PAUL HAY Capital Projects

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Rainwater Drainage

Author: Paul Hay

e-mail: paul.hay@phcjam.com

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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 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.

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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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29 July 2011