Rainwater disposal
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If rainwater flowed off the roof of a building in an uncontrolled way, then it could flood the areas around the building and inconvenience the occupants. Water can also flow off shallow eaves and find its way into a building through the walls and windows. For these reasons, it is better to collect water at the eaves in a gutter and direct it down to the ground in a rainwater pipe. The rainwater can then drain away underground and discharge into soakaways.
A soakaway is a rubble-filled pit in the ground which absorbs water quickly. The soakaway should be at least 3 metres from the building.
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Figure 5 Details of bracket gutters and a connection: (a) a gutter bracket connection;
a half-round gutter; (c) a box gutter.
Gutters. A gutter is a channel fixed to the long edge of a roof. The bracket fixings on the fascia should be at 900 mm intervals. A gutter will have an outlet which is a spout that points down to connect with a rainwater pipe.
In most domestic buildings, one outlet is usually enough to take the rainwater during normal wet weather. The distribution of water along the length of the gutter pushes it naturally towards the outlet. Any residue of water soon evaporates.
Some gutters are not attached to the edge of roofs such as:
- gutters formed in the surface of flat roofs beside parapets;
- valley gutters, which are formed at the junction of two roof slopes.
The most common gutter shapes are the half- round and the box (Figure 5).
Rainwater pipes. The rainwater pipes are attached to the gutter outlets and fixed vertically on the building. The roof overhang requires a rainwater pipe to bend in
more than one direction to reach past the gutter to the surface of the building. This bend is called a swan neck junction (Figure 6).
The pipes are fixed with brackets, plugged and screwed to the wall surface, which hold them off the surface to allow for painting and cleaning.
The water runs out of the bottom of the pipe from a shoe or angled pipe into a gully fitted with a grating. If this gully is connected to a soakaway by an underground pipe, then it does not need a trap. If it is connected to the soil drainage, then it must have a trap.
The flow should run down against the face of the rainwater sockets. The joints do not need to be sealed except to stiffen the pipes. If there is a blockage in a pipe, then the water can leak out through the joints and alert you to the problem. If the pipes are inside a building, then the joints must be watertight.
Materials. Gutters and rainwater pipes are most commonly made from:
uPVC; fibre cement; zinc.
UPVC is the most popular material because it does not need decoration, it is lightweight and easy to fix. UPVC does not rot or corrode. However, this material is more easily damaged and is unsuitable for areas where it could be hit by moving vehicles. UPVC also expands and contracts more than the other materials as a result of temperature changes.
Fibre cement is a durable and heavier material than plastic that requires more joints and supports. It is often used for larger roofs that take big gutters, like those on industrial buildings. Fibre cement is not affected by temperature changes and it can be painted or left in its natural grey colour. You need to be very careful if you cut it that you do not inhale toxic dust.
Zinc is not a strong material because it dents easily and corrodes in some climates. It is popular since it can be made almost anywhere by folding and soldering zinc sheets into the desired shapes.
Valley gutters. When pitched roofs meet at right angles, then the junction is called the valley. You make a gutter in the valley that runs from top to bottom. Since this is almost an internal gutter, water could enter the building if it overflowed. These gutters must be a minimum 300 mm across to avoid blockages and overflows.
Surface water.Water that falls on the hard surfaces surrounding a building such as paths, terraces and driveways can be collected by open channels or gullies. Hard surfaces should slope away from the walls towards these collecting points. If the drains running from the gullies go to a soakaway, then they do not have traps, but they should be fitted with removable silt buckets to make them easier to clean.