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Building floors may be as simple as the compacted soil present on the site before the building was constructed or as complex as attractively finished hardwood parquet. A well-chosen, well-built floor offers protection from vermin and rodents, is easy to clean, dry, durable and is a valuable asset to a building. For special circumstances it may be designed to be washable, particularly attractive, thermally insulated, sloped to a drain or perfectly net and level.
For farm buildings, including homes, simple floors offering hard, durable surfaces at ground level grade are probably adequate for the vast majority of situations. Floors may be built at ground level, i.e. on the soil within the building, in which case they are called solid or grade floors, or they may be supported on joists and beams in which case they are called suspended or above-grade floors. The finished level of a solid floor should be at least 150mm above outside ground level as a protection against flooding. The top soil should be removed and replaced with coarse material before the actual floor slab is constructed.
Figure 5.30b Plastered bamboo wan mats
Figure 5.30c Woven bamboo panels (Japanese wall panels)
Solid or Grade Floors
Tamped soil is often satisfactory for the floors of animal shelters and perhaps the homes of subsistence farm families. They should be designed a little above the ground level outside the building and will be improved by being stabilized with ant-hill clay, cow dung, lime or Portland cement.
A discussion of stabilizing materials to use for different circumstances will be found in Chapter 3.
Concrete makes a more durable, harder and cleaner floor. Properly constructed concrete floors can be made dry enough to be used for grain storage or the farm home. Figure 5.32 shows cross sections of stabilized soil and concrete floors. An earth floor suitable for a well-drained site is shown in figure a, while a concrete floor that needs to be moderately dry is shown in b. The single-size coarse aggregate shown in c, is used to prevent capillary movement of water to the underside of the floor. The polythene sheet prevents moisture from reaching the concrete slab and the layer of sand or mortar protects the sheet from being punctured.
Floor
- 2 layers of tamped stabilized soil (50mm)
- Tamped sand
- Existing subsoil (topsoil removed)
Figure 5.32a Well drained site
Floor
- Concrete slab (min. 75mm)
- Tamped sand (100-150mm)
- Existing subsoil
Figure 5.32b Well drained site
Floor
- Concrete slab (min. 75mm)
- Polythene sheet (750 guage)
- Sand (50 mm) or mortar (25mm)
- Coarse aggregate (150-200mm)
- Existing subsoil (eu. compacted)
Figure 5.32c Poorly drained site or a very dryfloor is required
Construction of solid floors built at ground level.
The concrete mixture chosen to be used in a solid floor will depend on the severity of use and type of loading. For a deep litter building or a subsistence farm dwelling a mix of 1 :3:6 by weight may be satisfactory for the light service to which it will be subjected. Floors that will be exposed to heavy loads, as in a bag grain store or a farm repair shop, will need to be stronger. A 1 :2:4 should be adequate over a good firm base. The floors in a creamery or slaughterhouse are subjected to acid erosion and require a richer mix of concrete ( 1 :2:3) to give a durable surface.
Pouring Concrete Floors
Solid concrete floors should be laid on a level and tampered base of hardcore or gravel. On well drained sites also sand or even laterite can be satisfactory. The base layer should be at least 100mm thick. While it is desirable for the finished floor level to be at least 150mm above the surrounding ground, some fill may be required under the base course. However, fill need to be thouroughly compacted to get the required stability and, generally, it is therefore more satisfactory to increase the thickness of the base course. Any material used for fill or for the base course must be free of organic matter. hence, the excavated top soil must be rejected as fill. If a damp-proof barrier (polythene or a 3mm thick hot bitumen layer) is to be installed, a layer of sand should be spread over the base. Sand can also be used as blinding on a hardcore base to reduce the amount of concrete that 'disappears' in the gaps between the stones. Finally, 75 to 150mm timber screeds are put in place to be used as guides in striking off and leveling the concrete and reinforcement bars, if advocated, are put in position. The thickness of the slab will depend on the expected loads, the quality of concrete used, reinforcement and the bearing characteristics of the ground.
A floor area that is larger than about 10mē should be divided into bays for concreting. This will help to prevent the development of shrinkage cracks during the curing process and will allow for each bay to be cast, leveled and finished within a managable time. Square bays are best and 2.5 to 4m sides allow a slab to be cast in one go.
The concrete can then be mixed and placed. Regardless of the mix chooser, the concrete should be kept as stiff as possible and the size of the coarse aggregate should not exceed a quarter of the thickness of the slab. The bays are' concreted alternately as shown in Figure 5.33. When the first set of bays have hardened the timber screeds are carefully removed and the remaining bays can be cast.
Once the concrete is placed it is leveled by moving a straight timber along the screeds, or, in case of the second set of bays, the already hardened concrete in adjoining bays, with a sawing action. The concrete can then be 'floated' slightly to smooth the surface. After the initial light floating, the bay can be left for a few hours before a final floating to give it a smooth surface. If a non-slip floor is desired, the concrete can be broomed soon after it is placed to give a rough surface. It will not be touched again until it sets. Once the concrete is set, it should be kept moist for a week.
Suspended or Above-grade Floors
Timber Floors
Suspended timber ground-level floors are useful on sloping sites where a great deal of filling would be required to level the ground for a solid floor.
Timber ground-level floors must be well protected against moisture, fungus and termites and must therefore be raised above the ground. The space under such a timber floor should be high enough to ensure good ventilation and to allow a person to crawl underneath for inspecting the floor. Termite protection is more likely to be effective if the floor is raised above the ground at least 45cm.
Figure 5.33 Concrete floor construction
The supporting piers are frequently built of timber but are better if made of stone, concrete or steel. Hollow concrete blocks reinforced and filled with concrete make a strong support. Metal termite shields should be fitted to the top of the foundation wall and to steeper walls and piers.
The foundation wall beneath a timber ground-level floor must be fitted with ventilation openings to ensure good air exchange in the crawl space below the floor. The openings should be covered with 10mm mesh screen to keep rodents out.
When the span is more than 5m, joists may be supported by cross walls built with 150mm solid concrete blocks laid about 80mm apart in a honeycomb pattern to allow free passage of air.
Beams of steel, timber or concrete may be used to support upper floors when the span is over 5m.
Figure 5.34 Suspended timber floor construction.
Suspended Concrete Floors
The main advantage of a reinforced-concrete suspended floor is its greater fire resistance and better sound insulation than that of a timber floor, but it is generally too expensive to find applications in farm buildings.
In its simplest form it consists of a cast-in-situ, one-wayspan slab with the reinforcement acting in one direction only between two supports not more than 5m apart. The reinforcement may be either mild steel main rods and distribution bars wired together at right angles, or reinforcing mesh consisting of main bars and distribution bars electrically welded at the crossings. The reinforcement must be designed by a qualified structural engineer or obtained from a reliable standard design.
Floor Finishes
In rural areas the extra cost of a floor finish is often considered unnecessary, the surface of a slab of concrete or stabilized soil having a durability which is satisfactory for most purposes. However, a floor finish can enhance the appearance of the room, cut down on noise or make the floor easier to clean, depending on the type of finish used.
A cement and sand screed or a granolitich finish (one part cement and three parts hard stone chippings laid about 30mm thick) may be used where an extremely durable finish is needed. Sheet materials and slab tiles are likely to be very expensive, but slab tiles are in exceptional cases installed in farm buildings because of their durability. A typical wood floor over a solid slab is shown in Figure 5.35.
Note that the space between the concrete slab and the wood flooring should be ventilated. a Cured and dried out concrete or stabilized soil slab, preferably with damp-proof course between slab and hardcore. b Joists 50 x 50mm c Bulldog floor clips d Wood flooring or chipboard
Figure 5.35 Wood floor on a solid concrete poor.
A roof is an essential part of any building in that it provides the necessary protection from rain, sun, wind, heat and cold. The integrity of the roof is important for the structure of the building itself as well as the occupants and the goods stored within the building.
The roof structure must be designed to withstand the dead load imposed by the roofing and framing as well as the forces of wind and in some areas, snow or drifting dust. The roofing must be leakproof, durable and perhaps satisfy other requirements such as being fire resistant, a good thermal insulator or high in thermal capacity.
There is a wide variety of roof shapes, frames and coverings from which to choose. The choice is related to factors such as the size and use of the building, its anticipated life and appearance, and the availability and cost of materials. Roofs may be classified in three ways:
Flat and pitched roofs: A roof is called a flat roof when the outer surface is within 5° of horizontal whereas a pitched roof has a slope of over 5° in one or more directions. Climate and covering material affect the choice between a flat or pitched roof. The affect of climate is less marked architecturally in temperate areas than in those with extremes of climate. In hot, dry areas the flat roof is common because it is not exposed to heavy rainfall and it forms a useful out-of-doors living room. In areas of heavy rainfall a steeply pitched roof drains off rainwater more quickly.
Two-dimensional roof structures have length and depth only and all forces are resolved within a single vertical plane. Rafters, roof joists and trussers fall in this category. They fulfill only a spanning function and volume is obtained by using several two-dimensional members carrying secondary two-dimensional members (purling) in order to cover the required span.
Three-dimensional structures have length, depth and also breadth, and forces are resolved in three dimensions within the structure. These forms can fulfill a covering and enclosing function as well as that of spanning and are now commonly referred to as 'space structures'. Three dimensional or space structures include cylindrical and parabolic shells and shell domes, multi-curved slabs, folded slabs and prismatic shells, grid structures such as space frames, and suspended or tension roof structures.
Long and short span roofs: Span is a major consideration in the design and choice of a roof structure although functional requirements and economy have an influence as well.
Short spans, up to 8m, can generally be covered with pitched timber rafters or light-weight trusses either pitched or flat. Medium spans of 7 to 15 or 16m require truss frames designed of timber or steel.
Long spans of over 16m should, if possible, be broken into smaller units. Otherwise, these roofs are generally designed by specialists using girder, space deck or vaulting techniques.
In order to reduce the span and thereby reduce the dimensions of the members, the roof structure can be supported by poles or columns within the building or by internal walls. However, in farm buildings a free span roof structure will be advantageous if the farmer eventually wants to alter the internal arrangement of the building. The free space without columns allows greater convenience in maneuvering equipment as well.
Ring beam: In large buildings e.g. village stores, that have block or brick walls, a 150mm square reinforced concrete beam is sometimes installed on top of the external walls instead of a wall plate. The objective of this ring beam, which is continuous around the building, is to carry the roof structure should part of the wall collaps in an earth tremor. It will also provide a good anchorage for the roof to prevent it lifting and reduce the effects of heavy wind pressure on the walls and unequal settlement.
Figure 5.36 Three-dimensional roof structures.Earth dome and vault
Figure 5.36 Three-dimensional roof structures. Grid structure
Types of Roofs
Flat Roof
The flat roof is a simple design for large buildings in which columns are not a disadvantage. Simple beams can be used for spans up to about Sm. but with longer spans it is necessary to use deep beams, web beams or trusses for adequate support. Because farm buildings often need large areas free of columns, flat roofs with built-up roofing are not common. Flat roofs are prone to leak. To prevent pools of water from collecting on the surface they are usually built with a minimum slope of 1:20 to provide drainage.
The roof structure consists of the supporting beams, decking, insulation and a waterproof surface. The decking, which provides a continuous support for the insulation and surface, can be made of timber boards, plywood, chipboard, metal or asbestos-cement decking units or concrete slabs.
The insulation material improves the thermal resistance and is placed either above or below the decking.
The most common design for a waterproof surface is the built-up roof using roofing felt. This material consists of a fibre, asbestos or glass-fibre base which has been impregnated with hot bitumen. The minimum pitch recommended for built-up roofs is 1:20 or 3° which is also near the maximum if creeping of the felt layers is to be prevented.
For net roofs two or three layers of felt are used, the first being laid at right angles to the slope commencing at the eaves. If the decking is timber the first layer is secured with large flat-head felting nails and the subsequent layers are bonded to it with layers of hot bitumen compound. If the decking is of a material other than timber all three layers are bonded with hot bitumen compound. While it is still hot the final coat of bitumen is covered with a layers of stone chippings to protect the underlying felt, provide additional fire resistance and give increased solar reflection. An application of 20 kg/mē of 12.5mm chippings of limestone, granite or light-coloured gravel is suitable.
Where three layers of roofing felt are used and properly laid, flat roofs are satisfactory in rainy areas. However, they tend to be more expensive than other types and require maintenance every few years.
Figure 5.37 Built-up roofing felt.
Earth Roof
Soil-covered roofs have good thermal insulation and high capacity for storing heat. The traditional earth roof is subject to erosion during rain, requires steady maintenance to prevent leakage. The roof is laid rather flat with a slope of 1:6 or less.
The supporting structure should be generously designed of preservative - treated or termite-resistant timber of poles, and inspected and maintained periodically, as a sudden collapse of this heavy structure could cause great harm. The durability of the mud cover can be improved by stabilizing the top soil with cement, and it can be waterproofed by placing a plastic sheet under the soil. Figure 5.38 and Figure 5.39 shows two types of earth roofs.
Figure 5.38 Cross-section of an improved earth roof
Figure 5.39 Earth roof with bitumen waterproofing.
However, the introduction of these improvements adds considerably to the cost of the roof. The improved earth roof therefore is a doubtful alternative for low-cost roofing and should be considered only in dry areas where soil-roof construction is known and accepted.
Monopitched Roof
Monopitch roofs slope in only one direction and have no ridge. They are easy to build, are comparatively inexpensive and are recommended for use on many farm buildings. The maximum span with timber members is about 5m, thus wider buildings will require intermediate supports. Also wide buildings with this type of roof will have a high front wall which increases the cost and leaves the bottom of that wall relatively unprotected by the roof overhang. When using corrugated steel or asbestos-cement sheets, the slope should not be less that 1:3(17 to 18°). Less slope may cause leakage as strong winds can force water up the slope.
The rafters can be of round or sawn timber or when wider spans are required, of timber or steel trusses which can be supported on a continuous wall or on posts. The inclined rafters of a pitched roof meet the wall plates at an angle and their load tends to make them slide off the plate. To reduce this tendency and to provide a horizontal surface through which the load may be transferred to the wall without excessively high compressive forces, the rafters in pitched roofs are notched over the plates. To avoid weakening of the rafter, the depth of the notch (seat cut) should not exceed one-third that of the rafter. When double rafters are used a bolted joint is an alternative. The rafters should always be thoroughly fixed to the walls or posts to resist the uplifting forces of the wind.
Figure 5.40 Pole framing for a monopitch building.
Double-pitched (Gable) Roof
A gable roof normally has a centre ridge with a slope to either side of the building. With this design a greater free span (7 to 8m) is possible with timber rafters than with a monopitch roof. Although the monopitch design may be less expensive in building widths up to 10m the inconvenience of many support columns favors the gable roof. The gable roof may be built in a wide range of pitches to suit any of several different roofing materials. Figure 5.41 shows a number of the elements that are associated with a gable roof. The following description is keyed to the figure:
Figure 5.41 Gable roof design.
The angle of the ridge and seat cuts can be laid out on the rafter using a steel carpenter's square and the appropriate rise and run values both on the outside of the blades or both on the inside of the blades of the square, 30 and 20cm in the example in Figure 5.42. The length may be found with the pythagorean theorem using the rise and run of the rafter. The length is measured along the workline.
When a gable roof must span more than 7 to 8m, trusses are usually chosen to replace plain rafters. For large spans the trusses will save on total material used and provide a stronger roof structure. For solid roof decks the trusses are usually designed to be spaced approximately 600mm on centre, while for rigid roofing mounted on purling, a truss spacing of 1200mm or more is common.
The agricultural extension can provide designs for the spans, spacings and loads that are commonly found on farms. Also, in Chapter 4 the theory of truss design is discussed. Figure 5.43 illustrates a simple truss design.
Figure 5.4.2 Laying out a common rafter.
Figure 5.4.3 A " W" truss design.
Due to large negative windloads, roofs are in danger of being blown off. Therefore it-is important to anchor the roof trusses properly to the wall plates. This can be done with strips of hoop iron, one strip tying the wall plate to the wall at every 90cm and the other tying the trusses to the wall plate. See Figure 5.44. In the coastal areas it is advisable to use galvanized strips. If the walls are plastered the strips can be recessed in the wall by cutting a channel and covering the strip with mortar.
Figure 5.44 Anchoring trusses to the wall
For stores or other buildings where tractors and lorries may be driven inside, considerable free height is necessary. Rigid frame structures are well suited for this purpose. A simple frame can be built of gumpoles or sawn timber connected with bolts as shown in Figure 5.45.
Rigid frames are also manufactured at factories in steel and reinforced concrete.
Hip Roof
A hip roof has a ridge in the centre and four slopes. It is much more complicated in its construction, necessitating the cutting of compound angles on all of the shortened rafters and the provision for deep hip rafters running from the ridge to the wall plate to carry the top ends of the jack rafters. The tendency of the inclined thrust of the hip rafters to push out the walls at the corners is overcome by tying the two wall plates together with an angle tie. At the hips and valleys the roofing material has to be cut at an angle to make it fit. The valleys are prone to leakage and special care has to be taken in the construction.
Four gutters are needed to collect the rain water from the roof, but that does not mean that there is any increase in the amount of water collected. Because this is an expensive and difficult way to roof a building, it should be recommended only where it is necessary to protect mud walls or unplastered brick walls against heavy driving rain and for wide buildings to reduce the height of the end walls.
Figure 5.45 Timber rigid frame.
Conical-shaped Roof
The conical roof is a three dimensional structure that is commonly used in rural areas. It is easy to assemble and can be built with locally available materials, making it inexpensive. It must be constructed with a slope appropriate to the roofing materials used to prevent it from leaking. The conical roof design is limited to rather short spans and to either circular or small square buildings. It does not allow for any extensions. If modern roofing materials are used there is considerable waste because of the amount of cutting necessary to obtain proper fit.
A conical-shaped roof structure requires rafters and purling, and in circular buildings, a wall plate in the form of a ring beam. This ring beam has three functions:
In the case of square buildings, the outward pressure on the walls from the inclined rafters cannot be converted to pure tensile stress in the wall plate. Instead, it resembles the hip roof structure and should be designed with the angle ties across the wall plates at the corners.
Figure 5.47 Conical roof design.
