|FCR: Fibre Concrete Roofing (SKAT, 1987, 185 p.)|
In chapter 5 Heinz Wehrli is treating the aspects
- Roof structure
- Protection against insects and decay
- Structural demands, wind loads
- Fixing the elements
- Ridge tiles
- Who is installing the elements
- It ist not possible to consider the FC technology in isolation of the roof structure.
- Most damages to FC products on roofs can be tracked down to faults in the roof structure and in the way the products were placed and fixed.
- The smaller the element, the more easily it adapts to the structure.
- The tile presents several advantages over the sheet, but usually needs imported equipment and electricity. On the other hand there is still room for improvement of the sheet technology.
- The design should be simple, e.g. gable roof with rafters and purling, or prefabricated trusses and purling. Minimum pitch of 20°, but up to 30° is required in areas where torrential driving rain occurs. Imposed loads, especially wind suction, may produce higher forces than self weight.
- Good quality timber is desirable but wood is getting scarce and expensive.
- FC tiles need approximately the same quantity of timber as FC sheets.
- Craftsmanship: Carpenters who traditionally build roofs for C.I. sheets will need special training to achieve a more demanding qualify execution. Purlins and tile laths must be straigt and parallel. FCR elements, particularly sheets, cannot adapt to inaccuracies of supports without risk of cracking.
Installation of roof cover
- Prior to installation on roof, check FC elements for good quality, exact size, properly fitting overlaps, correctly formed mitres, etc. The laying of sheets should preferably be done by a team of carpenters and sheetmakers. Edges and overlaps should be well aligned. The installation of the ridge tiles requires careful attention too.
- Sheets should be handled with great care.
- Fixing: the driving of nails through elements should be discouraged. If screws or J-hook bolts are used they must not be overtightened. Cast-in wire loops which allow fixing from underneath prove to be the best solution so far. Sheets should rest on both purlin and bottom sheet at each corrugation.
TREAT FC PRODUCTS LIKE CLAY TILES! NEVER GO ONTO A ROOF WITHOUT CRAWLING BOARDS!
14 questionnaires were scrutinized with regard to roof structure and installation of FC elements.
Most respondents build gable roots, usually with trusses and purlins. A few prefer rafters to trusses. No material other than timber is used. There is little information available on truss assembly. Two participants mention bolts, otherwise nails are used.
The span of the trusses, or width of building varies from 5 to 10 m, the distance between rafters, or trusses, from 0.8 m to 2.5 m; minimum required roof pitches mentioned are between 14° and 26°.
Protection against insects and decay
Only halt of the participants think that some sort of protection Is necessary. Usually, waste engine oil is applied.
Structural demands towards the roof
There is little awareness of the necessity to build a strong roof. Three of the questionnaires mention wind speed of 120-200 km/in. One participant thinks that the roof has to support only the self weight of FCR tiles; another one mentions earthquakes in his area. One mentions, that the roof should be able to support the weight of people installing or repairing the roofing elements.
FCR: sheets or tiles?
Ten participants make sheets, four make tiles. If given a choice, six would continue with sheets, but eight would prefer tiles.
How do the elements overlap?
Only five participants give a clear answer. For sheets the normal end overlaps are 6, and side overlaps probably according to the available asbestos moulds. For tiles the overlaps are as per standard I.T.W. specifications.
Method of fixing the elements
For sheets, two participants install J-hooks, two others fix them with nails, another one uses screws. Fixing through cast-in wire loops is used for both sheets and tiles.
Except three participants who install either galvanized iron sheet ridges, concrete ridges or clay tiles, all the others manufacture their own V-shaped FCR ridge tiles.
By whom are the elements Installed on the roof?
Eight participants say that the elements are installed by an experienced work group, in five instances this group being identical with the producers team. Five others go along with do-it-yourself methods. As for guidance, practical on-the-pb training as well as written guidelines are applied.
Ten participants claim that it is fairly easy to carry out repairs. Four participants, working with sheets, maintain that repairing is difficult. Commonly used repair methods are:
- very fine cracks are sealed with paint or cement slurry
- on small cracks, first paint with PVC glue (white
carpenters glue), then apply a strip of FC mortar, approx. 1/4
- if cracks are large the element has to be replaced.
Five participants claim that no maintenance is needed. The others mention checking of the wooden structure, e.g. sagging or badly warped purling. The fixings need to be checked, and cracks may need repairs. The roof cover should be kept clear of leaves, accumulated dirt, etc.
This chapter deals only with timber roofs. Steel structures, for a number of reasons, would not be regarded as appropriate technology (import of material, mechanical workshop facilities, special rust protection paint, higher cost).
The design should be simple and limit itself to lean-to roofs i.e. single pitch, and gable roofs i.e. double pitch.
Lean-to roof (fig. 1)
It comprises mainly rafters and purling. The span of the rafters i.e. the width of the building should normally not exceed 3,5 m. The advantages are: no trusses required, no ridge, simple construction. But on the other hand it might be difficult to get adequate timber sizes.
Fig. 1 Lean - to roof
Gable roof (fig. 2)
The traditional design consists of rafters and purling, the rafters resting either on a ridge purlin or spine wall, and on a wall plate. This system is suitable for buildings of approx. up to 6,0 m width. For wider buildings, triangular trusses provide an economical solution. (fig. 3)
Fig. 2 Gable roof
Fig. 3 Example of roof truss
Properly designed trusses are able to span very wide buildings. But for usual FCR purposes the practical limit would be around 10 m. The truss presents several advantages compared to the simple rafter, such as smaller timber sizes, a more stable roof structure and absence of horizontal loads on top of walls. But their manufacture require more professional skills. The distance between trusses or rafters will depend on the imposed loads, the purlin size and the quality of the available timber.
For usual conditions, using 2 x 3 purilns on edge, a span of about 1.5-1.7 m will be suitable for sheets. If tiles are used, the laths (small purlins) are of smaller size, and the distance between trusses or rafters is reduced. The roof pitch, or slope, should be approx. 4:10, or 22°, for average climatic conditions, with end overlaps of sheets of at least 6. A steeper pitch of up to 30° or even more Is necessary in regions of torrential driving rain to prevent water from entering between overlaps.
Loads on roof
It is commonly held that the roof structure to support FCR must be heavier than that to support a lighter cladding, e.g. G.C.I. sheet. In fact, It Is the Imposed loads (wind or people), not self weight, which produces the main forces in the roof structure. Very often traditional roofs intended for G.C.I. sheets are built too weak to carry Imposed loads.
It is useful to obtain climatic data on maximum wind speed, to refer to local building codes, if any, and to the experience of the inhabitants, e.g. local farmers, carpenters. In many cases the wind pressure is lower than the wind suction, the latter leading to considerable uplift forces, particulary on low-pitched roofs. (fig. 4)
Fig. 4 Wind load
An example of loads that may act on a gable roof, with a 22° pitch, covering a closed one-storey building with windows, and assuming a max. wind speed of 100 mph (160 km/h):
- self weight of timber roof structure: approx. 20 kg/m2
- self weight of FCR: sheets 30 kg/m2; tiles 22 kg/m2
- self weight of ceiling, if any: 10-20 kg/m2
- live load (e.g. workers during construction or repairs): 100 kg on mid - span of purlin 2x50 kg on 1/4th span of tile laths
- snow load depending on altitude (rare in the case of 3rd world countries)
- wind pressure on windward slope: practically nil
- wind suction on lee slope: 60-80 kg/m2
It should be noted, that at roof overhangs, ridges, protruding edges and corners, the suction forces are much higher (fig. 5). These must be resisted by especially firm fixing of the roof structure and coverings in these areas (see fig. 6, 7, 8, 14). Moreover, if a building has large openings. or in the case of an open shed, the wind forces increase further.
Fig. 5 Areas of more severe suction
A large overhang is good building practice. It increases the life of the walls by preventing the flow of rainwater and it helps to reduce the Inside temperature. For practical purposes, such as timber sizes, overhangs have to be limited to approx. 3 ft. at eaves and 2 ft at verges.
The size of sheets, or tiles, determines the lengths along the ridge and along the slope, overlaps being deducted. Whereas there is no limit to the lengt of the roof, its width is limited and for practical purposes each roof slope should not exceed 6-7 m.
The availability and the properties of the required materials should be taken into account already when designing the roof. Wood. Usually sawn timber is used, but reportedly poles have also been used. Depending on its species, bamboo too may have good structural properties, but connections (joints), accurate dimensions and preservative treatment are more of a problem than with timber. For purlins it will always be preferable to use sawn timber. It is wise to choose a wood species that has a proven record with local builders.
The timber has to be selected carefully. Cheap, low grade timber might not be economical in the long run. Bolts, nails or other devices for assembly (e.g. tin sheet connectors) should match the timber sizes.
Assembly and erection
This work should be done by skilled carpenters. The various parts of the truss are connected solidly. Often bolts are used, but properly designed and executed nailed joints provide better and more rigid connections. Multi-nailed galvanized tin sheet or plywood gussets may also be used. It should be noted that any inaccurancies in the roof structure can be accommodated much less by FC sheets than by FC tiles, asbestos cement or G.C.I. sheets. To ensure uniformity when assembling the trusses, the use of a template may be of help, e.g. wooden pegs driven into the ground, or pencil marks on the floor. For nailed hardwood joints it is necessary to drill pilot holes. A separate plywood template for each type of joint is of great help when positioning the nails.
The trusses must be securely fastened to the building by means of anchors resisting the wind uplift forces mentioned above (fig. 6). In the case of wallplates, they themselves should be fixed well to the wall (fig. 7). Anchors consisting of iron rods that penetrate just into a couple of rows of brickwork would be insufficient. In cyclon-prone areas it is not unusual that complete roofs are lifted and torn from the building.
Fig. 6 Example of anchoring a truss
Fig. 7 Fixing of wallplate and rafter
The purlins should be in line and correctly spaced according to sheet or tile lengths minus overlaps. As the timber is usually rough sawn it is helpful to plane the top edges straight, at least in the case of FCR sheets. When fixing the purlins make sure the top edges are parallel, for example by sighting over the purlins from the ground or from the scaffold. Typical purlin spacings for 1 m-sheets: 85 cm, for standard tiles: 40 cm.
The purlins should be well fastened to the rafters. Driving just a nail into the rafter is not sufficient in regions with high wind speeds. Adding tin straps, or wooden cleats about 1 1/2 x 1 1/2 size, nailed to both purlin and rafter are a good solution (fig. 8). They also prevent the purlin from tilting in the case of a greater roof pitch.
Fig. 8 Fastening of purlin to rafter
Wind bracing should not be omitted. It would be wrong to rely on the FCR sheets or tiles to stiffen the roof structure. The wind braces, approx. 3 x 1 1/2 size, are fastened diagonally onto the purling, adjoining both gable walls. All metal parts should be protected with a good red oxide primer paint.
Protection of timber
In tropical regions it is not easy to protect timber against attack by wood boring beetles, white ants, etc. Suitable wood preservation insecticides are often not readily available. <Dieldrln> or <Lindane>- type emulsion concentrates are very persistent and toxic to insects in extremely small quantities. The usual safety precautions when handling poisonous liquids should be taken.
- Decay, fungi
Whereas decay (rot) cannot take place in dry wood, a preservative treatment should be applied to all parts exposed to humidity and weathering, especially roof overhangs. Methods using pressure to ensure deep penetration are the most suitable but access to such installations will normally not be possible. A reasonably good protection can be achieved by two or three applications of <Creosote> on all exposed parts. Used motor oil can also be applied. End grain should be treated with special care. Coal tar too provides a good protection, but it is unsightly. It should be used on all wood surfaces in contact with masonry.
Installation of roof cover
- Preliminary remarks
The installation of FCR sheets is a delicate job and should be done by skilled workers. It is of advantage to have a team composed of sheetmakers and carpenters from the roof construction gang. They will be confronted with the various practical problems and thus be able to effect improvements during future roof building and sheet manufacturing.
- Overlaps, mitres
A precondition for proper installation is to have well made sheets of good quality. Properly fitting side and end overlaps are very important. Note that with thicker sheets the end overlaps will not fit as well as with thinner sheets. Rough <flashings> of cement along edges should be removed by running a wet mosaic stone along all arrises. Since cutting the corners of finished sheets is practically impossible, the mitres are usually incorporated during forming. This means that mitres are present also in unwanted positions, such as along eaves, ridges and verges. It is of course possible to make several types of sheets with mitres only at those corners where actually required. But this would lead to complications such as special frames for each type of sheet (costly), risk of errors during production, increased number of spare sheets, etc. Several trials at the manufacturing stage may be necessary in order to achieve correctly shaped mitres, i.e. with a clearance of 1/8 - 3/8. It is easier, and also cheaper, to use wooden screeding frames for these trials.
- Laying of sheets
In order to improve waterproofing some technicians recommend laying the courses by advancing toward the prevailing wind direction. This system makes it necessary to have at least two types of mitred sheets (left and right). But in many regions the prevailing wind direction may change with the seasons, thus making this complication fruitless.
Coming back to the simple method with just one type of sheet one starts with each row from bottom to top. Each side overlap should be checked for good fit. This is done by sighting from the ground or from the scaffold because the workers who place the sheets are unable to see the overlap without climbing onto the sheets (which should be stronly discouraged).
As a general rule, walking on the roof cover must be avoided. In exceptional cases, i.e. for repairs or cleaning, crawling boards should be laid carefully above the purling. Sliding of the boards can be prevented by placing jute cloth or similar material between boards and FCR. With sheets made true to size and square it should not be a problem to align the edges of the eaves sheets. For better appearance and fit, the eaves purlin should be approx. 1/2 higher than the other purling.
Once all sheets have been laid on one slope the work continues on the other slope, but in the opposite direction.
At this stage it is necessary to install the ridge tiles (fig. 9), gradually with each row of sheets. This in order to avoid the need of later on climbing onto the roof for ridge tile installation.
Fig. 9 Laying of ridge tiles
- Laying of ridge tiles
The FC ridge tiles are manufactured at the same time an the sheets or pantiles. The simple V-shape type is a good design. As it has no corrugations it should cover the sheet by about 10, or more, in regions of torrential driving rain.
The ridge tile can be made with an off-set at one end so as to be able to overlap, or by simply butt-jointing with FC mortar. To improve water-tightness the butt joint will be covered with a FC mortar strip. It is necessary to water each gap before applying mortar, and to cover the finished joint with wet rags for curing. A <weir, of cement mortar may be added at the top end of the rolls at the ridge sheet to prevent rain water being driven up into the building.
Fig. 10 Grouting together of ridge tiles
Fixing of the elements
The fixing can be done by means of nails, screws, bolts or cast-in wire loops.
In some countries, sheets are apparently nailed to the purlins. This practice Is however not to be recommended.
- Fixing of sheets with screws or bolts
This is best done right after the laying of each row, thus avoiding the need of climbing onto the sheets. Round head screws (preferably galvanized) of sufficient length, i.e. min. 4 1/2, are usually not available. The same applies to carriage bolts. It is preferable to fix the elements on the bottom rather than the top to avoid flapping in high winds. J-hook bolts can be made fairly easily from 6 mm (1/4) dia. M.S. rod. (mild steel bars). Great care should be taken when drilling the holes for screws or bolts:
- Mark hole exactly on top of corrugation, and above purlin center for screws, or purlin edge for bolts.
- Choose a masonry drill bit of a diameter about 2-3 mm larger than the screw or bolt. If a hand drill is used it may be easier to make a pilot hole first with a thinner bit.
- Protect FC sheet from sudden Impact shock of drill by a buffer material, e.g. a piece of 3/4 thick softwood board with a 3/4 dia. hole in its centre.
Water proofing of the hole requires special attention too. The J-hook bolt should sit at a right angle to the sheet, or the head just slightly inclined towards the apex of the roof. Besides a galvanized iron flat or cup washer,a bitumen or plastic or rubber washer should be fitted. Suitable thick rubber washers can be made from truck tire inner tubes. The nut should be tightened gently, using a spanner in preference to an oversize sliding wrench. Overtightening may result in cracks. For normal cases two fixing points per sheet suffice (see fig. 14).
This system, developed by JPA, presents several advantages:
- No holes in sheet: no waterproofing problems
- Work done from underneath: no temptation to climb onto roof cover
- No extra stresses in sheet because of overtightening
- Lower cost than screws or J-hook bolts
The cast-in fixing, usually a pair of galvanized wires, gauge 18 or 16, shaped into a loop, is well anchored in a small rib during sheet manufacture (fig. 11). One loop in every other corrugation (i.e. 3 loops per sheet) will be sufficient except in hurricane areas.
Fig . 11 Cast - in fixing (sheet)
From manufacturing of the sheets through curing, storing, transport and handling stages, the wire loops should be treated with care. They should not be touched until the concrete has set (place wooden <bridges> above freshly moulded sheet before covering it with polythene for curing), and should not be twisted unnecessarily afterwards. Fig. 12 shows a simple V-shaped ridge tile with cast-in ties
Fig. 12 Ridge tile with cast-In wire loops
The loop may be tied to the purlin with a pair of wires, although with this system there is the risk of the wires snapping because of overtwisting, or directly with a screw (fig. 13). The wire loop should be straightened first and care should be taken to avoid damaging It when drilling or punching the pilot hole for the screw.
The tied-down wire loops should not have any play, i.e. each fixing point should be tight, otherwise at high wind speeds the suction forces might cause the sheets to clatter. This could result in cracking of the sheets. As already mentioned before, roof overhangs are subject to severe wind uplift. Moreover, the wireloops along the overhangs are accessible from the outside of the building, thus making it easy for burglars to enter the house by lifting a couple of sheets.
Fig. 13 Fixing of sheets
For the above reasons it is advisable to Install one line of J-hook bolts (fig. 14) all along eaves and verges. Water- proofing will not be a major problem since occasional leaks will not affect the interior of the building.
Fig. 14 Installation of J-hook bolt
The ridge tiles too need to be fastened carefully, e.g. as shown in fig. 15
Fig. 15 Fixing of ridge tiles
Damages to FCR products due to faulty Installation and fixing
FCR elements have very litte flexibility and are therefore easily prone to cracking. Therefore, handbooks and practitioners insist on the necessity for each corrugation to touch the purlin. This is quite easily achieved with FC tiles, each one of them having only two supporting points, whereas in the case of FC shoots, the above requirement is usually not met for practical reasons:
- sheets cannot possibly be made with industrial precision
- the top edges of the purlins are never perfectly straight, or they do not remain so due to bending under load and natural movements of wood (warping etc.) There are other reasons for the sheets not being proferly supported:
- at the end overlap the lower sheet supports the upper sheet: inaccurately moulded corrugations will make it impossible for the two sheets to overlap properly.
- Poorly fitting sided overlaps will either leak (wind blown rain) or make the sheet crack if the overlap roll is forced down on the underlap roll but still leaving a gap between purlin and sheet.
Now, at each point where the lower roll of the sheet does not rest on its support, when pressed down either by self- weigth, wind pressure, fastening points such as J-hook bolts, the sheet tries to bend, and since it is quite rigid, this will lead to cracks.