Roofing for Pitched Roofs
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Desirable characteristics for roof surfacing materials are:
The roof shape, type of structure and slope determine the types of roofing material that are suitable. The minimum slope on which a material can be used depends on exposure to the wind, type of joint and overlap, porosity and the size of the unit.
When considering the cost of various roofing materials, it should be noted that those requiring steeper slopes will need to cover a greater area. Table 5.11 provides a guideline for the relative increase in roofing area with an increase in slope. The area for a flat roof has been taken as 100.
The weight of the roof covering material greatly influences the design of the roof structure and the purling. Table 5.12 shows some examples.
Table 5.10 Minimum Pitch Requirements for Roofing Materials
|Roof Covering||Angle||Slope||Rise in mm/m|
|Built-up bitumen felt||3°||1:20||50|
|Corrugated metal sheets (min. 150end laps)||12°||1:5||200|
|Corrugated metal sheets (min. 100 end laps)||18°||1:3||300|
|Corrugated asbestos cement sheets with 300mm end lap||10°||1:5.7||180|
|Corrugated asbestos cement streets with 150mm end lap||22.5°||1:2.4||410|
|Single lap tiles||30°||1:1.7||580|
|Plain tiles in burnt clay||40°||1:1.2||840|
|Slates min 300mm wide||25°||1:2.1||470|
|Slates min 225mm wide||35°||1:1.4||700|
|Thatch of palm leaves (Makuti)||34°||1:1.5||670|
|Thatch of grass||45°||1:1||1000|
|In-situ mud (dry climates only)||6°||1:10||100|
|Fibre-cement rooPmg sheets||20°||1:2.8||360|
|Concrete tiles, interlocking||17.5°||1:3.2||320|
The spacing of the purlins which support the roofing depends on the size and rigidity of the roofing material. The dimensions of the purlins depends on the spacing of the rafters and purling, the weight of the roofing material and the loading on the roof from wind, persons constructing and maintaining the roof and in some areas snow. Timber, round or sawn, is the most common material used for purlins since roofing material can be easily attached by nailing. Where the spacing of trusses is in excess of 2,5 to 3,0m, timber purlins are not feasible and steel profiles are used instead. The profile can be an angle iron or a Z-profile made from plain iron sheets;
Small units such as slates, tiles and shingles are affixed to closely spaced battens of rather small section which means that the rafters must be closely spaced.
Table 5.1 1 Relative Areas of Roofs of Various Sloped
|Angle||Slope||Relative Area of Roofing|
Table 5. 12 Weights of Roofing Materials
|Corrugated aluminium sheet||2.5-5 depending on gauge|
|Corrugated steel sheets||6-9 depending on gauge|
|Corrugated steel sheets, laid||8-12 depending on gauge|
|Asbestos cement sheets||14|
|Asbestos cement sheets, laid||16|
Thatch is a very common roofing material in rural areas. It has good thermal insulating qualities and helps to maintain rather uniform temperatures within the building even when outside temperatures vary considerably. The level of noise from rain splashing on the roof is low but during long, heavy rains some leakage may occur. Although thatch is easy to maintain, it may also harbor insects, pests and snakes.
A number of different plant materials such as grass, reeds, papyrus, palm leaves and banana leaves are suitable and inexpensive when locally available. Although the materials are cheap, thatching is rather labour intensive and requires some skill.
The durability of thatch is relatively low. In the case of grass, a major repair will be required every 2 to 3 years, but when well laid by a specialist and maintained, it can last for 20 to 30 years or longer. The supporting structure of wooden poles or bamboo, although simple, must be strong enough to carry the weight of wet thatch. The use of thatch is limited to rather narrow buildings since the supporting structure would otherwise be complicated and expensive and the rise of the roof very high due to the necessity of a very steep slope. Palm leaves should have a slope of at least 1 :1.5, but preferably 1:1 and grass thatch a minimum of 1: 1 but preferably 1 :0.6. Increasing the slope will improve the durability and reduce the risk of leakage. The risk of fire is extremely high but may be reduced by treatment with a fire retardant as described later.
Grass Thatch Grass for thatching should be: .
For easy handling the grass is tied into bundles. The thatching is started from the eaves in widths of about 1 m. A number of grass bundles are put next to each other on the roof, with the base of the stems to the bottom. The grass is tied to the purlins with bark fibre or preferably tarred sisal cord. In subsequent layers the bundles are laid to overlap the layer underneath by half to two-thirds of their length, which means there will be two to three layers in the finished thatch.
A long needle is used to push the string through and tie the bundles of grass onto the roof-laths. Then the bundles themselves are untied and with the hands the grass is pushed into the right position giving a smooth surface to the roof. Then the string is pulled and this fixes the grass securely in place. Another method leaves the bundles of grass as they are, which gives the roof a stepped surface. The thickness of the new thatch layers varies between 15 and 20cm but later on this will become somewhat thinner because of settling.
Figure 5.48 Thatching wish grass.
Stitch at bottom of first thatch on lowest batten. The second layer must overlay the stitching of the first row and include the top section of the underneath layer in the actual stitch. it is better to have each layer held by 3 rows of stitching. The stitching of every row must be completely covered by the free ends of the next layer above it.
The grass or straw is bound in bundles to the battens forming thatch boards. These boards are manufactured on the ground and bound to the rafters beginning at the eaves and continuing to the ridge. Each board covers with it's free ends the board underneath.
Palm Leaf Thatch (Makuti)
Palm leaves are often tied into makuti mats which are used for the roof covering. They consist of palm leaves tied to a rib (part of the stem of the palm leaf) using the dried fibre of Doum palm leaves or sisal twine.
The mats are laid on the rafters (round poles) and the stems tied to the rafters with sisal twine. The mats are usually produced to a standard size of 600 x 600mm and laid with a 100mm side lap, thus requiring a rafter spacing of 500mm. For a good quality mat 600mm wide, an average of 75 blades will be required. Spacing up the roof slope, i.e. the distance between the ribs of the makuti mats, is usually 150 to 100mm thus forming a 4 to 6 layer coverage, 5 to 8cm thick.
First a papyrus mat is placed on top of the purling, then a layer of black polythene and finally another one or two papyrus mats to complete the roof. These materials are fixed to the purlins with nails and iron wire. Nails are fixed to the purlin at 1 5 to 20cm spacing and the iron wire is then stretched over the top of the papyrus mat and secured to the nails. The papyrus has a life span of about three years but that can be extended by treating the papyrus with a water-repellent paint.
Figure 5.49 Methods of grass thatching.
Figure 5.50 Assembling makuti mats.
Fire Retardant for Bamboo and Thatch
Fire retardant paints are available as oil-bourne or waterbourne finishes. They retard ignition and the spread of flame over surfaces. Some are intumescent, that is, they swell when heated forming a porous insulating coating.
A cheap fire-retardant solution can be prepared from fertilizer-grade diammonium phosphate and ammonium sulphate. The solution is made by mixing 5 kg of diammonium phosphate and 2.5 kg of ammonium sulphate with 50 kg of water. The principal disadvantage is that it is rendered less effective by leaching with rain. Therefore the fire retardant impregnation must be covered with a water repellent paint. The entire roof construction, i.e. bamboo trusses, strings, wooden parts and thatch, should be treated with the fire retardant. The following procedure is recommended:
Impregnation of Thatch
Such roofs do not catch fire easily and fire spreads very slowly.
Figure 5.51 Alternative ridge caps for thatch roofs.
Figure 5.52 Papyrus-polythene roof
Galvanized Corrugated Steel Sheets
Galvanized Corrugated Steel Sheets (GCS) is the cheapest of the modern corrugated sheeting materials and is widely used as roofing material for farm buildings. Unprotected steel would have a very short life, but a zinc coating (galvanizing) adds substantial protection at a relatively low cost. Alternative coatings for steel sheets are bitumen, polyvinyl chloride (plastic) on zinc, asbestos, felt and polyester. If the coating is damaged the steel will rust. When the first signs of rust appear, the sheet should be coated with a lead-based paint to stop the rusting.
The main advantages of GCS are:
The main disadvantages of GCS are its poor thermal properties and the noise caused by heavy rainfall and thermal movements. The thermal and sound properties are improved by an insulated ceiling.
Most corrugated steel sheets have corrugations with a 76mm pitch and 19mm depth. Thickness varies between 0.3-1.6mm of which 0.375-0.425mm are recommended for farm buildings.
Standard widths normally marketed are 610, 762mm and 1000mm. Lengths range from 2 to 4m. See figure 5.53 and Table 5.13 and 5.14.
Figure 5.53 Corrugated roofing with overlap
Table 5.13 Recommendations for Slope, End Lap and Side Lap for Corrugated Steel Roffing
|Type of position||min slope||min end lap||min side lap|
|Sheltered site||1:5||150mm||1.5 corrugation|
|Normal site||1:3||100mm||1.5 corrugation|
|Exposed site||1:3||150mm||2 corrugation|
Figure 5.54 Methods of fastening corrugated roofing to purling.
Table 5.14 Covering width for different side lap and type of corrugated iron sheets
|Type||No. of Corr. mm||Overall Width||Covering
Number of corrugation side lap
|C.S. Nominal 8/7||6 8||610||533||495||457|
|C.S. Nominal 10/76||10||762||686||645||610|
Laying the Sheets
The spacing of the purlins will depend on the thickness of the sheets used. As a guide, maximum spacing of purlins for 0.475mm sheets is 1500mm. The purlins should be a minimum of 50mm in width in order to be easily nailed.
The laying of the sheets should commence from the eave and away from the prevailing wind. The side laps will then be away from the wind preventing water from being forced into the lap.
It is very important that the first sheet be laid at right angles to the eave and the ridge for by so doing, all the rest will also be perpendicular with the ridge. The first row of sheets is laid with a 50mm overhang beyond the facie board.
Special roofing nails are used to fix the sheets to timber purling. They are 67mm long and average about 100 nails per kilo. Under average conditions, the nails should be placed at every second corrugation on the purlin at the eave and then at every third corrugation on other purling. A stretched string along the purlin makes it easier to nail the sheets. Extra nails are needed along the verge (gable end overhang). The nails should always be placed at the ridge of the corrugation to prevent risk of leakage. Roofs in exposed positions require closer nailing. All end laps must occur over purling.
Ridging is normally available in pieces of 1800mm length. They should be fitted with a 150mm overlap. Other accessories such as close-fitting ridges, eaves-filler pieces and gutters are available from some suppliers.
The number of sheets to be purchased for a roof can be calculated by using the following formula:
No. of sheets = (Length of roof x width of roof ) / ( Length of sheets x covering width )
Note that the length of the sheets in the formula is the nominal length minus end lap. When making the bill of quantity for a building, the calculated number of sheets should be increased about 10% due to the waste during transport and installation.
The disadvantages are:
Corrugated asbestos-cement sheets are normally marketed in a variety of corrugations and sizes. However, the most common corrugation which is used for farm buildings has a pitch of 177mm and a depth of 57mm. The sheet width is 920mm. It is supplied in lengths ranging from 1.5m to 3m. The effective coverage width is 873mm.
Storage and Handling
At the building site the sheets should be stacked on timber bearers levelled with each other at not more than 1m centres on firm, level ground. The sheets can be stacked to a height of approximately 1.2m without risk of damage. The sheets should be handled by two men - one at each end.
During installation roof ladders or crawl boards must be used to ensure safety and avoid possible damage to the sheets. Under no circumstances should anyone walk on the sheets between two purling.
Laying the Sheets
Corrugated A-C roofing should be installed with a slope of 1:2.5 (22° ) and an end lap of at least 150mm under normal conditions. Under exposed conditions a 200mm end lap is better. The sheets are designed for a side lap of half a corrugation in all situations.
Purlins must be of sawn timber in order to provide a flat support for the sheets and must be designed with a minimum of deflection. For the type of sheets described here, a maximum purlin spacing of 1.5m is recommended. If used as wall cladding the spacing can be increased to 1.8m.
Sheets should be laid from left to right or right to left depending on the direction of the prevailing wind. Side laps must always be sheltered from the main wind direction.
Figure 5.55 Lapping the roofing against the prevailing wind
Mitring the corner of the sheets at the overlaps is essential to ensure correct positioning and to allow the sheets to lie flat. The smooth surface of the sheet should be laid uppermost. Laying of the sheets should commence at the eaves (or from the lowest course for cladding). The necessary mitring is shown in Figure 5.56.
The correct mitre is most important. This should be made from a point along the edge of the sheet equivalent to the end lap, i.e. either 150mm or 200mm, to a point along the end of the sheet equivalent to the side lap 47mm. The gap between the mitres should be at least 3mm, but not to exceed 6mm. The sheets can be cut with a handsaw or a sheet hacksaw.
Fixing the Sheets
Holes must be drilled 2 to 3mm larger than the diameter of the roofing screws to be used to allow for movement within the framework of the building and the sheets themselves. All holes must be on the crown of the corrugation. It is important to remove all drilling dust before washers are put in position, otherwise water may be allowed to penetrate. Screws should be finger-tight until the correct alignment of the sheets in relation to the purlin has been checked. They should then be tightened until some resistance is felt. Screws should be located in the crown of the second and fifth corrugation of a sheet of seven corrugations. All end laps must occur over the purling.
Sisal-cement Roofing Sheets
These sheets are normally heavier and more brittle than asbestos-cement sheets which means that they will require a stronger roofing structure and even more caution during handling and laying. In all other respects they are similar to and used for construction in the same way as asbestoscement sheets.
Figure 5.56 Mitring asbestos-cement sheets.
Figure 5.57 Mitre dimensions.
Figure 5.58 Screws and cops for asbestos-cement roofing.
Corrugated-aluminium Sheets- CA
CA sheets are lighter and more durable than GCS sheets, but are more expensive. When new, the sheets have a bright reflective surface, but after a year or more oxidation of the surface will reduce the glare. There is never any need to paint aluminium sheets for protection.
The reflective surface will keep the building cooler than with GCS sheets, but since aluminium is softer, the roof is more likely to tear away in a heavy wind storm. Aluminium also has a greater thermal expansion than steel resulting in noisy creaks and more stress on fasteners.
CA sheets are normally supplied with the same corrugation and in the same sizes as GCS. For use in farm buildings, a thickness of 0.425mm is recommended. The sheets are laid and fixed in the same manner as GCS.
Figure 5.59 Examples of single-lap tiles and slate.
Fibreglass-reinforced Plastic Sheets
These sheets are shaped like those of steel, asbestos cement or aluminium and are used to replace some of the sheets in a roof to give overhead light. They are translucent and give good light inside large halls, workshops etc. They are long lasting, simple to install and provide inexpensive light, though the sheets themselves are expensive. They are combustible and must be cleaned occasionally.
Roof Tiles and Slates
Tiles were originally made by hand of burnt clay, but they are now manufactured by machine from clay, concrete and stabilized soil in several sizes and shapes. Plain tiles are usually cambered from head to tail so they do not lie flat on each other. This prevents capillary movement of water between the tiles. The shaped side lap in single-lap tiling takes the place of the double end lap and bond in plain tiles or slates. Many types of single-lap tiles are available, examples of which are shown in Figure 5.59.
Slates were originally made from natural stone, but now are also manufactured from asbestos cement and sisal cement. Since plain tiles and slates have similar properties and are laid and fixed in the same manner, they will be discussed together.
Tiles and slates are durable, require a minimum of maintenance and have good thermal and sound properties. The units themselves are water tight, but leaks may occur between the units if not properly laid. However, handmade tiles tend to absorb water and stabilized-soil tiles may erode in heavy rains. They are fairly easy to lay and fix, but being very heavy, they require a very strong supporting roof structure. The weight is, however, advantageous in overcoming uplifting wind forces. The dead weight of the covering will normally be enough anchorage for the roofing as well as the roof structure.
When rainwater falls on a pitched roof, it will fan out and run over the surface at an angle which is determined by e pitch of the roof. The steeper the pitch, the narrower the angle, while the lower the pitch, the wider the angle. Wider slates will be required for low pitch roofs.
Water running off tile A runs between B and C and spreads between tiles B and D and C and D as shown by the hatched area. It then runs because of the lap, onto the tiles E and F close to their heads. Note that tile is normally laid close together at the sides.
Figure 5.60 Water drainage on tiles.
Table 5.15 Slate and Tile Size, Pitch and Lap
|Unit||Min.||Min.||Minimum lap (mm)|
|Size||Pitch||Slope||Normal sites||Exposed sites|
|305 x 205mm||45°||1:1||65||65|
|330 x 180mm||40°||1:1.2||65||65|
|405 x 205mm||35°||1:1.4||70||70|
|510 x 255mm||30°||1:1.7||75||75|
|610 x 305mm||25°||1:2.1||90||100|
|610 x 355mm||22.5°||1:2.4||100||N/A|
|Concrete and Machine pressed||35°||1:1.4||65||75|
* standard size 265 x 165mm. laid with 32mm side lap
* standard size 265 x 165mm. laid with 32mm side lap
Figure 5.61 Installation of slates.
Plain tiling and slating provides an effective barrier to rain but wind and dust penetrate through the gaps between the units. Therefore boarding or sheeting may be placed under the battens on which the tiles or slates are to be hung. Roofing felt is the material most commonly used for this purpose.
In laying plain tile or slate there must always be at least two thicknesses covering any part of the roof, butt jointed at the side and placed so that no vertical joint is immediately over another vertical joint in the course below. To ensure this, shorter length units are required at the eaves and the ridge and each alternate course is commenced with a tile or slate of one-and-a-half units in width. The ridge is capped with special units bedded in cement mortar.
The hips can be covered with a ridge unit, in which case the plain tiling or slating is laid underneath and mitred at the hip. Valleys can be formed by using special units.
Plain tiles are ordinarily fixed with two galvanized nails in each tile at every fourth or fifth course. However, in very exposed positions every tile should be nailed.
Slates, which do not have nibs securing them to the battens, should be nailed twice in every unit. Plain tiles and small slates are nailed at the head while long slates are sometimes nailed at the centre to overcome vibrations caused by the wind. Centre nailing is mainly used for pitches below 35° and in the courses close to the cave.
Figure 5.62 Plain tile roof
The battens upon which the slates or plain tiles are fixed should not be less than 40mm wide and of sufficient thickness to prevent undue springing back as the slates are being nailed to them. Thus the thickness of the battens will depend upon the spacing of the rafters, and for rafters spaced 400 to 460mm on centres, the battens should be 20mm thick.
The distance from the centre to centre of the battens is known as gauge and is equal to the exposure of the slate or tile.
Figure 5.63 Single-lap tiles.
Wood shingles are pleasing in appearance and when made from decay-resistant species, will last 15 to 20 years even without preservative treatment. Cedar and cypress will last 20 years or more. Wood shingles have good thermal properties and are not noisy during heavy rain. The shingles are light and not very sensitive to movements in the supporting structure, which means that a rather simple roof frame made of round timber can be used. The shingles are laid starting at the eaves, touching on the sides and doubled lapped. This means that there are three layers of shingle over each batten. Each shingle is fastened with one galvanized nail to the batten. No nail should go through two shingles. The shingles can be laid either with the core side of the timber alternating up and down in the successive rows, or with the core side down in all rows, thereby using the cupping effect of timber after drying to produce a roof cover less prone to leakage.
Figure 5.64 Core-side effect on wood shingles.
The simplest form of bamboo roof covering is made of halved bamboo culms running full length from the eaves to the ridge. Large diameter culms are split into two halves and the cross section at the nodes removed. The first layer of culms is laid side by side with the concave face upwards. The second is placed over the first with the convex face upwards. In this way the bamboo overlaps as in a tile roof and can be made completely watertight. Several types and shapes of bamboo shingle roofing may be used where only smaller sizes of bamboo culms are available.
Rainwater Drainage from Roofs
The simplest method is to let the roof water drop onto a splash apron all around the building. This method also protects the walls from surface ground-water. The water is then collected in a concrete ground channel or allowed to flow onto the ground surrounding the building to soak into the soil. This latter method can only be recommended for very small buildings since the concentrated flow from a larger building may cause considerable soil erosion and damage to the foundation. The water from ground channels is drained into a soakaway or collected and stored. Blind channels are frequently used. These are simply trenches filled with stones that act as soakaways either for a ground channel or for a splash apron.
Figure 5.65 Roofing with wood shingles.
Figure 5.66 Bamboo shingles.
Pitched roofs are often provided with eave gutters to collect and carry the rainwater to downpipes which deliver the water to ground drains or a tank. Flat roofs are usually constructed with a slight fall to carry rain water directly to a roof outlet.
The sizing of gutters and downpipes to effectively remove rainwater from a roof will depend upon:
Pitched roofs receive more rain than their plan area would indicate due to the wind blowing rain against it. An estimate of the effective area for a pitched roof can be made by multiplying the length by the horizontal width plus half the rise.
In order to find the flow, the area is multiplied by the rainfall rate per hour. The rainfall intensity during a heavy rain will vary between areas and local data should be used where available. As a guide, rainfall values of 75 to 100mm per hour may be used. Gutters should be installed with very little fall, 0.3% being recommended. Falls which are too steep cause difficulties because the water flows too rapidly leaving trash behind. Also gutters with more than a slight fall do not look well.
Table 5.16 Flow Capacities in Litres per Second for Level Half-round Gutters
|Gutter size mm||Flow l/s|
There is always the possibility that unusually heavy rain, or a blockage in a pipe, may cause gutters to overflow. With this in mind, it is always advisable to design a building with a roof overhang so that in case of overflow the water will not flow down the facade or make its way into the wall where damage may result.
Common material for gutters and downpipes are galvanized steel, aluminium and vinyl. The galvanized steel is the least expensive. Aluminium is long lasting but easily damaged. The vinyl is both durable and resistant to impact damage.
Two major types of gutter brackets are normally available. One is for fixing the gutter to a fascia as illustrated in Figure 5.67. The other is used when there is no fascia board and the gutter is fixed to the rafters. The roof cover should extend 50mm beyond the ends of the rafters or the fascia board in order to let the water drop clear.
Figure 5.67 Gutter and downpipe fastenings.
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