image207Pitched or sloping roofs are the oldest form of roofs in building, sheds and several temporary structures. The covering of shed roofs to match the triangulated support structure like trusses is normally done by tiles or corrugated Galvanized sheets or Asbestos cement corrugated sheets. These days awareness is growing about effects of components of Asbestos on the workers handling the manufacture of these sheets and several other types of sheets such as Aluminum corrugated sheets, stainless steel corrugated sheets are being used. Apart from the different material and process of manufacture, the method of making the support structure and fixing the sheet remains mostly same.
Various components of slope Roofing:
Barge boards – Boards mounted under the roofing at a gable end traditionally timber, nowadays usually UPVC. These are generally mounted flush on the end wall. It is about wide and thick.
Dormer – A vertical window built into the slope of a pitched roof.
Eaves – The overhung area just below the lower end of the roof on the outsides of building – includes the fascia, soffit and guttering. The eaves board is about wide and thick, it is made in timber, however in AC sheet roofing, the board is made of AC sheet.
Fascia – The vertical board secured to the ends of the rafters under the lower end of the roof to which the guttering is normally fixed – traditionally timber, nowadays usually AC or PVC is used.
Flashing – A weather seal usually made of A.C. sheet, Galvanized iron sheet is fixed where part of the roof abuts a vertical surface – one side of the flashing is normally embedded in a joint between two rows of bricks, the other side being shaped to fit onto the roof surface. Plain flashing is where the roof meets the vertical surface horizontally. Stepped flashing is where the roof meets the vertical surface at an angle – the steps in the flashing align with the joints of different courses in the brickwork
Gable – The vertical wall at the end of a pitched roof, is like an inverted +V,.
Hip – A sloping ridge formed by the junction of a pitched roof and a hip end.
Hip board – The board along the line of a hip from the fascia to the ridge of the pitch.
Hip end – A sloping end to a pitched roof which is covered with tiles or sheets.
Rafter – A vertical, sloping timber or steel angle used to form the shape of the roof – the side of a truss.
Purlins: The horizontal member between the spacing of rafters, supporting and holding roofing material like G.I. sheets, A.C. sheets etc.
Ridge – The horizontal line at the top of a pitched roof – applies whether there is a sloping roof on both sides (a Duo ridge), or if there is just one (a Mono ridge).
Ridge board – The horizontal board along the line of the ridge – normally along the top of the rafters or trusses.
Soffit – The horizontal board used to seal the space between the back of the fascia and the wall of the building – traditionally timber, or Asbestos cement board nowadays PVC with air vents is also used.
Trusses: A triangulated truss structure is generally used for long spans. However, light trusses work out cheaper than beams even for small span of 3.6m and above. The spacing of trusses depend upon the load of roof, position of cross walls, span and material of truss. Some of the usual trusses are given below.
King-post truss: In this type of truss, the vertical central post is known as King post.image208 The Tie beam is the horizontal member at the bottom of the truss. King post rests on the tie beam. The inclined members known as struts, which divide the span of the principal rafter, thus prevent it from becoming beam. Normally, King-post trusses are used up to the span of 9m. The figure below shows the details of the King-post truss. If the truss is made from hard wood, the sizes of various components are adopted as per the table below.
No.
Span
m
Members
King-post (mm)
Principals (mm)
Struts (mm)
Tie Beam (mm)
D          B
D          B
D          B
D          B
1
3.60
80 x 80
130 x 80
70 x 80
130 x 80
2
4.20
80 x 80
140 x 80
70 x 80
130 x 80
3
4.80
90 x 80
150 x 80
70 x 80
130 x 80
4
5.40
90 x 90
150 x 90
70 x 90
140 x 90
5
6.00
90 x 90
180 x 90
80 x 90
150 x 90
6
6.60
100 x 90
190 x 90
80 x 90
160 x 90
7
7.20
100 x 100
190 x 100
80 x 100
160 x 100
8
7.80
100 x 100
200 x 100
90 x 100
180 x 100
9
8.40
100 x 120
200 x 120
90 x 120
180 x 120
10
9.00
100 x 120
210 x 120
90 x 120
190 x 120
Queen-post truss: This type of truss has two vertical post known as Queen-posts placed at 1/3 rd length of the beam from either ends. image209The upper ends of the queen posts are kept in position by means of a horizontal member known as ‘straining beam,. A ‘straining sill, is also placed on the tie beam between the queen-posts to counteract the thrust of struts. This form of truss is used for longer span and generally up to 15m. The below given figure shows the details of the queen-post.
If this truss is made by hard wood, the approximate sizes of various elements are as under.
No.
Span
m
Members
King-post (mm)
Principals (mm)
Struts (mm)
Tie Beam (mm)
D          B
D          B
D          B
D          B
1
7.80
130 x 100
200 x 100
70 x 100
180 x 100
2
8.40
130 x 120
200 x 120
70 x 120
180 x 120
3
9.00
140 x 120
210 x 120
70 x 120
190 x 120
4
9.60
150 x 120
230 x 120
70 x 120
190 x 120
5
10.20
150 x 120
240 x 120
70 x 120
200 x 120
6
10.80
150 x 120
240 x 130
80 x 130
200 x 130
7
11.40
160 x 130
240 x 130
80 x 130
200 x 130
8
12.00
160 x 130
250 x 130
90 x 130
210 x 130
The details of the joints used in wooden trusses are shown below:
Spacing of purlins: Wooden or steel purlins, as per design, are fixed over the rafters spacing the consecutive trusses. image210The spacing of purlins are so decided that each support point on roof receives one purlin and additional purlins may be required to suit the length of sheets. The spacing of purlin shall not be spaced at more than the following distances for different thicknesses of A.C. and CGI sheets:
Spacing for A.C. sheet
Thickness of A.C. sheet
Distance between Purlin Centres
For roof covering
For side cladding
4 mm
0.8m
1.35m
6 mm
1.4m
1.70m
Spacing for CGI Sheet:
Spacings of Purlins
Thickness of CGI Sheets
1.60 metres
0.63 mm – 24 gauge
1.80 m
0.80 mm – 22 gauge
2.00 m
1.00 mm – 20 gauge
2.40 m
1.25 mm –        18 gauge
2.80 m
1.60 mm –        16 gauge
If the smaller sheets are available and accordingly purlin spacing is required to be smaller, it should be done near the eaves, as more strength is required at this location.
Corrugated Galvanized Iron Sheet / A.C. Sheet Roofing:
Sloped roof coverings using galvanized steel sheets or A.C. sheets are generally used for storage sheds, godowns and factory buildings besides their extensive use in remote areas. These are easy and quick to erect and economical.
Slope: Slope of the CGI / A.C. sheet roofing is normally 1:3 and should not be flatter than 1:5.
Overlap: The A.C. Sheets are laid with a minimum end lap of 150 mm, generally it is kept as 200 mm, side lap of half corrugation. In the CGI sheet roofing the end lap is same as A.C. sheet i.e. minimum 150 mm. but the side lap should be minimum two ridges of corrugations at each side. The minimum lap of sheets with ridges, hips and valleys should not be less than 200 mm.
Size of Sheets: The CGI sheets are available in the width of 910mm and various lengths like 2.44m, 3.05m, 3.66m, 4.27m and 4.88m. The thickness of the CGI sheet available for use of roofs varies from 0.63mm to 1.60mm (24 gauge to 16 gauge).

The A.C. sheets are available in the width of 1065mm and length varying between 1980 to 3050mm as per table below.

Sizes
Thickness
1980 x 1065
5 mm
1980 x 1065
6 mm
2440 x 1065
5 mm
2440 x 1065
6 mm
3050 x 1065
5 mm
3050 x 1065
6 mm
Laying of corrugated sheets: This procedure is applicable to CGI as well as A.C. sheets.
Choosing Correct size of sheet:image211
Step — 1: The size of sheet should be selected as per the inclined length of the roof and spacing between the purlins. Overlap and projection over the wall, also needs to be considered.
Step – 2: Top surface of the purlins should be painted before sheets are fixed. Embedded portions of wooden purlins should be coal-tarred by two coats.
Step — 3: Decide the projection of sheets over the front wall. The unsupported length of overhang should not exceed 300mm.
Step — 4: Then a line dori should be stretched horizontally, on the projection line of sheet, between first and last rafter parallel to front wall and perpendicular to the rafter.
Step — 5: In case of A.C. sheets wherever four sheets join at a corner (at the overlap corner) mitering of corners is necessary, to provide a snug fit. Mitered corner is a triangular cut at the end of sheet equal to length of end lap and width of side lap. The cutting of corners should always be done on ground by hack-saw. A complete sketch should be made out, working out; the sheets and their corners to be mitered. This is very important or else, leakage from such joint can be a chronic problem.

Step — 6: The work of laying sheet should be commenced by laying the bottom sheets first and then work up towards the ridge. The laying of sheet should start from the side away from wind direction so that the side laps will come opposite to regular wind direction and wind will not enter in the gap. In the A.C. sheet smooth side of sheet should be kept on the up side. The overlaps are kept as mentioned earlier for each type of sheet.

image212Step — 7: Holes in CGI and A.C. sheets shall be drilled and not punched on the ridge of the corrugations. The holes should be made from bottom side and not from the top of roof so that the line of purlins will be matched and there will be no gap or mismatch between hole and purlin edge. The hole drilled from top can also cause a source of leakage. The holes in the sheet, holes of the washers shall be of the exact diameter of the hook bolts, maximum 2mm bigger dia hole in the sheet and washer is allowed than the dia of bolt. The nuts shall be tightened from above. No hole should be within 75mm from the edge of sheet and maximum spacing between two hook bolts should be 300mm.image213
Step — 8: Sheets shall be fixed to the purlins or other roof members such as hips valley rafters, with galvanized ‘J, and ‘L, hook bolts and nuts, 6mm or 8 mm dia bolt with bitumen washer of 35mm dia and galvanized Iron ‘limpet, (dome) washers of 25mm dia. The diameter of bolt is depending on the spacing of purlins, 6mm dia bolts can be used for up to 1.8m spacing of purlins and 8mm dia bolt be used for spacing more than 2m. The ‘J, hooks are used for fixing sheets on iron (MS) angles and ‘L, hooks are used for RS Joist, MS Channel and wooden members. The length of the ‘J, or ‘L, bolt should be 75mm longer than the height of purlin in case of single sheet fixing and 90mm longer than two sheets (laps) fixing. However, after fixing the bolt it should project above the top of their nuts not less than 12 mm. In CGI sheets length can be reduced to meet this requirement. The grip of J or L hook bolts on the side of the purlins shall not be less than 25 mm. The bolt should have exact width at the bottom of hook equal to the size of purlin so that it will fit into the purlin. The bolt should be fixed on the ridges of the corrugations of the sheet. There shall be minimum three hook bolts placed in each sheet on every purlin and their spacing shall not exceed . Wood screw shall not be used for fixing sheets to purlins.
image214 image215 image216Special Conditions:
Wherever gables or parapet or wall is existing at the end, the sheet shall not be built into gables and parapets. The sheets shall be bent up along their side edges close to wall and the junction shall be protected by suitable flashing or by a projecting drip course, the latter to cover the junction by at least by .
If there is break in the level of roof i.e. some valley ridge may have to be introduced in the roof, the G.I. plain sheet gutter should be fixed below roofing sheets in addition. The G.I. sheet is bent to proper shape and then fixed below roofing sheet. The overlap of G.I. sheet should not be less than on either side. The rain water coming from the roof sheets will properly flow down to the gutter.
The roofing sheets should be cut as per the angle subtended on the roof due to valley. The special attention should be given to fixing sheet to valley to prevent leakage.
image218Fixing of Ridges and Gutters:
Ridge: The ridge is the apex part of the sloping roof. The both side sloping panels are joining at the ridges. For the A.C. Sheet roofing serrated adjustable type ridges are available in pairs as inner and outer ridges as shown in figure. For CGI sheet roofing the ridges should be made with minimum 18 gauge GI sheet. The GI sheet ridge is shown in figure. The ridge hips should be covered with ridge and hip section with a minimum overlap of on both side of roofing sheet. The end laps of ridges and hips, and between ridges and hips also not less than .
 image220 image221Gutters: Gutters are provided to collect and carry rain water coming from roof and put it in the down take pipe. The gutters are laid in the slope of 1 in 120. The gutters are made up of plain A.C. and G.I. sheets. Generally the G.I. gutters are half round in shape. It is readily available in the diameter size of , and . The following shapes are available in A.C. gutters.image223
The gutters are supported with the bracket. The brackets are made of M.S. flat wide and thick. The brackets are fixed to the rafters at one end with nut bolts and other end has shape according to the shape of gutters. The slope in the gutters shall be given in the line of supporting brackets. The brackets shall be placed at not more than . The bracket is twisted at the neck as shown in figure.
The gutters are laid in the slope of 1 in 120. The clamps are provided at the distance of with the slope of 1 in 120 as shown in the figure. The clamps should be placed at the distance and below the previous clamp to achieve the slope of 1 in 120. image225For any further minor adjustment, the gutters can be raised to achieve proper slope by providing steel packing plates duly tack-welded to the clamps below the gutter.
The down take pipe from the gutter to the outlet drain on ground, needs special attention and should be fitted using funnel shape connection piece as shown in figure.pitched image226
Wind Ties:
pitched image228In coastal areas and other storm affected areas a steel flat of 50 x is fixed over the length of the roof both sides. The wind tie is fixed closer to eaves. The M.S. flat is fitted to first and last purlins with the same hook bolts which are holding the sheets.

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