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DRWH Technology: Gutters

The water from the roof must be conveyed to the store in some way, this is usually by way of a system of guttering. Other systems such as roof slides (Waller, 1982) or ground level drains (Zhang et al., 1995) can be used but are less popular for rainwater harvesting systems as they either spill water or allow ground borne contamination into the conveyance system. Gutters in developing countries are often the weak link in the rainwater harvesting system (Mwami, 1995) and installations can be seen with gutters coming away from their mountings, leaking at joints or even sloping the wrong way. Beyond the mere functional failure, poor guttering can also be a health hazard if it allows water to remain in the gutter and become a breeding ground for mosquitoes. This situation is exacerbated by the common NGO practice of supplying a free water store but insisting that beneficiaries install the guttering. As a general rule, the cost of the gutters is not considered important as the storage tends to dominate the system cost, however with VLC DRWH systems the storage cost becomes less dominant and gutters can even demand over half the total investment.

Analysis of gutters

The two main criteria for guttering are to catch the water from the roof and to transport it to the tank. On the surface this seems simple enough, however the relative complexity of achieving this simple aim often confounds, resulting either in poor designs that fail to deliver water to the tank or overly conservative designs with a high cost.

Water conveyance

The flow performance of a gutters varies along its length resulting in a spatially varying flow, however for a long gutter it can be approximated by the manning formula:

(1)

Where:

Q = flow in channel (m3 s-1)

A = cross-sectional area (m2)

V = velocity of flow in channel (m s-1)

n = Manning roughness coefficient (usually between 0.01 and 0.015 for gutters)

P = Wetted perimeter (m)

R = Hydraulic radius (m) ()

S = Slope

Using this formula an idea of the actual size of gutter needed can be developed for any gutter profile. Figure 3 shows the sizing in terms of material needs for the most popular gutter profiles. The shaded area represents the range of typical domestic roof sizes.

Figure 3: Material required for gutters to service various roof area

For a typical household roof of 60m2 the guttering requirement is shown in Table 2. Typical gutter widths for such a roof quoted in the literature are shown in Table 3 and are generally larger (sometime much larger) than are necessary for water conveyance.

Table 2: Guttering for a 60m2 roof

 

Square
0.5% slope

Square
1.0% slope

Half round
1.0% slope

45° Triangle
1.0% slope

Material use

214mm

189mm

150mm

175mm

Gutter width (at top)

71mm

63mm

96mm

124mm

Cross sectional area

47cm2

39cm2

36cm2

38cm2

Table 3: Gutter sizes quoted in literature

Source

Section

Roof size

Slope

Cross sectional area

(Herrmann & Hasse, 1996)

Square

40 - 100m2

0.3 - 0.5%

70cm2

Half Round

40 - 60 m2

0.3 - 0.5%

63cm2

(Nissen-Petersen & Lee, 1990)

45° Triangle

Not specified

1.0%

113cm2

(Edwards et al., 1984)

Not specified

Not specified

0.8 - 1.0%

70-80cm2

Interception

When water falls from the roof, it can curl, around the edge under surface tension, it can drop vertically down or it can follow a roughly parabolic path off the edge depending on the rainfall intensity and the roofing material. Wind also complicates this as storms are often accompanied by high winds that can blow the stream of water from the roof from its natural path. Work at the University of Warwick (WP56 - in preparation) indicates that to intercept the water for rainfall intensities from 0 to 2mm/min requires the mouth of the gutter to be 60 mm wide for a 10cm drop from the roof edge. This distance can be reduced by the use of "splash guards", "upstands" and by enclosing the gutter-roof junction. The splashguard (Figure 4a) developed in Kenya and described by Nissen-Petersen and Lee (Nissen-Petersen & Lee, 1990) consists of a piece of downward pointing sheet metal at the lip of the roof. The off-shooting water hits the splashguard and is diverted vertically downwards into the gutter. The upstand (Figure 4a) effectively raises the interception height of the gutter allowing it to be mounted lower while still effectively intercepting the runoff. Enclosing the gutter-roof junction (Tapio, 1995) effectively makes interception loss impossible but uses considerably more material and makes the gutter almost impossible to clean as well as making evaporation of any retained water less efficient (Wade, 1999).

Figure 4: Methods of augmenting interception

a. splashguard
c. upstand
a. enclosed gutter

Gutter Type

There is a staggering variety of guttering available throughout the world. From prefabricated plastics to simple gutters made on-site from sheet metal and even bamboo.

Plastic

Gutters made from extruded plastic are popular in developed countries; they are durable and relatively inexpensive. Mounting is usually by way of purpose built brackets and there is an array of hardware for joining, downpipe connection and finishing ends. They are less available in developing countries and tend to be expensive there, however in countries with a good industrial base, such as Mexico, India and Sri Lanka, plastic gutters are readily available for reasonable prices.

Aluminium

Aluminium guttering is extremely popular in countries such as Australia and the USA where it dominates the market. It is rolled on-site from coils of sheet metal in lengths to suit the house, eliminating in-line joints. Aluminium is naturally resistant to corrosion and so the gutters should last indefinitely. In developing countries where it is available, the cost of the sheet is over 1.5 times the cost of steel of the same gauge and the material is less stiff so for a similar strength of gutter a larger gauge of material is required, resulting in gutters up to three times the price. This makes aluminium gutters prohibitively expensive, however aluminium sheet is a growing market in developing countries so the price will almost certainly come down over time.

Steel

In Africa galvanised sheet steel gutters dominate. They are either made in small workshops in lengths and joined together or can even be made on-site by builders. Workshop-made gutters are usually square section and can employ an upstand to aid interception. The cost of these gutters tends to be in the order of 2 - 3 times the cost of similar gutters made on-site but they are readily available in a number of configurations (open lengths, lengths with closed ends and with attached downpipe connectors), standard mounting hardware is available and their quality is usually slightly better.

On-site gutters are usually of a vee shape as described by Nissen-Peterson (Nissen-Petersen & Lee, 1990) and adopted by several agencies such as CARE Zimbabwe (CARE Zimbabwe, 2000). The shape is quite efficient but reportedly has a tendency to block with debris. Mounting the vee shape is also more difficult and they are usually tied directly under the roof or onto a splashguard.

Wood and bamboo

Wooden Planks and bamboo gutters are widely described in the literature (Agarwall & Narain, 1997) (Pacey & Cullis, 1996) (Institute for Rural Water, 1982), They are usually cheap (or even free) and all money tends to stay in the community. They do, however, suffer from problems of longevity as the organic material will eventually rot away and leak. The porous surface also forms an ideal environment for accumulation of bacteria that may be subsequently washed into the storage tank.

Half pipe

Half pipes have been proposed as an inexpensive form of guttering (Hapugoda, 1999) and are used in many areas. The manufacture is relatively simple, and the semi-circular shape is extremely efficient. The cost of these gutters depends on the local cost of PVC pipe, which may be more expensive than an equivalent sheet metal gutter and the opening size at the top is fixed to the standard sizes of pipe available which may not be appropriate. A variant on the half pipe is a full pipe with either a slit is a groove cut into it and mounted over the edge of the roof enclosing the edge. The design is adept at catching the water, however less so at transportation as the gutter can have no slope.

Flexible guttering

The challenge of unusual shaped houses has confounded many gutter designers. The best solution so far appears to be in the area of flexible sheet material. Polythene has been tried, but UV radiation eventually degrades them and they become brittle and fail. Morgan (Morgan, 1998) has experimented with shade cloth (a tarpaulin-like material) in Zimbabwe and developed a bag-like flexible gutter with a nominal slope. The material is connected directly to the roof by wires on top and bottom as with an enclosed gutter.

Mounting

Mounting gutters to roof in developing countries presents particular problems. The roof edge is very often not straight, facia boards are frequently missing and eaves end at a random distance from the edge of the roof. Any mounting system must account for these deviations and also must allow the gutter slope to be controlled within fairly fine limits.

The usual method of gutter mounting is to use the fascia board usually used to finish the edge of the rafters. Brackets can be mounted to this or nails can simply be put through the top of the gutter with a short length of small pipe as a stand-off. The mounts can be at different heights along the line of the roof to give some adjustment in height, however no adjustment is possible for distance from the roof edge unless packing material is used. The result can be a little hit-and-miss and often requires wider gutter to intercept the water falling from a crooked roof.

The fascia board is often missing so brackets have to be mounted on the top or side of the rafters themselves. The bracket can sometimes be rotated to give height control but the rotation is strongly limited by the rafter width. More often the bracket is mounted parallel to the rafter and projected by varying amounts to adjust both planes simultaneously; so that to achieve a drop the gutter will increasingly project from the building. To combat this the brackets can be bent or individual brackets can be made for each support but this is a time consuming process and can result in the gutters having a varying slope and even points of negative slope. It is also worth bearing in mind that the adjustment will take place on an empty gutter whereas a full gutter will flex the brackets somewhat altering their position.

The roof edge itself is an attractive place to mount the gutter. The gutter will automatically follow any lateral movement of the roof and the length of the mountings can be adjusted to give fine control of the drop. The mounting is also very cheap as only wire is required. The difficulty is in mounting the gutter firmly. Most under-roof systems presently employed use a triangle of wires to tie the gutter under the edge or to a splashguard. This results in a gutter that can be blown from side-to-side which interferes with good water interception (the gutters do however move out of the way of ladders automatically). The gutter is also naturally suspended with its centre below the roof so only half the gutter is available to intercept falling water. The wires themselves are an obstruction when cleaning the gutter, as a brush cannot simply be swept along the length of the gutter. Finally, care must be taken to ensure there are enough tie wires so that the full weight of the water does not damage the roof edge.