User Guidelines: Clay and Earth

Earth has, probably the longest history of all traditional building materials, it also has a number of different names depending on the geographic area that it’s being used, the form it takes ‘on site’ as well as variations in the mix. Clay lump and cob could be taken as a similar background, the main difference being that the West Country cob is shuttered and formed in layers whereas clay lump involves the use of pre-formed blocks.

Daub, is traditionally found as an in-fill between the studs on historic timber framed buildings. It would have been hand placed, surrounding the framed in timber wattles. The wattles generally consist of upright Hazel poles about an inch in diameter tied with hemp, vine or bramble to oak staves. The staves are locked into place between the upright studs with one end in a notch and the other pulled down a groove.

It’s safe to say that the upright Hazel poles must have been regarded as almost sacrificial, although it’s common to find them still in situ, they very often consist of thin bark containing nothing but powder! The all important, structural staves however are usually still strong and holding the daub panel in place.

Clay/Earth plasters

These again are common to historic timber framed buildings, often used over lath or latterly clay lump, in order to achieve a plastered finish with similar elastic properties as the background.

Clay Lump/Cob

The clay used is often light and friable, this may especially be the case externally if the clay has been subject to a cement render or freeze thaw action. In all situations, it is likely that the background will be high suction and possibly dusty/ friable.

1. Firstly, gently brush down any loose material. A soft brush is advisable, so as not to cause further damage to the clay.
2. Lightly mist the clay using clean water delivered with a pump type garden spray.
3. With a bucket and plasterers whisk, knock up an NHL limewash, either NHL2 or 3.5. Add the powdered lime to the water and mix to milk consistency.
4. While the clay is still dark in colour from the mist spray, the NHL limewash should be worked well into the surface with a circular, scrubbing action. This will have two effects, firstly it will consolidate the surface, binding together all of the loose material. Secondly, it will give a degree of suction control. The best ‘off the shelf’ brushes for the job are the cheap ‘shed & fence type that are rectangle with a screw-in central handle.
5. If dubbing out/repair is required, it is possible to use any damaged clay that has been removed from the building. Just break up with a hammer, add water, knock up to a plastic consistency and place back into the void. If there isn't any clay available, it is possible to dub out to at least 250mm with Warmcote. It should be mixed ‘stiff’ for this purpose, with a minimum of water. It can then be pushed into place.
6. On clay-lump and cob, once the areas in need of repair have been brought up flush, leave for 12 hours to set.
7. Externally, damp down, then apply a scratch coat to a minimum of 12mm depth, the key should be added, by lightly drawing a standard ‘render scratcher’, horizontally across the surface. Internally, the Warmcote scratch coat could be applied at 6-8mm
8. Leave the scratch coat to set firm, but not necessarily dry, before applying the finish coat. The finish coat, both internally and externally is Limecote. Externally Limecote needs to be applied at a minimum of 8mm, internally it should be possible to apply it at 3mm.
9. Lightly damp down the scratch coat if it has dried. This shouldn’t be necessary if the plaster background is still green (able to push in a thumbnail).
10. Apply the Limecote with a flexible trowel. If required it can be straightened and flattened in with a flexible straight edge, such as those produced by Speedskim or Refina.
11. As the Limecote firms up, take out any remaining trowel lines and leave alone. Neither product requires damping down or repeated aftercare subsequent to finishing.

Variations on Clay backgrounds:

Wattle & Daub.

Limecote is ideal for use directly onto wattle and daub, based as it is, on the medieval chalk plasters found commonly on timber framed buildings. It is very common to find a gap around the edge of the daub panel, sometimes up to an inch wide and the full depth of the frame. This occurs due to the use of green oak for the frame and wet clay for the infill. When the building dried out they both shrunk back away from each other.

As with other clay backgrounds, remove as much dust and loose, friable material as possible. A vacuum with the soft ‘upholstery’ head is very effective especially if the daub has holes pushed into the surface for a plaster key, though be careful not to lose any historic artefacts.

Lightly damp down the daub with a garden spray and clean water. Push a stiffer mix of Limecote into the gaps and backfill up to flush. (Warmcote could be used if there are large deep voids to fill, this will give a quicker set as well as the increased insulation value). Apply a single 8-10mm coat of Limecote over the whole panel, allow to ‘pull in’ and trowel flat.

Variations in depth with both Warmcote and Limecote are perfectly acceptable, this allows the plaster to be rolled in where it meets the timber frame if required.

Clay Plaster Ceilings.

Plaster ceilings consisting of clay, chalk and chopped straw/animal hair are relatively common in medieval buildings, they are usually onto a riven lath background, or directly onto the underside of thatch.

It is not unusual for the iron nails to have rusted away over time, this results in the plaster sagging as the laths spring forward. The relatively lightweight nature and flexibility of the plaster means that it is still well attached to the laths, but will move considerably when pushed. It is often the case that ceilings such as this are condemned, taken down and replaced. This however, is nearly always unnecessary, resulting in the loss of historic fabric.

Using a narrow gauge screw and penny washer, it is usually possible to locate a lath through the plaster. Choose a section of lath beneath a joist and gently start to re-fix the ceiling. It doesn't have to be pulled back flat, usually just enough to put the washer under tension will secure the ceiling.

Very gently remove loose material and lightly mist the clay with clean water. Limecote can then be applied directly onto the clay as a finish plaster, probably no more than 6mm deep. As the Limecote sets, it will stiffen up the clay, it will no longer feel ‘live’, but the character and history will be intact.

Skimming over existing plaster.

As with plastering over clay ceilings, Limecote can be applied directly onto the existing plaster background.

Surface preparation should include the gentle removal of any loose, friable material such as failed plaster, soft distemper, wallpaper or wallpaper paste. Often gently planing with a window scraper, or scouring with a scotch type abrasive cloth will remove the loose material.

For thick areas of distemper, a wallpaper steamer can be used, distemper is mostly chalk and will soften when steamed, it can then be gently planed off with a scraper. Thoroughly clean off residue prior to plastering, a large garden spray, clean water and a jumbo sponge will suffice. Limecote can then be applied at any thickness required down to 2mm.

If the background is high suction, it is worth applying the Limecote in two ‘tight’ passes, one straight after the other.