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


==Felt==
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 fib 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.

==Deck==
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.


==Felt 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]]

On 22/12/2010 14:15, Tabby wrote:
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

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

Nightjar wrote:

On 22/12/2010 14:15, Tabby wrote:
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

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

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

On Dec 22, 4:11*pm, John Rumm wrote:
On 22/12/2010 14:15, Tabby wrote:


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 fib 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]]