Farm Building Construction

New Zealand Journal of Agriculture, Volume 94, Issue 6, 15 June 1957, Page 589

Farm Building Construction

By

H. W. T. EGGERS,

Engineer, Department of Agriculture, Wellington \

THIS article is the third in a series about " farm building construction and it deals with roofing materials. Next month's article will describe types of wall sheathings tor buildings. The previous article in this series dealt with the construction of the framework of a building, which acts as a support for the sheathing and roof.

THE roof is probably the most important part of a building. To fulfil their function as a protection against sun and rain, materials used for roofing must have the following characteristics: — A minimum of movement due to temperature changes. No likelihood of cracking. High resistance to weathering. High degree of permanence. Ease of maintenance and ability to shed water. Types of Roofing Materials The four main types of roofing materials available are as follows: Sheet roofing . . (a) Metallic sheets: corrugated iron, corrugated aluminium, aluminium strip, treated steel either corrugated or flat (b) Asbestos cement ... . .sheets, large or

small corruga- ■ tions (c) Plastic corrugated sheet Bitumen- or (a) Plain fabric asphalt-satu- (b) Mineral-surfaced rated fabric fabric (roll roofing)' Tiles . . .. (a) Clay tiles (b) Concrete tiles Asphalt (built- (a) Plain asphalt up roofs) . . (b) Built-up asphalt and impregnated felts Choice of Material Savings in constructional costs that can be effected by the use of some of these materials should be offset against the first cost of the materials themselves and should be considered in the choice of a type of roofing. Other factors which must be weighed against first cost are durability (effective life) and frequency and cost of maintenance. Generally the more expensive roofings are the more durable they are and the less maintenance they need.

Sheet roofing and bitumen and asphalt roll roofing can be fixed by the layman, but tiles and built-up asphalt roofs require, expert fixing. Corrugated Iron Corrugated iron is sheet steel in various gauges, corrugated and galvanised (zinc coated). As the zinc coating, which is provided to prevent rapid oxidisation of the steel, itself oxidises in time, corrugated iron needs a protective coating of paint to increase its durability. New material should, however. be permitted to oxidise slightly by weathering before painting to ensure a good bond for the paint. If immediate painting is necessary, artificial oxidisation can be carried out with the use of a suitable acid preparation. Corrugated iron must be fixed with sealing head fasteners such as leadhead nails. Corrugated Aluminium The effective life and advantages and disadvantages of corrugated. iron have been proved and are well known, but the general use of aluminium' is rela-

tively new. Aluminium roofs over 50 years old are in existence and evidence suggests that under equal conditions the effective life of aluminium would exceed that of corrugated iron. It has the same advantages and disadvantages as corrugated iron as a roofing material and greater care is necessary in its fixing to ensure that no other metal is in contact with it.

Sheets of aluminium alloy corrugated to the same dimensions as corrugated iron are available in various gauges.

Very thorough laboratory tests have been made of this material, which suffers pitting-type deterioration in weathering rather than uniform attack from which most other metals suffer. This pitting proceeds rapidly at first and then slows down, producing a selfstifling effect, so there is much more danger of perforation in thin sheets than in thick.

Installation Aluminium is very susceptible to attack by galvanic action; it has been known to fail merely from contact with rain-water which has flowed over copper sheet before reaching the aluminium. For this, reason the only metals which can safely touch the aluminium sheets are zinc or hot-dip galvanised iron.

The sheets should be fastened with aluminium nails or alternatively 1 hotdip galvanised nails. All saddles, brackets, spoutings, valleys, flashings, and similar parts must be of aluminium or hot-dip galvanised iron. Soft aluminium sheet only should be used for flashings.

Piercing the sheet with a drill or sharp steel probe before nailing is done will avoid denting of the surface with blunt nails. This drilling is especially helpful at the overlap of four sheets. A loose washer of soft aluminium or oil-resistant rubber under the head of the nail is a help in weatherproofing.

The slope, of an aluminium roof is an important factor in removing moisture from the laps and so avoiding undue corrosion. A minimum slope of 3in. per foot is recommended. Ventilation of the space under. the aluminium sheeting is also important. If there is any possibility of prolonged wet conditions without free access of air, it is advisable to give the under side . -of the sheets a coat of zinc chromate primer.

The sheets must not come into contact with wet cement or plaster and these materials are best weathered for a month or so and. then protected by bitumastic paint and a layer of bitumen-impregnated paper before the aluminium is laid. Contact with green or resinous timber should also be prevented by a layer of tar-free, bitumenimpregnated paper. Alternatively the timber could be given a coat of bituminous or aluminium paint.

When sheets are bent along the lines of corrugations the bend should be so

located as to continue the curvature of the corrugation. Do not scribe the bending line or restraighten or rebend the sheet, as such treatment may cause cracking. Painting Aluminium Roofs For aesthetic reasons many users will prefer to paint aluminium roofs, and if this is correctly done, it will provide additional protection for the metal. Painting removes one of the advantages claimed for aluminium—that its brilliant reflection keeps the space beneath it cooler than that under a painted galvanised iron roof. Against this must be balanced the unsightly appearance of the polished aluminium roof and the fact that in a sea coast atmosphere the metal will in any case lose its lustre ■in time. Painting the laps with zinc chromate primer before the sheets are laid is advisable. i Both contact surfaces should be so treated. . To ensure good paint adhesion the metal should be cleaned and etched. This is best done before the sheets are fixed to avoid the chemicals used being trapped in crevices. The Department of Scientific and Industrial Research has carried out extensive investigations with aluminium roofing and has published information concerning its use. It can advise about cleaning and etching. Any specific queries on painting aluminium can be answered by the Paint Section of the Dominion Laboratory.

Collection of Drinking Water Where rain-water is used for domestic purposes water from an unpainted aluminium roof is entirely suitable for human consumption. If the roof has been painted, it is advisable to wait until it has been well washed by several heavy showers before the water is collected for culinary purposes. Effects of Destructive Agencies Bird droppings: Apart from a slight surface roughening of the metal, bird droppings have no effect on unpainted aluminium. Artificial manures: There is no risk of aluminium sheeting deteriorating seriously when used to roof sheds where superphosphate or agricultural lime is stored even when dust can rise and collect in the laps. Lack of access of air: If aluminium roofing sheets are stacked in wet conditions so that free access of air to the surface of the sheets is prevented, they may undergo rapid pitting, which can cause perforation of the sheets in a few weeks. In single sheets freely exposed to the air this action , does not take place. If lengthy stacking is necessary, the sheets are best placed on end and preferably oiled. Effect of Timber on Aluminium Nails Many green, sappy timbers are corrosive to aluminium, especially if it is in intimate contact with them without free access of air. It is therefore wise to use a greater number of

aluminium nails than that required when normal lead-head nails are used; this number can be up to 25 per cent, extra unless the underlying timber is known to be thoroughly dry.

Contact with Rusty Galvanised Iron

Though new hot-dip galvanised iron is not affected by corrosive action through contact with rusty iron, aluminium in such a contact is likely to be pitted. Aluminium roofing sheets are therefore not recommended for the repair of deteriorated galvanised iron roofs.

Aluminium Strip

Both corrugated iron and aluminium are suitable materials for pitched roofs; aluminium strip can be used for both flat and pitched roofs. Aluminium strip of suitable gauge and fixed width can be factory cut to lengths to suit each order (within reason) and has therefore no horizontal joints. The only limitation to length is the “tailing out” space in the factory and difficulty of transport. For roofing, strips are fastened together With standing seam joints. '

The strips are pre-formed, one beaded edge fitting over the turn-up of the next strip, and after the strips have been placed the bead and turn-up are folded over to make a welted joint.

Expansion and contraction of strips are provided for by anchoring in the middle, and fixing with sliding expansion clips at regular intervals toward the ends. No ridging is required, as the strips on both sides of a ridge can be folded and welted in a similar manner to longitudinal joints. Orthodox aluminium ridging can of course be used, but the edge of the strips is folded back to form a tray and no ridging nails must penetrate the strips, which must be free to expand or contract under the ridging.

Treated Steel

Treated steel sheets are either round corrugated in two sizes or V beam corrugated which are treated with permanent external coatings of asphalt and asbestos felt that keep all destructive agencies entirely away from the steel.

Flashings, ridge caps, corrugation closers, etc., are available in the same material.

This roofing is permanent and requires no underlay as a precaution against condensation. The sheets are fixed with screws, hook bolts, or clips and are available in lengths up to 12ft.

Asbestos Cement Sheets

Asbestos cement sheets are manufactured from cement mortar reinforced with asbestos fibre and are available with small, large, or angular corrugations. Standards are laid down for manufacture which specify dimensions, breaking loads, water absorption, acid solubility, etc., all of which are covered by inspection and test.

AH' auxiliary equipment such as ridging, ventilators, guttering, and down-pipes is available in the same material.

The sheets are fastened with galvanised screws or bolts having flat under sides to the heads under which washers with a coating of bitumastic material are used to make the fastenings water-tight. Holes for the screws or bolts must be drilled and not punched.

Plastic Corrugated Sheet

Plastic corrugated sheet is of standard size and corrugations and is a laminate composed of resin-impreg-nated wood fibres bonded together under great pressure and regulated temperature to give a very dense, homogeneous, and tough material.

It is fireproof and, as far as tests can ascertain, permanent. Its heat transmission is low in comparison with metallic sheetings and it has a high strength value while still retaining lightness. Acid fumes have no effect on this material; direct contact with acid solutions causes only slight staining and for all practical purposes it is unaffected by acids or caustic soda, which makes it suitable for fertiliser sheds.

Asphalt-saturated Fabric

Asphalt-saturated fabric is usually termed roll roofing, as it is supplied in rolls 3ft. wide. It is a rag felt saturated with asphalt or bitumen and then coated with bitumen. It can be obtained in various weights usually termed ply, though this term has no relation to its construction, as the material is not a laminate.

Asphalt-saturated fabric is a very inexpensive and convenient form of roofing material, as it can be laid in horizontal lengths from the eaves to the ridge with the lengths lapping over each other. If all sharp angles are avoided on the ridge and additional layers provided, it requires no special ridging. It is fixed by use of galvanised clouts staggered along the laps, the upper lap being sealed with either hot bitumen or bitumen adhesive.

As it is virtually a very thin layer of reinforced asphalt, this roofing is not permanent and is more suitable for sheds or buildings where a leak may not be detrimental than for dwellings. The same material, however, when used in conjunction with other materials to produce a thick layer of reinforced asphalt becomes a permanent roofing.

Mineral-surfaced fabric is the same as the other fabrics described but with a mineral incorporated in the outer bitumen coating to give colour.

Tiles

Tiles in some form or another have been in use as a roofing material for

several centuries. Tiles of several patterns may be obtained in glazed or unglazed baked clay or in coloured or plain concrete. All patterns have tongues and grooves to ensure angled mating surfaces at side and end overlaps. The tiles are laid in rows on battens, the higher rows overlapping the lower rows and each tile overlapping its neighbour on one side. They are usually fixed by wiring to the battens and the top and bottom overlaps may be cemented with a lime mortar to assist in waterproofing.

As tiles absorb moisture, they must be laid with sufficient slope to shed rain quickly. Special ridging of the same material as the tiles is usually provided to suit each type. This is usually of the half-round type with female spigot, the spigots and joints to the roof being filled with cement mortar. Half tiles at the ends of gables are usually fixed by drilling to accommodate galvanised screws.

Plain Asphalt

Plain asphalt roofing forms a good permanent protection and consists of an asphalt-base hot mix applied in two gin. layers to give a lin. thick coating of asphalt over the. whole roof area. This method is particularly suited to relatively flat roofs, where creep cannot be accentuated by slope. . The layers of asphalt are placed hot over impregnated paper or felt laid on the roof deck and are screeded off,, the upper coat being smoothed off with a float in a manner similar to that for finishing concrete.

Built-up Asphalt

Built-up asphalt roofing consists of alternate layers of felt and asphalt. Different types or weights of felts may be used for alternate layers, the lower layer being firmly attached to the deck by clouting, with the succeeding layers pressed into the coatings of hot asphalt. By use of perforated felts for the top layers air trapping under the felt is avoided, the hot asphalt on which the felt is* laid - squeezing through the perforations with resultant better bonding.

These roofs may be finished with a gravel coating, a mineral-surfaced cap sheet, or a bituminous paint, preferably aluminium or silver.

Factors Affecting Cost of Roofing

The cost of a roof cannot be assessed by the . cost of the material. Several factors affecting the cost must be considered as spread over a period of reasonable life expectancy for the roof. They are cost of materials, cost of labour, type of structure required to support protecting fabric, subsequent maintenance, and economic life. These factors are all inter-related; for example. some materials incur no main-

tenance costs for the full period of their economic life; others give substantial economic life if well maintained, but deteriorate rapidly without maintenance. Deterioration of some materials cannot be prevented even by maintenance, and their economic life. is snorter. The type of structure required to support the protecting fabric is important in,that though some materials are heavy and require a relatively strong structure, other materials are light and strength of structure can be reduced accordingly. Again some materials require no deck and no under sealing as a protection against condensation which also effects a reduction in the cost of supporting structure. Table 1 gives the lengths of sheets of different roofing materials and the area they cover. J ’ The type and purpose of the building will also influence any decision

as to material; for example, though roll roofing may be suitable for an implement shed, something more durable is required for a dwelling. Table 2 shows the characteristics of various roofing materials The figures given in column 2 are approxima t ionS) as these costs can be affected by various factors; for example, the combination of lengths of sheet of corrugated iron or aluminium will affect the cost, Similarly the design of roof, placing of ridges and valleys, etc., can affect cost of both material and labour. though aR roofs re rafters and some materials require sarking, others require pur i ins O nly. The relative cost di^erenc P e betwe en purlins and close sarking - s representd by a figure of 9 2 hich an be added * o the materials requiring sarking to bring .■ them on to a comparative basis with material requiring no sarking. For example, item 4 in the table requires

no sarking, whereas item 2 does; therefore 9.2 is to be added to item 2 (14.1 -j- 9.2 = 23.9). A comparison of the two sets of figures shows that a roof in treated steel will cost approximately half as much again as a roof of the same size in corrugated aluminium. As tiles need no sarking, a tiled roof costs only slightly more than aluminium, though more timber is needed in the supporting structure to give greater slope.

As the cost of roofing is a capital charge which must be spread over the reasonable life expectancy of the building, choice of material should be influenced by maintenance costs. The following example illustrates this.

From the table two materials are chosen, the initial cost of one being more than half as much again as that of the other, and their cost over a period of 30 years is compared.

Corrugated Treated iron steel Area of roof 1000 sq. ft. 1000 sq. ft. Type of roof Pitched Pitched Initial cost .. £lOO £156 Maintenance Painting 2 Nil coats at 5-yearly periods at £25 per time for 30 years £l5O Cost after 30 years .. £250 £156 This example shows that the initially more expensive roof is in the long run the cheapest by a wide margin, particularly as after 30 years the corrugated iron roof would probably require extensive repairs, whereas the treated steel roof should be as satisfactory as when it was first erected.

Reference D.P.L. Information Circular No. 1187

• Material Width Effective width allowing for overlap End lap Length of sheets in. in. Corrugated iron .. 27 21 6ft. to 10ft. by multiples of 1ft. . Corrugated Aluminium .. 32 28 1 4ft. 6in. to 12ft. by 24 225 ) multiples of 3in. Treated steel Small corrugations 275 23J Large corrugations 26J 235 1 corm1 5ft. to lift, by multiples gation lap Dependent on | of 6in. V beam . . 24J 201 1 corrupitch of roof; gation lap 6in. minimum Corrugated asbestos in most cases Large section .. 415 . 35| I 43 37J I 4ft. to 10ft. by multiples 38 32 j of 6in. 365 295 Small section .. 29J 265 30 27 . 1 4ft. to 10ft. by multiples ■ 42 39 C. of 6in. . 315 285 ' j of 6in. Tiles Average of 120 tiles per 100 sq. ft.

TABLE 1—COVERAGE OF ROOFING MATERIALS

Type of material Comparative cost index of laid materials Type of roof Structure to support Under sealing Maintenance Approximate life 1. Corrugated iron .. 14.1 Pitched Purlins or close or Requires water-proof Regular painting 30 years or more, deopen sarking underlay pending on maintena nee 2. Corrugated Corrugated aluminium 14.7 Pitched Close or open sarking Requires water-proof Painting for aesthetic Indefinitely permanent underlay reasons only 3. Aluminium strip 25.9 Pitched or flat Close sarking Requires water-proof Painting for aesthetic Indefinitely permanent underlay reasons only 4. Treated steel 37.8 Pitched Purlins only Not required Not required Indefinitely permanent 5. Corrugated asbestos cement Large .. 16.5 Pitched J Open sarking Optional Not required Indefinitely permanent Small 15.3 Pitched ( 6. Plastic corrugated sheets .. 48.0 Pitched Purlins or open Optional Painting for aesthetic Indefinitely permanent 7. Fabric roofing, sarking reasons only 1 layer only .. 2.5 Pitched Close sarking Not required Regular dressing Temporary only 8. Tiles K Clay Clay 27.0 27.0 Pitched j Pitched j Close spaced battens Close spaced battens Not required Not required Not required Not required Indefinitely permanent Indefinitely permanent Concrete .. 25.8 Pitched ( 9. Asphalt roofing . . 53.0 Flat Close sarking Forms part of roofing Occasional dressing Indefinitely permanent 10. Asphalt and fabric may be required (standard) 26.6 Flat Close sarking Forms part of roofing Occasional dressing Indefinitely permanent may be required

TABLE 2—CHARACTERISTICS OF ROOFING MATERIALS