Earth walls - Load Bearing or non load bearing

Earth walls - Load Bearing or non load bearing The arguments for and against Load bearing or Non load bearing walls has been going on for as long as earth wall construction. Load bearing means that the weight of the roof or upper floors is supported directly on the walls. This generally requires the walls to be constructed before the roof or upper floors are built. Non-load bearing implies that the roof or upper floor loads are supported on a structure that carries the load independent of the earth walls. This allows the earth walls to be built after the roof is on. Either method is viable and a combination of both can often provide a practical solution to the problem. There would appear to be quite a polarised debate on the advantages of either method. Many “experts” have very strong opinions about the way a mudbrick house should be built. When a prospective client, self-builder or owner looking for a contractor, is confronted by two opposing views on the methods of construction, it can be quite confusing and often very discouraging. A decision on the type of construction is generally made for most people by circumstances around them. Environmental pressures form the key to this decision. High rainfall areas are often hostile environments for building earth houses, as is evidenced by the lack of historical earth buildings in high rainfall areas of Australia and other countries. More recently, this trend has changed with the increasing use of non-load bearing methods, enabling a sheltered area to make and lay mudbricks despite high and often frequent rainfalls. The benefits of having the roof up first are manifold. Shelter from the hot sun or rain makes work comfortable all the time. Materials such as mudbricks, doors, windows, finishing timbers and PC items can be stored out of the weather. It is good to plan your storage requirements ahead. Time spent building racks and organising materials properly will save you moving everything three or four times because it is forever in the way. From a construction point of view, it is simpler to build a mud brick house with load bearing walls. It is a more logical sequence of construction to start at the floor, then build the walls and finally put a roof on top. This is certainly the case with rammed earth as it is very difficult to ram walls from above when there is a roof in the way. For commercial builders, or owner builders who can afford to pay a professional team, it is fair to presume that the building will proceed without too many holdups. IF the weather is kind, and the time frame for construction fits within the normal dry season, all materials, including windows, timber and roofing, are on site and YOUR god is on YOUR side, THEN it is possible there will be no problems. However, as self-builders it is unrealistic to presume that your house will be finished or at least have the entire roof on within one dry season. The only thing worse than wet mud bricks is wet mud brick walls. Many, if not most, professional mud brick builders prefer to get the roof up first for these very same reasons. It also can give the owner and builder time to assess the project during construction and make changes to suit their needs or whims. This flexibility is what can give a mud brick house its own personal charm and individuality. If there is a cost difference between the methods, it is minimal and debatable. (A comparison of material costs between load bearing and non-load bearing walls can be read in OB50 something???????Ed?) After the footings or slab have been poured, the next step is to build the walls up to the damp proof coarse level. Generally, structural walls under the damp proof coarse should not be built using unstabilised earth. Moisture absorption from the ground will destabilise the soil and create movement problems in the structure above. In slab on ground construction, a 20 to 25mpa slab forms the damp proof coarse and the earth walls and timber frames can be built directly on the slab. For houses with elevated floors, dwarf walls will need to be built up to floor level from the footings. If the floor and roof frame is to be supported on these walls, they will need to be stabilised, and meet engineers specifications. Generally, fired brick or concrete is used for this job, however a properly constructed earth wall, stabilised with bitumen or cement will support a timber floor or suspended concrete slab. Particular attention must be paid to damp proofing and weather proofing if stabilised earth walls are used, as chronic moisture problems will eventually break down the soil structure. There are many different methods used to support roofs and floors of “non-load bearing walls. The now almost traditional timber post and beam has been popularised in the last fifty years in Australia. Self-builders particularly, have used it due to the simplicity of construction and the speed of getting a shelter up. This style has lent itself well to modular construction and kit houses like “Post and Beam”. The system is very simple to manufacture and erect and easy to infill with earth, windows and doors or other paneling. A major plus for first time self-builders and the finish can be as varied as the clients wishes. In the simplest form, poles or posts supporting bond beams, are set at regular intervals along north and south walls and the ridge. These bond beams support the upper floor or roof structure of joists, rafters or trusses. The posts must be tied down to the slab, using steel brackets set in during the slab pour or fixed to the slab with “chemsets” or “loxons”, or tied down well into the dwarf walls. These fixings must be strong enough to resist uplift from wind and the frame braced to resist horizontal forces from wind and wayward trucks, until the earth infill is finished. Bracing is critical when the roof is on but there are no walls. The roof is like an aeroplane wing and in a high wind the forces on it are enormous. Heavy bracing, at least 100x50 should be used and be fixed to the base of posts where they are bolted to the slab and the bond beam at the top using 100mm nails that will not pull out or move. There should be at least two braces, on every line of poles in each direction. Many houses in the past were built in this way but with the posts set into the ground. This is easy for construction and bracing purposes but a disaster in terms of moisture absorption and termite attack. Any timber or cellulose material in the ground is a termite attractant and should be avoided at all cost. The logic of digging a hole and filling it up with timber still escapes me, except perhaps for fence posts. (OB70 Poles in ground.) Modular framing tends to limit design flexibility but for most people this is not a major concern. Having posts interrupting earth walls means a lot more cutting of bricks and short walls which will slow laying to a certain extent. It also creates more work finishing up to the post and frames. The employment of load bearing window and door frames reduces this work and markedly reduces the quantity of materials needed. Instead of having load-bearing posts at regular intervals, you can use sawn timber posts 150mmx75mm from floor to bond beam, each side of window and door frames, to carry the roof loads. Bond beams, usually from 200mmx75mm up to 300mmx100mm, will need to be ordered to the correct lengths to span between the posts. If a beam is over a wall, the size can be reduced to allow the mud bricks to carry the load. Temporary props can be used if the beam is likely to sag before the mud bricks have finished settling. If you are not sure of the final size of a door or window, one of the posts can be fixed temporarily so it can be moved to suit the door or window when you find one. Steel frames are also becoming popular but there is a lot of extra work involved in concealing steel posts in mud brick walls. One successful method is to use a steel portal frame, made up of lightweight galvanized steel beams welded and bolted together to form the posts and rafters in one piece. Large spans can be covered cheaply and effectively and mud brick or poured earth can be used to infill between the posts. If a steel purlin roof is used, posts can be up to 5 meters apart. Ceilings and linings can either be fixed directly to the steel using glue and screws, or timber battens can be screwed to the steel, and linings fixed to the battens with glue and nails. Unfortunately, steel has a very high embodied energy, that is, it needs an enormous amount of power to produce. This is becoming an important equation in the cost effectiveness and environmental awareness in modern building methods. Timber, if harvested wisely, is a renewable resource, and earth has the human embodied energy it takes to dig it out of the ground, mix it with water and place it in moulds. Once the frame is complete and the roof on, you have a storage area for mud bricks and materials. If you are really desperate you can even move in at this stage but it can be a messy and dusty way to live. A very good way to test a relationship to the limit.