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29 July 2011

Rainwater Drainage - Site, Roof Design, Gutters, Drains & Leaders


PAUL HAY Capital Projects


Rainwater Drainage

Author:            Paul Hay
e-mail:             paul.hay@phcjam.com
profile:             www.linkedin.com/in/phcjam




1.0       SURFACE DRAINAGE IS NEEDED ON THE SITE


Figure showing 4 pipe locations for Foundation Drains


Figure 1: Foundation Drains  [Source :- JLC Field Guide to Residential Construction (Vol. 1)]


1.1    Surface drainage systems are designed to collect and drain rainwater.

1.2       There are two basic systems:
          1.2.1   An open system comprises natural or man-made swales/ditches, and is generally used on sites with predominantly natural surfaces; but
          1.2.2   A closed system which comprises inlets, catchbasins, pipes and manholes is used otherwise.

1.3       The two systems can also be used collectively.

1.4       Permeability of natural surfaces depends on soil composition:
           1.4.1   Coarse soils, like gravel, are most permeable and fine soils, such as clay, are least permeable;
        1.4.2   If soil readily forms a ball when moist and does not fragment when dropped, it most likely contains a large proportion of clay; but
            1.4.3   Soils laboratory tests give a more conclusive analysis of soil composition.

1.5       Vegetation on site should be protected and maintain wherever possible.

1.6       Runoff should be diverted where required by digging ditches or mounding berms:
            1.6.1   Swales/ditches with slopes less than 4% should be sodded; and
            1.6.2   Steeper slopes must be lined with fabric or rock to minimize erosion.

1.7       Catchbasins have a sump below the outlet pipe to trap debris, and should not be interconnected.

1.8       Manholes should be located where pipes join, change size, slope, or alignment, but should be no further than 180 m apart.


2.0       ROOFS SHOULD HAVE FALLS FOR DRAINAGE


2.1       Roofs need to have falls to facilitate the removal of rainwater:
            2.1.1   Falls should be towards the eaves of the roof, gutters or drains;
            2.1.2   Drain-pipes and leaders can be located within a building;
            2.1.3   Slope of the roof does not influence the size of gutters and leaders;
            2.1.4   Each roofing material has a recommended range of slopes for application;
            2.1.5   If gutters are not at roof eaves, provision should be made to prevent water from running along the soffit of the roof.

2.2       Exposed slabs should have a fall of at least 1:50 to prevent “ponding”:
            2.2.1   Cement and sand screeds may be placed on flat roofs;
            2.2.2   Structural roofs may be tilted; or
            2.2.3   Roof insulation can be installed with the required falls.


3.0            GUTTERS REDIRECT FLOW TO LEADERS & DRAINS


3.1       Gutters and leaders are used to prevent flooding around the perimeter of a building:
            3.1.1   Gutters are required if grounds, doorways, and foundation footings are prone to flooding;
            3.1.2   Leaders transfer water from gutters to ground.

3.2       Only small porches and canopies should use gutters with width smaller than 100 mm.

3.3       Exposed gutters are available in many shapes and sizes:
            3.3.1   Shapes are classified as rectangular, half-round, or ogee;
         3.3.2   Gutter size should be determined from the plan area of the section of roof proposed to be drained by the gutter.

3.4       Exposed gutters are hung from straps.

3.5       The back of gutters should be 12 mm higher than the front.

3.6       Gutters can be built into roof surfaces:
            3.6.1   Gutters should be detailed to prevent leaking indoors; and
            3.6.2   Expansion joints must be built into their linings to permit movement.

3.7       Gutter should have a minimum fall of 1:200 for proper drainage:
            3.7.1   Increasing fall permits the use of smaller gutters; and
            3.7.2   Expansion joints should be located at high points.

3.8       Gutters can empty into leader heads or drains.


Figure showing Gutter & Drain Detail

Figure 2: Gutter & Drain Detail  [Source:- Mechanical & Electrical Equipment for Buildings]


4.0     DRAINS & LEADERS CARRY WATER AWAY FROM BUILDINGS

4.1       Roof drains are installed at low points of flat roofs and roofs with parapets.
4.2       Drains usually have strainers to keep leaves out of drain-pipes.
4.3       Leaders can be round or rectangular.
4.4       Leaders are secured to walls with metal straps.
4.5       Leaders should be installed 6m - 15m apart.
4.6       Drains and leader sizes should be determined from the plan area of the section of roof proposed for drainage.
Figure showing 3 ways to terminate rainwater leaders

Figure 3: Termination of rainwater leaders  [Source:- Mechanical & Electrical Equipment for Buildings]

4.7       Leaders should terminate at ground level in an elbow.
4.8       Rainwater must be directed away from the foundation of the building:
            4.8.1   A splash pan can be located at the end of the elbow;
            4.8.2   A gravel-filled hole is more effective; or
            4.8.3   If soil is very permeable, a soak-away pit can be used.
4.9     Footing drains can lead water away from foundation footings to prevent leakage through basement walls:
           4.9.1   This is especially useful where drainage from higher ground increases flow against underground walls; and 
          4.9.2   Pipes can lead water away to distant locations, if there is sufficient land space and fall is mild; but
          4.9.3   Drains are not permitted to connect to sanitary drains.


5.0       ROOF DESIGN & RATE OF HOURLY RAINFALL INFLUENCE SIZING

5.1       Roof area is determined from location of leaders and respective roof falls:
           5.1.1   Only the portion of roof which falls towards a gutter or drain is considered;
           5.1.2   Area of eaves are also included if they fall towards a gutter or drain.

5.2       Slope of gutters need to be established.

5.3       The maximum hourly rainfall needs to be determined for the location.

5.4       Diameters of half-round gutters, round drains and leaders are sized from tables provided:
            5.4.1   Table 1 should be consulted for gutter size;
            5.4.2   Table 2 for drains and leaders, and
            5.4.3   Table 3 for drain-pipes.

5.5       Rectangular gutters should have areas equivalent to size determined for half-round gutters.
                         _________________________________________________

FURTHER READING     Mechanical and Electrical Equipment for Buildings, Benjamin Stein & John S. Reynolds, John Wiley & Sons Inc., U.S.A.
Construction Materials & Processes, Don G. Watson, McGrawHill Book Co., USA.

Ramsey/Sleeper Architectural Graphic Standards, AIA, Robert T Packard (ed.),

            John Wiley & Sons Inc., USA.

_________________________________________________



TABLE 1: GUTTER SIZES

Maximum Plan Area of roof, m2
DIA. @
MAXIMUM RAINFALL (mm/hr)
1:200 fall
50
75
100
125
150
75
32
21
16
13
10
100
67
45
33
27
22
125
116
77
58
46
39
150
178
 -
89
71
59
175
256
171
128
102
85
200
370
247
185
148
123
250
669
446
334
268
223


DIA. @
MAXIMUM RAINFALL (mm/hr)
1:100 fall
50
75
100
125
150
75
45
30
22
18
15
100
95
63
47
38
32
125
163
109
82
65
54
150
253
169
126
101
84
175
362
241
181
145
121
200
520
347
260
208
174
250
948
632
474
379
316


DIA. @
MAXIMUM RAINFALL (mm/hr)
1:50 fall
50
75
100
125
150
75
63
42
32
25
21
100
134
89
67
53
45
125
232
155
116
93
77
150
357
238
178
143
119
175
513
342
256
205
171
200
739
493
370
295
247
250
1,134
892
669
534
446


DIA. @
MAXIMUM RAINFALL (mm/hr)
1:25 fall
50
75
100
125
150
75
89
59
45
36
30
100
189
126
95
76
67
125
329
219
164
131
110
150
515
343
257
206
172
175
725
483
362
290
241
200
1,040
693
520
416
346
250
1,858
1,238
929
743
619



TABLE 2: DRAIN & LEADER
Maximum Plan Area of roof, m2
MAXIMUM RAINFALL (mm/hr)
DIA.
50
75
100
125
150
50
134
89
67
53
45
75
409
272
204
163
137
100
855
569
427
341
285
125
1607
1071
804
643
536
150
2508
1672
1254
1003
836
200
5388
3591
2694
2155
1794

TABLE 3: DRAIN-PIPES
DIA. @
MAXIMUM RAINFALL (mm/hr)
1:100 fall
50
75
100
125
150
75
153
102
76
61
51
100
349
233
175
140
116
125
621
414
310
248
207
150
994
663
497
398
331
200
2,137
1,424
1,068
855
706
250
3,846
2,564
1,923
1,540
1,282
300
6,187
4,125
3,094
2,476
2,062


DIA. @
MAXIMUM RAINFALL (mm/hr)
1:50 fall
50
75
100
125
150
75
215
144
108
86
72
100
492
328
246
197
164
125
877
584
438
351
292
150
1,403
935
701
561
468
200
3,028
2,019
1,514
1,211
1,009
250
5,425
3,618
2,713
2,169
1,807
300
8,732
5,815
4,266
3,493
2,912


Dia. @
MAXIMUM RAINFALL (mm/hr)
1:25 fall
50
75
100
125
150
75
305
213
153
122
102
100
699
465
349
280
232
125
1,241
827
621
494
413
150
1,988
1,272
994
797
663
200
4,274
2,847
2,137
1,709
1,423
250
7,692
5,128
3,846
3,080
2,564
300
12,374
8,249
6,187
4,942
4,125

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