Wet Lay – Cornish Scantle Roofing


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

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 in the pic!), 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.  This is possibly where the word Scantle comes from (Scant as in small) but nobody really really knows, I don’t care who they are!

There is a Scantle stick which has a series of marks on it. One set of marks would be for the lengths of slate from 14″ down to 7″ or thereabouts, 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 at around 90 to 110 years of age.   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, failing mortar 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.

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.)

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.   Scantle always was a lime/sand mix, but after The Second World war when cement became prevalent, cement was used for speed.   Too much cement makes the mix 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 completed post war usually fail due to the mortar.

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 gone 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 pozallan (hardener).  Non hydraulic lime can be used, otherwise moderately Hydraulic Lime (3.5 NHL) with fine sand.   The reason a lot of more modern Scantle roofs fail is due to the cement content.  So don’t use cement!

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 that 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 could 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 definitely 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!

Just goes to show what can be repaired though…

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 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.

We have learnt (and paid) for 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!).

An alternative to wet laid roofing is nail fixed Scantle roofing.   It gives a very similar look to wet lay, is quicker and cheaper and has far less potential problems.


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.

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 and corroded the timbers.    Similarly a lot of people make the mistake of using a vapour permeable underlay, which allows moisture from within the property to escape into the void between the underlay and the slates creating the same problem.   People think lime is ‘breathable’ so this should not be a problem.   Lime is permeable to a certain extent, but permeability is not in the same league as ventilation.

A 20 year old roof that looks absolutely fine but leaks badly.

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)

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