Asbestos cement sheets are popularly used as a low-cost roofing material. The improved properties attained by the addition of asbestos in cement make it suitable to be used for insulation, fireproofing, roofing, and flooring jobs.
In this article, we'll discuss the specifications for laying and fixing of corrugated asbestos cement roofing sheets.
Laying of Corrugated Asbestos Cement Roofing Sheets
- The sheets are laid on the purlins and other roof members as indicated in the working drawings or as instructed by the engineer.
- The maximum spacing of purlins under the sheets shall be 1.40 m for 6 mm thick sheets, and should not exceed in any case.
- The ridge purlins shall be fixed at a distance between 75 mm to 115 mm from the apex of the roof.
- The top-level of all purlins and other roof members shall be in one place so that when the sheets are fixed, it shall not be required to force down the sheets to rest on the purlins.
- The finished roof shall have a uniform slope, and the line of corrugations shall be straight and true.
- The sheets must be laid with the smooth surface upwards.
- The sheets must be laid with a side lap of half a corrugation, and an end lap of 150 mm, minimum in the case of roofs with a pitch less than 1 vertical to 2.5 horizontal (approx. 22 degrees).
- In the case of very exposed situations, the minimum permissible end lap shall be 200 mm.
- The side laps should be laid on the side facing away from the prevailing monsoon winds.
- The free overhang of the sheets shall not exceed 300 mm.
- The corrugated sheets shall be laid from left to right direction, starting at the eaves.
- The first sheet must be laid uncut, but the remaining sheets in the bottom row shall have the top left-hand corners cut or mitred.
- The sheets in the intermediate rows, except the end sheets, shall have both the top left-hand corner and bottom right-hand corner cut.
- The last or top row sheets shall have the bottom right-hand corner cut, with the exception of the last sheet, which shall be laid uncut.
- If, for any reason, such as on the consideration of the direction of prevailing winds, laying is to be started from the bottom right-hand corner; then, the whole procedure should be reversed.
- The ‘mitre’ described above is necessary to provide a snug fit where four sheets meet at a lap. It is cut from a point 150 mm up the vertical side of the sheet to a point 50 mm long along the horizontal edge.
Fixing of Corrugated Asbestos Cement Roofing Sheets
- The asbestos roofing sheets shall be secured to the purlins, and other roof members by means of 8 mm diameter galvanized iron J or L-hook bolts and nuts.
- J-hooks are used to fix angle iron purlins, L-hooks are used for fixing to RS joists, timber, or precast concrete purlins.
- The grip of the J or L-hook bolt on the side of the purlin shall not be less than 25 mm.
- Each galvanized iron J or L-hook bolt shall have a bitumen washer and a galvanized iron washer placed over the sheet before the nut is screwed down from above.
- On each purlin, there shall be one hook bolt on the crown adjacent to the side lap on either side.
- The GI flat washer must be of 25 mm in diameter, 1.6 mm thick, and the bitumen washer must be 35 mm in diameter and 1.5 mm thick.
- The length of the J-bolt or crank bolt shall be as specified in Table-1 below.
- Each nut shall be screwed lightly at first. After laying down the other adjacent sheets, the nuts shall be tightened to full capacity to ensure a leak-proof joint.
- The holes for hook bolts shall be drilled and not always punched through the crown of the corrugation and not in valleys, to suit the purlin's position.
- The diameter of holes made for fixing of bolts shall be 2 mm more than the diameter of the bolt.
- Care must be taken to check that no hole is nearer than 40 mm to any edge of a sheet or any accessory.
- The roof ladders or planks shall always be used when laying and fixing the sheets to avoid damage to the sheets and provide safety to the workmen.
Table-1: Details of Length and Number of Bolts and Washers to be used in different Asbestos Roofing situation
|Sl. No||Situation||No. of Bolts & Washers||Length of Bolts|
|1||At horizontal (end) laps of sheets. At eaves when filter pieces are used. At ridges when sheets and ridge pieces are secured by the same bolt.||Twice the No. of sheets on one horizontal course.||Depth of purlin plus 90 mm.|
|2||At eaves when filler pieces are not used. At ridge when corrugated sheets and ridge pieces are secured by the same bolt.||Twice the No. of sheets in the horizontal course.||Depth of purlin plus 75 mm.|
|3||At intermediate purlins where horizontal laps do not occur||Twice the No. of sheets in the horizontal course.||Depth of purlin plus 75 mm.|
Wind Ties in Asbestos Cement Roofing Sheet
- The wind ties may be provided where the situation justifies their provision.
- These shall be a flat iron section of 40 x 6 mm or of other sizes as specified.
- These shall be fixed at the eave ends of the sheets.
- The fixing shall be done with the same hook bolts which secure the sheets to the purlins.
- The wind ties shall be paid for separately unless described as included in the items of the roof work.
The maximum spacing of purlins under the sheets shall be 1.40 m for 6 mm thick sheets, and should not exceed in any case.
The free overhang of the sheets shall not exceed 300 mm.
It is cut from a point 150 mm up the vertical side of the sheet to a point 50 mm long along the horizontal edge.
The roofing sheets shall be secured to the purlins, and other roof members by means of 8 mm diameter galvanized iron J or L-hook bolts and nuts.