Cover Image
close this book Handbook for building homes of earth
View the document Table of contents
View the document Foreword
View the document Chapter 1: Introduction - Types of earth houses
View the document Chapter 2: Soils - And what can be done with them
View the document Chapter 3: Stabilization of soils
View the document Chapter 4: Where to build
View the document Chapter 5: Foundations
View the document Chapter 6: Lightweight roofs
View the document Chapter 7: Preparing the soil
View the document Chapter 8: Making adobe blocks
View the document Chapter 9: Making pressed earth blocks
View the document Chapter 10: Making walls with earth blocks
View the document Chapter 11: Making rammed earth walls
View the document Chapter 12: Roofs for earth houses
View the document Chapter 13: Floors
View the document Chapter 14: Surface coatings
View the document Suggested references
Open this folder and view contents Appendix

Chapter 14: Surface coatings

A surface coating is a layer or film of material placed over an earth wall.

This is what surface coatings do:

(a) They can make some earth walls look better.

(b) They can help earth walls shed rain water.

(c) They can make earth walls last longer.

Decide whether you want to use surface coatings when you are planning your house - not after it is constructed.

APPEARANCE - Houses built of carefully made pressed blocks look better without surface coatings.

Rammed earth walls will have a pleasing appearance if the inside of the forms is smooth and in good condition during construction. In fact, some rammed earth house owners feel that a few form marks are pleasing to an earth wall and they do not care to use surface coatings. But if the insides of the forms were rough and in poor condition, a surface coating will probably improve the wall's appearance.

Most adobe home owners prefer surface coatings on their houses.

One way to decide on a. surface coating is to look at other homes in your area. See if surface coatings helped their appearance.

Remember this:

A cheap or poorly applied surface coating that quickly peels off looks worse than none at all.

The owner must decide how often he is willing to repair the surface coating to keep it looking good. Most cheap surface coatings need repairing every 2 or 3 years. If the owner is young and healthy and does not mind the time and small expense of repairing the coating, a cheap surface coating may be suitable. But if the owner cannot perform frequent repair work, his walls will look bad, and they may suffer severe damage. Then a more expensive coating will be necessary.

DAMAGE FROM RAIN WATER - If you follow the instructions in Chapter 2 for selecting soils for earth houses, you won't have to worry about rain damage unless you have to use very clayey soils. If your test blocks, even when stabilized, do not pass the spray test, try a surface coating.

Even the most expensive surface coatings are likely to develop cracks and let some water through them and into the wall. If your wall contains a lot of clay, the water will cause it to swell and crack surface coatings that are stiff. Check to see whether this will happen to your surface coating. Completely coat a few test blocks of your soil mix and subject them to both the spray test and absorption test. If the surface coating cracks or peels, the same thing will happen to your house when coated.

INCREASING THE LIFE OF THE WALL - Other things beside rain may wear down an earth wall. Strong winds carrying dust, children throwing rocks or sticks, and even animals, can scar or damage earth walls - especially unstabilized ones. If these things occur in your neighborhood or area, then unstabilized walls should be coated. Most stabilized walls can resist them.

COST OF SURFACE COATINGS - Since most surface coatings require some type of stabilizer, you must decide whether it is best to use the stabilizer in the surface coating or in the wall itself. Surface coatings normally take less stabilizer than for stabilizing the entire walls. But it also takes additional time and money to apply surface coatings and keep them repaired. Unless you can get by with a thin, cheap surface coating, the total cost will be nearly the same whether you use a surface coating or stabilize the entire wall.

TYPES OF SURFACE COATINGS - There are four main types of surface coatings:

(a) Plasters

(b) Slurries

(c) Paints

(d) Special chemicals

Plasters are placed on the walls in fairly thick layers - about ½". Below is a high-quality plaster mix that has worked well on earth houses:

1 part portland cement

4 to 5 parts clean sand

Sufficient water to make a thick mixture

Add a small amount of lime if the plaster is hard to spread.

Apply this plaster in two coats. each about ¼" thick. Moisten the wall and then throw or "splatter" the first coat against the wall with a large brush. This will not only bond the plaster to the wall, it will leave it rough so the second coat will bond to it. (See page 143 concerning reinforcement of plasters.) Cure this coat at least 12 hours (more in wet areas) before applying the second coat. The second coat may be brushed on also, but if a smooth surface is desired, put it on with a metal trowel. Rough surfaces have less tendency to crack and they look better on most homes.

Don't apply cement plasters on a sunny wall; wait until it is shaded. When a section of wall is completed, cover it and keep it moist for a day or two, until it gains strength.

Cement plasters should be used only on stable walls. Remember that walls containing a lot of clay will swell and crack the coating.

Low-cost mud plasters can be made by combining a soil mix with water. These are called Dagga plasters. Dagga plasters made with the right kind of clay and having sufficient sand in them may be as good as any other plasters. Most good ones contain about 2 parts sand to one part clay. Dagga plasters will not last if they contain too much clay, the wrong kind of clay, or when they are too soft. The red and brown laterite clays found in the tropic zones make excellent Dagga plasters.

Dagga plasters can be improved by adding a stabilizer to them. Lime or cement work well. Try one part lime or cement to 9 parts of soil. Asphalts can also be used in Dagga plasters to waterproof them but they will not harden the plaster and they may make it too dark to look good on a house. Many of the other stabilizers mentioned in Chapter 3 can be used. Experiment by trying them out on small sections of your wall or on test blocks of your soil mix. Watch for damaging cracks as the plaster dries.

Dagga plasters are usually applied to a previously moistened wall with a trowel to a thickness of about ½". Roughening the dagga surface will help to keep cracks from forming when the plaster dries.

Slurries are mixtures of cement or lime and water. They are brushed on the wall in thin coats like paint. Good ones may last 5 to 10 years.

Some of the best slurries are made from equal parts of cement and lime mixed with enough water to make a thick liquid. Adding a small amount of clean, fine sand will give the slurry a gritty texture.

Moisten the earth wall before the slurry is applied. Then brush on two coats at intervals of 24 hours. Keep the slurry mixed while applying it or some of the materials may settle to the bottom of the container.

Slurries make economical surface coatings, but they must be applied to the right kind of wall. Earth walls that shrink and swell will cause them to crack and peel off.

Paints make good surface coatings for some walls. Cement-based masonry paints work best. They are tougher and last longer than plain cement slurries. Present oil-based house paints and aluminum paints do not work on earth walls. However, new ones may be developed that work as well as cement-based paints.

It is not practical to include a list of manufactured paints that have worked on earth walls because they may not be available in your area. If you want to try them, see the nearest paint salesman. Ask him for a demonstration and samples of his paints. Then see how they work on test blocks of your soil mix. Perform the spray test on completely painted test blocks and if you have enough time, allow the blocks to weather outside. The performance of these test blocks should tell you how well the paint will work.

You can make a cement-based paint but it takes several chemicals. Here is one that has been used:

1 part calcium stearate (powdered).

2 parts calcium chloride (powdered).

50 parts portland cement.

25 parts clean, fine sand.

Mix these ingredients together to obtain a uniform mixture. Then add 50 parts water anti mix with a hoe until a creamy mixture is obtained.

Pour the mixture through a fly screen to remove lumps and large particles.

Use white portland cement in the mixture if a white coating is desired. For other colors, add 3-4 parts of a powdered, oxide pigment.

Apply the paint in two coats with a large brush. Moisten the earth wall before applying the first coat. Paint only on shady walls and keep the painted surface slightly moistened so the cement does not cure too quickly. The second coat should not be applied sooner than 12 hours after the first.

Only the second coat needs to be colored. It can also be thrown on the surface with a large brush or broom to. obtain a pleasing surface texture.

White-wash paints - or water mixed with lime - can improve the appearance of earth walls. But they offer little, if any, protection against water or wear, and they only last for a short time. However, they are not costly and they are easily applied with a large brush.

A whitewash which offers some protection against water can be made with the following materials:

1½ gallons unslaked lime.

3½ gallons of water.

½ pound of melted tallow.

Boil the water in a large can, add the lime, and stir well. Then add the melted tallow to the boiling mixture and stir again.

Apply the mixture with a large brush. Add a little water if it is difficult to spread.

This white-wash will last about a year in most climates.

Certain chemicals, applied similar to paints, will make good surface coatings. One mentioned in Chapter 3 is a mixture of 1 part sodium silicate (40° Baume) and 3 parts water. The solution is brushed into the wall with a fairly stiff brush to get good penetration. Apply two coats a few minutes apart. A suitable wetting agent or surfactant (see Chapter 3) added to the solution will increase the penetration of the treatment in some walls.

The only way to find out whether chemicals will work is to try them. Spray tests and weathering tests should be performed on treated test blocks of your soil.


Figure 92.

PREPARING THE SURFACE - Surface coatings applied to poorly prepared walls will soon peel off. Before applying any surface coating, the entire thickness of wall must be completely dry. For properly cured blocks, this may take only a few weeks until the mortar dries. Rammed earth, wattle and daub, and cob walls will take much longer. In rainy area', walls may be damaged before they get dry enough to apply surface coatings. If this is liable to happen to your wall, use the stabilizer in the wall so it will be protected from the time of construction.

When completely dried, the wall must be brushed or swept to remove dust and other loose particles. Just before slurries or plasters are applied to a wall, moisten it slightly. Painting

the wall with a thin mixture of portland cement and water improves the bond between the wall and surface coating.

REINFORCEMENT - To strengthen plasters and improve their bond to the earth wall, wire reinforcement should be used. Woven wire fencing works well. Fasten the wire securely to the top of the earth wall by placing it in the last mortar joint or between the bond beam and the top of the earth wall. For houses with bunker fill roofs, the wire should be attached near the roof fill, brought over the top row of blocks, and bent down along the wall. This method is shown in Figure 65.

The reinforcement should extend into window and door openings so the plaster will be reinforced at all points. (Figure 92.) Nail it to the wall every 6-8 inches. Special nails with roughened surfaces work best.

When two coats of plaster are used, the first coat should be thrown or spattered through the wire reinforcement. This improves the bond between wall and plaster and also leaves the reinforcement approximately midway in the plaster coat.

When wire reinforcing is not available, the bond to the earth wall can be improved by leaving a deep opening in the vertical mortar joints. Chips of broken rock or concrete, inserted into wet mortar, will also bond thick surface coatings to the wall. (See page 102 for discussion of these methods.) For rammed earth walls, cut shallow holes 6 to 8 inches apart and drive nails in these holes until their heads are even with the outside wall surface. Another way is to cut long, narrow grooves 6 to 8 inches apart in the wall.

Remember that the strongest plasters are reinforced with wire. Use it if you can.