rot of the roof timbers since they will now be cold enough to condense moisture out of the air that penetrates from the living space.

There is a section in the roof construction article that probably needs linking in somehow (or bits moved / replaced etc)

--
Cheers,

John.

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thanks, lots of good stuff there. I know nothing about the different types of non-bitumen backing btw.
NT
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
==Felt== Roof felt is simply fibres in [[bitumen]]. Sometimes sand or 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 ** polymer modified backings like SBS and APP make the backing handle and perform more like a rubberised layer
===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.
==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.
==Deck== 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 [[fill]]ed 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.
==Felt layers== 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.
==Fixing== 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.)
Roof structures too light to have clout nails [[hammer]]ed in, ie some sheds, can use waferhead [[screw]]s instead of clout nails. These are seldom used though.
[[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=== Felt is sometimes applied with a blowtorch. This melts the felt surface, making it stick to the deck. Use only felts designed for this. Good practice with torch-on felts is described in the expensive BS 8217:2005.
The basic idea is to sweep the torch across the underside of the rolled felt, creating an advancing wave of molten bitumen. Where the surface melts, the felt is pressed down onto the roof with a brush. 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.
==Improving longevity== 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=== 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 [[paint]]s 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.
==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. Or torch-on felt can be applied with a blowtorch.
==Insulation== ===New roof=== 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 ** Ventilation of the cold timber is advisable to avoid condensation & rot.
Warm deck: * 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
===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. 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.
==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.
==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== * [[Roof construction]] * [[Bitumen]] * [[Insulation]]
[[Category:Basics]] [[Category:Construction]] [[Category:Roofing]] [[Category:Sheds]]
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