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An explanation of Scantle roofs.
Reasons why
old wet laid (Scantle) roofs fail
Reasons
why new wet laid roofs fail
The mortar.
General stuff about
lime.
Natural Hydraulic lime.
Non Hydraulic Lime.
Hydrated lime.
Lime/cement wash.
Bitumen coats (Turnerizing).
Mortar staining the
slates.
Joining
2 roofs.
Example wet lay.
Scantle.
Whether you were born and
bred in Cornwall, or like me are an
'Emmet'
(Cornish name
for all things imported! Literal meaning - 'Ant') You
will at some stage come across the term 'Scantle' or 'Wet lay' or if
reading a surveyors report - 'Wet Laid Delabole Scantle
slate laid to diminishing courses on a lime based mortar bed
and hung with an Oak peg' usually followed by 'coming
to the end of its useful life!'. That last bit I sometimes take issue with, as I have seen
some roofs written off by a surveyor that I would consider
to have 20 years or more left in them.
In my
opinion a wet lay roof is easily the best roof to have -
A property like this deserves a Scantle roof - Listed or
not.
(All pictures of roofs that you see on this site
are roofs by us.)
It lasts a
long time, you can walk on it for maintenance to other parts
of a building (not on a regular basis though, and not if its
as steep as the one above!), it will withstand extremes of
weather, and it looks great. Unfortunately it is also
the most expensive roof you could choose. Essentially,
a Scantle roof is small Cornish slates which have been laid
to a triple lap gauge rather than a standard double lap
gauge (like usual slating). The slates are hung from the
batten with an oak peg or copper nail, and they are laid on
a mortar bed which is shaped like a one inch wide horseshoe.
The idea was to use up small slates from the quarry, so the
slates are sorted on site according to length, and then laid
on the roof with the longest first diminishing to the
shortest at the ridge. The term 'Scantle' comes from
the stick which would have a series of marks on it.
One set of marks would be for the lengths of slate from 14"
down to to 7", and another set of marks for the
corresponding set of slate gauges. So you could offer
a slate up to the Scantle Stick and easily see the gauge for
that particular slate. A compass could then be set to
that gauge and transposed to the rafter. These days we
have calculators and more education, so the Scantle stick is
pretty much obsolete. A shame really, but then you
don't use a map if you have a Satnav!
As a general
rule we are usually replacing wet laid roofs on turn of the
century properties, which puts the useful lifespan at around
90 to 110 years. There are reports that the slate will last 300
years. Yes they possibly will, but a roof will only last as
long as its weakest component (rusting fixings, rotten laths
etc). Other people have said
that a wet lay roof has a life of 150 years. I can
only go by my own experience and any that I have seen that
are 150 years old have been in an awful condition.
Older
wet lay roofs usually fail because: -
- The laths that the slates are
hung on deteriorate and rot away, causing sections of slate
to slip, locally known as a
'riffle'
- The iron nails used to fix the laths corrode
through (known as 'nail fatigue'), causing sections of
slates to slip.
- The area of the slate between the nail hole and
the top of the slate has eroded, causing individual slates
to slip.
- The mortar mix is wrong. Too much cement in the mix
(we do not use any cement at all these days just a Hydraulic
Lime/sand mix) would
have made the mortar bed too hard and prone to crack and fall
away, too little cement in the mix would have made the mortar bed
too soft and could be weather beaten out and prone to frost
damage. Wet laid
roofs that start to fail well before their expected 100
years is usually due to the latter - wrong mix.
This roof had hardly any visible mortar to the slates.
The mix was far too weak when it was laid and the mortar was almost
completely away and it was losing slates. It was just 40 years
old! Only possible remedy - new roof.
The Mortar.
Traditionally the mortar bed would have been lime putty (non
hydraulic) and
sand, with or without the addition of a small amount of
cement (pozallan). This addition of cement probably started
during the 19th century and was usually a trade secret
amongst builders and roofers, and they all had their own
'secret recipe' about which ratio they used. After the
second world war cement became very prevalent in the
building trade and much cheaper, and lime mortars were used less and less.
These days we use a Moderately Hydraulic Lime
(3.5 NHL) with fine sand and no cement, which takes the guess work and
mysticism out of the gauge. The reason a lot of more
modern Scantle roofs fail is due to the cement content.
General stuff about lime.
Lime
comes from burning limestone which is actually Calcium
Carbonate to a high temperature, which
carbonates (releases Carbon Dioxide and any moisture to the
air) and forms lumps or quicklime - or Calcium Oxide.
This can then
be 'slaked' with a small amount of water, or Hydrated
to form Calcium Hydroxide. This can be left as
a powder and bagged and sold as Hydrated lime (or 'bag'
lime), or slaked with more water and left to mature for
months on end, and is known as putty lime.
If you use
Hydrated 'Bag' lime, it would not be mature, so it would
need to be mixed with water and sand and them left to
mature. Putty lime is already matured and can be used
immediately it gets to site, or left as long as its protected from the air
for as long as you like. Clean limestone will produce non
hydraulic lime which is the purest and most breathable. When
its used its only the carbon dioxide in the air makes it set -
apparently it takes more carbon out of the atmosphere to set than it
adds to the atmosphere when its burnt - so is the greenest
lime as well as being the most breathable.
Limestone which contains impurities (like clay) would be a
hydraulic lime, and the amount and type of impurities would
mean it would be feebly Hydraulic, Moderately Hydraulic or
Eminently Hydraulic. The addition of water to the dry powder
reacts with the impurities and cause it to set - its because
it sets with water it gets the term 'Hydraulic'. It achieves the
Hydraulic set within a few hours to a day or two of being used
(depending on temperature), but it still
takes carbon from the air to achieve a full carbon set.
So
it is still environmentally green - just not as green as non hydraulic lime which
sets just with air (carbon dioxide). So, Hydraulic sets with water - Non Hydraulic
doesn't. Easy!
All lime
after it is used and hardens goes back to its original state
of being Calcium Carbonate - or limestone or chalk. As an aside, when lime is burnt to a
very high temperature it emits a bright glow which they used
for lighting for plays etc before electricity, hence the
term - in the limelight!
In a bit
more detail if you can bear it -
Natural
Hydraulic Lime.
These days
we use a Hydraulic lime with sand as a
standard mix for wet laying slates so there is no room for
variation of the mix. Natural Hydraulic Lime comes in different
grades according to the amount and type of impurities that
it has within it which is governed by British Standard 459,
so are suitable for many applications. It
sets initially through a chemical reaction with water giving
a hydraulic set. It should be protected from sun and wind in
this stage. After the initial set it will be
fairly resistant from light showers (in fact a light shower
can help it), and then over
the next few days to few weeks it will set further with a
carbon set. Again, care should be taken to prevent
this happening too quickly. A cloudy week with little wind
at about 12 degrees is perfect. A bit like one of our
usual summer days really.
Non
Hydraulic Lime sets with exposure to the air only (Carbon
Dioxide) and not water, so you can have a large amount of
putty lime delivered to site and store it under water, or
sand or anything to stop the air getting to it. This is the
purest lime & most breathable. This can
be
gauged with a Pozzolan (hardener) to give it an initial set
with water, so would then become a Hydraulic lime. A
Pozzolan can be brick dust, clay, fly ash or natural cement
(Not Portland Cement - natural cement). Without the Pozzolan it takes
a very long time to set, months usually, so is usually only used for render to
internal walls or cob wall mortar, as it would
be impractical to protect it for the amount of time it would
need to achieve good firmness outdoors. It is this lime
that used to be used with a small amount of cement as the Pozollan when the old Scantle roofs were laid. The National
Trust and English Heritage will not (as far as I know)
condone the use of any OPC (Portland cement) with a lime mortar as
its just considered to be too harsh and hard. Sometimes they
may have an old mortar tested and try to replicate the mix which
sometimes will have natural cement in it. In tests carried
out by English Heritage (Smeaton Project), it was found that
a lime mortar mixed with a very small amount of cement
will fail.
Hydrated
Lime can be hydraulic or Non Hydraulic, but is usually
the latter. This is also known as 'bag lime' and is
basically a form of lime putty. It is lime
that has been slaked with water to form a powder, but not
enough water to form a putty. It is not as good as
lime putty, as you never know just how much air has got to
it (carbon), so it is always 'gone off' to some extent.
Best not to use this for lime washing.
As a rule
of thumb, (I know, as a rule of thumb don't use rule of
thumbs), you can add (non hydraulic or hydrated) lime to a
cement mortar to enhance it, but you shouldn't add cement to a lime mortar
- if that makes sense.
Lime wash/Cement wash
Sometimes when wet lay roofs start to fail, owners will
have the roof cement washed, which is
basically brushing a very wet cement/lime mix all over the
roof which is designed to keep all the slates in place.
It does do this but as it deteriorates it will hold dampness
within the
wash and hasten corrosion of the slate laths and other
timbers. Also on an old Scantle roof the lath fixings
will almost definately be in a much weakened state, so the
extra load a cement/lime wash would put on those fixings may
not be a good idea. As soon as a surveyor for a house
buyer sees a cement wash on a
roof he should write it off and insist on a new roof (or
monetary value) as a
condition of sale. If you HAVE to do it to keep
the roof going a bit longer until you can afford to do it -
don't use cement at all or Hydrated Lime, use non Hydraulic putty lime mixed
with water to a milky consistency for best results. If
rain is expected in the next few months or so, or there is
the slightest risk of the sun coming out, or a bit of a
breeze coming up, then you may be better of with a hydraulic
lime wash as it goes off quicker. Its a bit less breathable than putty lime,
but at least it stands half a chance of setting. As
this is going to be a stop gap repair until the roof is
replaced, the trade off of breathability will be worth it to
have a coating that doesn't need a battalion of roofers
standing over it for weeks protecting it from - well,
everything really. As far as I'm concerned, lime
washes are good for walls and non hydraulic lime is good for
internal work only. Be warned though, I would only use a
lime wash on a lowish roof that does not border foot
traffic. The wash has the effect of binding all the
slates together. So instead of getting few slipped
slates, you get a major slide.
When a Scantle roof gets very old the
fixings will be quite corroded and the laths corroded to a
certain extent. The last thing you want to do is put
extra load on the fixings and laths with a big heavy coating
of lime mortar or cement wash. This roof fronted a
pedestrian alleyway in the Down-along area of St Ives. The
slates were bound together with a cement wash, and the total
weight of the load was too much for already stressed
fixings.
Not good! You can see more about
riffles here.
Just goes to show what
can be repaired though..
 This
picture shows a cement washed roof that actually looks quite
reasonable. It is sometimes only upon close
examination that you can see the cracks and fissures that
will hold dampness within it.
This shows the new roof - good for another 100 years.
Lime Summary.
Just to be clear about this, what I have talked about in the
last few paragraphs is just touching the surface about lime
and how to use/handle it. We do a lot of Scantle wet
lay roofing with lime mortar and through research and
experience we know the best way to use it to get the best
results. Some things are not discussed here, such as
the best way to protect the work, the best way to minimise
leaching, the best sand to use, the different hydraulic lime
to use for different applications such as verges, ridges,
valleys etc. If I discussed everything here, my
competitors could just read this page and know as much as
me! (They already copy and paste a lot of it! True!). A man called Phil Brown from
The Cornish
Lime Company is an expert on lime and does courses and
gives good advise, (he doesn't know much about roofing though!) If you need
more info on lime he would be only too pleased to help you out (I
know because he told me!) For best results, drop him an
email.
I have
learnt (and paid) from mistakes made with lime. Mainly underestimating problems
caused by not protecting it enough or using it when its been
too cold or warm or windy or there's an 'r' in the month etc etc etc. I have always made good any issues - and learnt from
it! I now treat lime with more respect than my own
wife (and she's started to notice!).
Bitumen coats. Worse than a cement wash is a bitumen coat, where the
whole roof will be coated with layers of bitumen and a
reinforcing sheet (Process also known as 'Turnerizing'). Again this will prevent slipping
slates but within a very short period of time the bitumen
will start blistering and cracking and hold a lot of
dampness within it, again resulting in hastened corrosion of
the roof timbers. Any non-breathable covering will
also not allow for the escape of enough condensation created
by everyday living - cooking, bathing, washing etc, resulting
in condensation rising through the house and evaporating
on the underside of the sealed roof surface causing more
extensive corrosion.
This is an extreme case of a bitumen coating that has
obviously deteriorated. The only options here are to
trim back the loose coating, clean and re-coat or have a new
roof. It is really not worth re-coating unless you
absolutely cannot afford a new roof.
Once the roof has been either cement washed,
lime washed or coated in
bitumen the choices are to either have a complete new roof
or keep over coating until you can afford a new roof.
Either way a new lid is the only long term solution, and any
surveyor looking at a house for mortgage purposes will
insist on it.
It is not
known exactly what the
original purpose of the mortar bed was - it does provide a
good anchor for
the tail of the slate in exposed conditions, but then the
weight of the slate with triple lap roofing does that, and
it does help to keep wind
driven rain from entering the loft space before the days of
roofing felt, though triple lap roofing stops wind driven
rain. My own view is that when the slates get smaller and smaller
which they do with Scantle, the battens get closer and
closer together, and if you try to dry lay a roof to the
triple lap system the smaller slates start to 'rock' on the
slate below, meaning the top of the slate does not sit on
the batten but hovers above it. This is especially
true with new quarried slate from Delabole and Trevillet. The only way around this is to pack up
the batten somehow or raise the tail of the slate
so the top of the slate sits firmly on the batten. By
laying the slates on mortar it has the effect of raising the
tail of the slate so the top of the slate always sits firmly
on its batten. But I have to say, the mortar bed also
does stop wind driven rain, and helps prevent slate loss in
strong storms. People are always amazed to see that
the slates are just hung over the batten and not nailed down
- but you cannot argue with the longevity of this proven
roof system. (If carried out correctly!)
The inside of the loft space
is often lime rendered with non hydraulic lime mixed with
horse hair, or 'torched'
as its commonly known as. Larger nailed slate roofs were often just pointed behind the
battens (semi-torched).
Reasons
for failure in modern wet lays. This
could be one of many reasons or a culmination of many
errors. It could be too much mortar,
the wrong mix, the introduction of cement to the lime, the
wrong lime, contaminated sand. The gauging of the
slates is also all important to a long lifespan. Don't
forget, wet lays are hung from an wooden peg or nail, so you
cannot treat the gauging like nail fixed roofs. The headlap
has to be much bigger for it to stand any chance against
strong winds and deflection. I have seen a few nice
looking wet lays on Listed Buildings of note and I know that
they will be lucky to still be in one piece in 20 years time
because the slate gauge has been worked out as if they were
being nailed - and so is wrong.
To go into
detail about one of the above points and its the sort of
thing a builder would do thinking he is doing a very good
job, is to put more mortar than is necessary as the bed. If the mortar bed
extends to the top of the slate below the bed, the
mortar will act as a sponge and drip into the loft space (if you imagine holding a sponge under a slow running tap it
will absorb water until it gets so saturated it will start
to drip). We have stripped a fine looking roof that
was suffering from this problem, it was installed
around 20 years ago but apparently has had increasing
problems with leaks over the last few years. On
inspection of the loft you can clearly see where the felt
has rotted through and see mortar hanging over the back of
the slates.
A 20 year old roof that looks absolutely fine but leaks
badly.
Every course was like this - the mortar bed is far too
excessive resulting in a path for water to get in.
Note the poor condition of the timber laths also.
The addition of the underlay created a void, and as there
was no ventilation between the top of the underlay and the
underside of the slates, any dampness that got in just
festered.
The owners wanted to keep the original look as much as
possible without laying out a lot of money on a full wet
lay. They were given samples of Brazilian, Chinese & 2
Spanish slates and decided on Chinese 12 x 8 slates nailed.
These slates are not as uniform as some of the others and I
must say that although it could not be mistaken for a wet
lay, the colour and size of the slates are quite similar and
I think that at a glance it does not look at all out of
place with the rest of the buildings on the surrounding
properties, all of which are wet laid roofs.
Mortar staining the slates..
A problem with wet laying slates is mortar leaching from the
slates and staining them, giving a light bloom. This is always more
noticeable if using a new darker slate like a new Delabole
or continental slate than if we use the old silvery Scantle
slates. We did a small wet laid front roof in Lelant with
new slates which only took about a week to complete. Rarely, it didn't rain
for the whole week! Temperatures were ideal, we kept the roof covered on the evenings
to protect it from dew (lime is even scared of a bit of
dew!), the mix was not too wet and it looked lovely when finished. I
looked at it about 3 months later and it had leached all over
it with a lime bloom. I looked at it again over the following months and
about 9 to 12 months later it was pretty much clean again and
back to its original loveliness. The point being, it is
practically impossible to stop lime from blooming over the
slates to a certain extent, but it will wear off. I
have to say though, there is a difference between a bit of lime
bloom and lime leaching. Leaching is bold ugly white
staining that you see if certain criteria were not met when
laying the roof. I have seen some new wet laid roofs horribly
stained which will
take years and years to weather off.. {shudder}
Joining
2 roofs. What
happens if you want a new roof but you are a semi-detached
or terraced house? Simple - we just install a secret gutter
also known as a bonding gutter between the houses - this
keeps your roof completely separate to the roof next door
that's had it!
Property on the left of
the picture has a new roof (complete with bloom!). You can just make out the
join between the 2 roofs - beneath this is a 12" wide secret
gutter formed from lead (or can be GRP)
Example
wet lay with old slate, old clay ridges and new lead to
replace felt. See more examples on the
PROJECTS page.
Pictures below show Biggleston's
shop roof in Hayle, which we completed by stripping the slates, saving as many
of the good ones as possible, dressing the slate (cutting
the top of the slate back to a solid state) and re-holing.
We then laid the slates to diminishing courses triple
lapped.
This picture shows part of the original roof -
Scantle
slate cement washed in places & cement fibre slates laid to
too low a pitch.
We replaced the felt flat roof dividing the pitched
elevations with code 7 lead
dressed over mop stick.
Re-use of the old ridge tiles finishes the original look.
Courses are from 12" slate to 8" slate.
One more word of caution. If you are trying to decide whether
to go for a wet lay with new slate or pre-used slate.
While I am all for using new slate as it supports the quarry
and the employment etc therein - make sure you know the
head-laps and gauge of the slate for your particular job
before you obtain a quote from the quarry. In order to be
competitive they will quote you for the minimum possible
head-lap of 75mm, (meaning far fewer slates per square
metre), which means that the courses of slate will
be much wider apart than shown in the picture above
(assuming you started with 12" slates) which would defeat
the object of trying to replicate a traditional looking
roof. The technical department of
Delabole Slate
are very good for information though, and as long as you
know what you want they will be very helpful.
The work on Biggleston's was funded partly by the owners of Biggleston's
shop and by the
Hayle Townscape Heritage Initiative
To see a
video of using new Delabole slates on a wet lay roof we are
doing at POrthmeor Studios, St Ives - click on this link
http://www.youtube.com/watch?v=WwqRCtf0f7A
This
picture has nothing to do with anything, I have just put it
here so I can point architects to it when I am trying to
explain how we do our verges!
WHAT IS
A 'RIFFLE'?
This is a local term to describe
what has happened when a large section of slates parts
company from the rest of the roof. By the very nature
of the system of wet laying - the slates are simply hung on
a peg, not nailed down - they are at the mercy of strong
winds as they get older. Sometimes you can tell a
riffle has happened because of a large number of slates on
the floor, but sometimes it may only be a jagged crack in
the roof which can be quite hard to spot. A riffle
usually happens on older roofs, sometimes the roof is so old
that it is inevitable, and sometimes an
otherwise good roof can be hit by a strong gust of wind in
the right (or wrong!) direction and be a casualty.
When insurance companies have to deal with this they have to
decide if the roof was past its sell by date anyway and
could not defend itself against strong winds, or if the
other elevations are in good condition and have been well
maintained, i.e. not covered in weeds or cement wash
or bitumen etc. If the other elevations appear to be
reasonable an insurance company should and will often pay to
have the affected elevation replaced. This can be a
grey area though and very often is, resulting in disputes
between policy holders and insurance companies.
Doesn't look much does it? Your typical
insurance company would look at this and allow a couple of
hundred pounds for a bit of pointing.
Luckily I know its a good idea to get a closer look at
these things...
...and It looked like this 2 days later. On
closer examination of this elevation it was decided it was
not cost effective to repair it so it was replaced 'like for
like'. The insurance company paid for all costs.
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