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!),  it will withstand extremes of weather, and it looks great.  Unfortunately it is also the most expensive roof you could possibly choose.

As a general rule we are usually replacing wet laid roofs on turn of the century properties, which puts the useful lifespan at around 100 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.  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. 

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 would make the mortar bed too hard and prone to crack and fall away, too little cement in the mix would make the mortar bed to soft and could be weather beaten out.  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 and the mortar was almost completely weather beaten away.  It was just 40 years old! Only possible remedy - new roof.

Another reason that wet lay roofs will fail long before their expected 100 years is because the pitch they they are laid on is too low. The minimum recognised pitch for wet lay in coastal areas is 25 degrees.  I have seen many  roofs laid to a pitch below this and they will leak.  If it is a modern roof the under felt will take the water away to the gutter, which means you will not know of any problems until the felt corrodes, but as felt is not designed to cope with getting wet and drying out on a continuous basis it will start to corrode, and when it does you will see the signs of the leak on your ceilings.

This picture shows a roof pitch of around 5 degrees!  The under-felt seems to last anything from around 10 - 20 years before it corrodes from the continual dampness.

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 cement wash and hasten corrosion of the slate laths and other timbers.  As soon as a surveyor see a cement wash on a roof he will write it off and insist on a new roof as a condition of sale.  

  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.

Worse than a cement wash is a bitumen coat, where the whole roof will be coated with layers of bitumen and a reinforcing sheet.  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 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.

Another potential problem (this list is getting quite exhaustive, sorry!) with a wet lay is if too much mortar was used to bed the slates. 

The original purpose of the mortar bed was as a good anchor for the tail of the slate in exposed conditions and to keep wind driven rain from entering the loft space (before the days of roofing felt), it is supposed to get wet and dry out quickly due to the lime content.  In other words the mortar is very porous.  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 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. 

Another problem these days is efflorescence from the mortar staining the slates.  A better option - in fact the National Trust insist on it - is to use Natural Hydraulic Lime with a high silica sand and not use cement at all, as this form of lime has its own catalyst.  Its a standard 5:2 sand/lime mix, so there is no room for variation of the mix, and there is no efflorescence or staining..  Natural Hydraulic Lime comes in different grades which are suitable for many applications.  For roofing we use 3.5 NHL for laying the slates and the stronger 5 NHL for ridging and flaunching chimneys etc.

Pictures below show Biggleston's roof in Hayle, which we recently 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 to diminishing courses, maintaining a minimum 100mm head lap.

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, 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

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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