Godolphin cider house Cornwall  2011

The first sign of wholesale failure - cracks in the grouting



Grouting or slurrying is a repair technique for leaking and failing slate roofs.  It is seen predominately along the western seaboard of the UK particularly in Cornwall, Devon, Pembrokeshire and Anglesey.  In the past it was also used on porous stone slates such as the Permian sandstones of Dumfriesshire.  The technique involves applying a lime or cement mortar wash over the whole roof to form a skin.  This might be done annually gradually building up a thick coat and producing distinctive roofs which are (or were) important landscape features and were often remarked on by Victorian picturesque tourists such as Celia Fiennes. 

The technique seems to have evolved to deal with two modes of roof failure.  In the West Country where generally slates were very durable it was adopted because of failure of the laths and fixings especially of lath nails.  In Pembrokeshire (and possibly in Anglesey where Anglesey slates were used) it is more to do with the failure of the local, less durable slates.  It has also been described historically as draught proofing.

As the layers of mortar build up with successive groutings the fixings become unable to carry the increased load and eventually large sections or even a whole slope start to slip.  This is aggravated by the consequential loss of ventilation through the slating, trapping moisture in the lath space and leading to rotting of the laths and rusting of iron nails. 

The first signs of wholesale failure are the appearance of cracks  known as riffles in Cornwall.  To restrain the slippage wire (often barbed wire) or steel rods are laid over the roof and either wrapped under the eaves or nailed through into the rafters.  The wire is then covered with a mortar fillet to form distinctive ribs.


Technically anything which reduces the flow of air through slating is likely to be detrimental because moisture can become trapped in the lath space.  If however the underside of the slating is open with good ventilation through the roof space, this effect may be mitigated.  But a roof is to be reconstructed with fully bedded slating or grouting and insulation and underlays or vapour permeable membranes are to be installed along the rafter line, ventilation of the lath space is essential. The simplest system will probably be to instal eaves ventilators and vent ridges.  Listed building consent may be required for this.

From a visual and conservation point of view there may be good reason to retain the technique especially where it is a significant aspect of a region’s landscape.  This is the approach taken by Pembrokeshire Coast National Park even to the extent of requiring newly re-slated roofs to be grouted.

In contrast in the West Country, it is regarded as a technique of last resort when the roof has failed and because the slates will probably be reusable if they are not grouted it is not recommended as a repair technique nor to be conserved when re-roofing.

Although the grouting of roofs will eventually destroy them the slating is more or less retained in its original condition and if the deterioration under the grouting hasn’t been too serious it is possible to record and reconstruct the original details.  This was achieved with the roof of Addislade Farm in Devon even though the roof had been grouted and subsequently Turnerised - covered with hessian and bitumen.  Where slates which had slipped and been subsequently held by grout it was possible to digitally reposition them to demonstrate the original construction.




Barbed wire hanger

Wire hangers Pembrokeshire

Two mile from Plymouth we Come to the river Plym just by a Little town all built of stone and the tyling is all slatt, which with the Lime its Cemented, which makes it Look white Like Snow, and in the sun shineing on the slatt it Glisters.

Through England on a Side Saddle in the Time of William and Mary. Celia Fiennes 1698


Mortar is used in slate roofing for several purposes.  It is usually visible as

  1. -tail bedding in for example West Country scantle slating or on Horsham stone roofs or as

  2. -torching on the underside of the slating

  3. -bedding for ridges and hips

  4. -flaunches and fillets at abutments and verges

Less visibly it is used as bedding between uneven slates to prevent them rocking and to hold fixing pegs in place (pin pointing). 

It is also used to point up slates externally to prevent draughts and as grouting or slurrying.  The latter applications are repairs and would not have been part of the original construction.

Pointing and bedding in Cornish slating

Addislade Farm showing the slates slipped from their laths

The use of unsuitable mortar has resulted in its loss from the roof surface leaving only the wire ribs in place


For a review of Pembrokeshire grouted roofs see The White Roofs of the St David’s Peninsula, Philip Roach in Stone in Wales 2005 CADW.

Flies in Amber > slating details hidden under grout


In 2012 the construction of an old out-building roof was recorded and a series of videos made of the reslating during a training course.

  1. Recording the roof

  2. Slate making

  3. Lathing

  4. Slating

  5. Abutments and listings

  6. Collar and tie valley