Cladding the interior walls of log houses with natural materials such as clay has been a sustainable insulation method since Viking times. The clay made the walls tight and draft-free. The smooth clay surface provided a good base for glue painting and a smooth base for paper wallpaper. Clay clay has also been used to seal blind bottoms in floors, as a filler for lime plaster outdoors, etc.
Clay lining is a method used in many restoration contexts. Due to the high thermal insulation and windproofing capacity of the clay liner, the method is also suitable for private houses and cottages. The method is even more relevant in today’s quest for energy-efficient, environmentally friendly, healthy and sustainable building materials. And that’s not even mentioning the low cost! One square meter of clay cladding costs around SEK 100 in materials (clay, sand, mesh, nails etc). And if you’re handy, you can do the work yourself.
The best time to use clay is in the fall and spring. If the temperature is too high, the clay mixture dries too quickly, cracks and does not adhere well to the substrate. If the temperature is too low, moisture and mold problems can arise, as the clay liner makes the walls very wet. Sometimes even shoots start to grow in the wall. A funny sight but completely harmless. The clay contains natural substances from grain and straw etc. and as soon as it dries it stops “growing”. You are left with only natural reinforcement.
Clay cladding should be carried out as late as possible in the construction process, as the material is sensitive to strong frame movements. The installation of exterior panels, linings and interior mouldings should therefore be completed before the cladding is carried out. Also expect the cladding to build up about 2.5 centimetres and install the joinery accordingly. Gaps and cavities more than 4-5 centimeters deep are first filled with wooden wedges, moldings or the like.
First, clear the walls of old, loose clay tiles, wallpaper etc. Solid mortar may remain. Take the loose clay liner and soak it for 3-4 days in a plastic tub or vat before reusing it. Do not add more water than can be absorbed by the mortar.
Canvas used to be attached to diagonal ribs or wooden pegs that had been laid down. They can be used again if they are in good condition. Usually, the clay liner has been attached directly to the surface of the timber that has been chipped with a special axe. Such a surface cannot normally support a new clay liner.
Cutting new ‘scales’ is also difficult in old, dry timber. Instead, supplement with a galvanized wall net or a sturdy chicken wire.
Fix the net with galvanized pegs or stainless steel staples at 10-20 centimetre intervals, so that the net is as rigid as possible against the substrate. Galvanized 3″ wire nails are also excellent (see picture) as an alternative to pegs and are easier to attach. The nail is driven into the wall at intervals of 15-20 centimeters and then bent so that it overlaps at least two meshes in the net (see picture).
Cover the surfaces that are not to be tiled. Flooring felt or masonite is good for floors, water-resistant tape is suitable around linings, moldings, etc. The clay can be washed off with water, so clay spills are not a disaster.
Fix the net with galvanized nails or stainless steel staples. Galvanized wire nails also work well.
8 liters of clay flour (about 10 kg)
24 liters of sand (about 35 kg)
2 liters of glue water
4-8 liters of water
8-16 liters of chopped flax straw
possibly old soaked clay liner
This recipe gives a mixture of about 25-30 liters, which is enough for about 2 square meters of wall surface, a normal-sized batch the first time you try to repair broken old clay tiles. The following quantity calculation, on the other hand, gives about 25 square meters of clay cladding:
5.5 bags of clay flour of 25 kg
500 kg of sand
40-80 liters of water
4 bags of soaked, chopped flax straw of 5 kg
20 liters of glue water (400 g animal glue water)
Mix all the ingredients in a cement mixer, except the flax straw, until smooth. Finally, add the soaked flax straw and process for about 10 minutes. You can also stamp the clay mixture into a wooden box, covered with a strong, waterproof tarpaulin. Tough, but fun. When it’s ready, the mixture should be smooth and easy to spread, without being watery.If you want to reuse old clay, you can mix it with an equal amount of new clay. However, the material you soak must not contain wallpaper residue, cakes of lime mortar and the like.
The clay is the main binding agent. The amount given here is for sand-free clay flour, which is available in bags in Gysinge. Tile kiln clay cannot be used, as it already contains sand. As with all other mortars, you must keep to certain proportions between binder (clay, lime, cement, etc.) and aggregate (sand, gravel, etc.). Normally you have 1 part binder by volume to 3 parts aggregate. Too little binder results in weak mortar with poor adhesion. Too much binder causes shrinkage and cracks. So do a test first and let the clay line harden!
The glue water is a pure binder and is prepared from animal glue.
200 g bone glue + 10L hot water. Stir the bone glue into the hot water in a spacious vessel. Leave overnight to allow the glue to melt properly. Stir from time to time.
The sand should be fine-grained and salt-free masonry sand. Do not use sea sand. The flax straw must be rooted and chopped into lengths of about 5 cm. Soak the flax straw for at least a couple of hours, preferably 24 hours, before use.
Ground floor
Moisten the surfaces to be clayed by sprinkling water over the surface with a lime brush or paint brush. The moisture will prevent the clay from drying too quickly. First, fill deeper cavities and cracks in the substrate with mortar. Smooth out the mortar with the trowel, making sure that the clay penetrates well into all the cavities. Leave to dry until the next day.
The clay liner cracks easily if it is laid in layers thicker than 2 cm at a time. Therefore, allow the base layer to dry overnight. However, it should not be completely dry before the next layer is laid. It is therefore advisable to moisten the walls by splashing water with a paintbrush over the surface before starting again. If old clay tiles remain on the walls, it is particularly important that the edges of the tiles are well moistened so that the clay tiles stick together. When patching old clay tiles, it is a good idea to lay the new mortar slightly thicker than the old. This is because the clay liner sinks slightly when it dries.
Apply a layer of just over 1 centimeter of mortar. Smooth it out with a trowel and, if possible, polish it with a “Spanish”, a wooden or metal disk with a handle that you move around in circles to smooth the surface. Leave to dry until the next day.
Add another layer of clay liner. The recommended thickness is about 2.5 cm. Smooth the surface with the trowel and spatula. Feel free to reduce the amount of flax straw in the finishing mortar, or omit it altogether, if you want a completely smooth surface, with no visible streaks in the mortar.
Tips! A professional bricklayer is often a good help in clay lathing, even if he has not clay lathed before. But he knows the technique of plastering and that’s the main thing.
On the left, newly sealed timber surface, before netting.
On the right, old clay linen on rib.
