Installing Roof Drainage

Rainwater drainage from roofs is an important aspect of domestic plumbing for the design of which, in the past, rule of thumb methods have always been applied. On the whole rule of thumb methods have proved to be satisfactory. ‘A little learning’ might suggest that, since rainfall varies tremendously throughout the United Kingdom, gutter sizes and the number of downpipes provided should vary correspondingly. The roof of a house built in the Lake District, for instance, may be expected to receive, during the course of a year, three to four times as much rain as a house built in north-east Essex.

Design of roof drainage does not however depend upon total annual rainfall but upon rainfall intensity—and this varies very little from one part of the country to another. The average intensity recommended as a basis for rain water drainage design is 3in per hour. This has been found to occur, in any given locality, over a period of five minutes about every other year. It will occur over a period of ten minutes only about once in eight years. As an indication of the safety factor that is provided by designing on this basis it may be mentioned that an intensity of rainfall of 4in lasting for five minutes occurs only once in five years, for ten minutes only once in about nineteen years. For roof pitches up to 50° the actual area of the roof surface is taken as a basis for calculation. The pitch and the angle at which the rain falls is ignored. At 3in per hour the flow load will be 0.026 X actual roof area in square feet, giving a result in gallons per hour.

Prior to World War I rainwater gutters were almost invariably made of cast iron, painted internally and externally, in either half-round, square or ogee pattern. Downpipes were made of the same material. For a brief period during the post-war years asbestos cement gutters and downpipes enjoyed a certain popularity because, since they needed no painting, they materially cut the cost of house maintenance. They were however rather heavy, clumsy in appearance and subject to accidental damage.

P.V.C. guttering and downspouts are now in standard use for both new and replacement work. The smooth internal surface of p.v.c. gives a better flow. No painting is required either for decoration or for protection against corrosion and the lightness in weight of the material makes for easy installation without the risk of damage to the material or injury to the installer.

The only disadvantage of rainwater systems of this material is that p.v.c. guttering does not offer adequate support for a ladder. When access to the roof is required the ladder must be placed against either the fascia board or the wall below it. Although p.v.c. guttering is available in a number of sizes and shapes 100mm wide, half-round gutter has become the standard choice for general domestic use.

On the basis of the design considerations already set out a gutter of this size, set dead level, will drain a roof having an area of approximately 425 sq. ft. Where the gutter is laid to a fall of lin in 50ft the area that can be drained is increased to 600 sq. ft. 68mm downpipe is used with 100mm guttering and a downpipe of this size, centrally situated, will cope with the drainage of a roof area of 1200 sq. ft. For all practical purposes this means that 100mm half round guttering draining into one 68mm downpipe will cope with the drainage of the front roofs of a pair of terraced or semi-detached houses and that similar provision will be needed at the back. 100mm guttering with 68mm downpipes at front and back will cope with the roof drainage of all but very large detached houses.

As has been suggested, rainwater guttering can be fixed dead level. However the slight fall of lin in 50ft improves drainage capacity and guards against the possibility of a slight accidental back-fall-perhaps the commonest cause of overflowing gutters.

All manufacturers of plastic waste and soil drainage systems include roof drainage systems within their range and supply full installation instructions. The details given below relate to the Marley system of 100mm half-round guttering with 68mm down-pipes. The first thing that must be established is the position of the gutter outlet. In a new building this will be decided by design considerations bearing in mind the effect of appearance of the downpipe on the facade of the building. For replacement work the deciding factor will be the position of the existing yard gully. A plumb-line dropped from the fascia board to the centre of this gully will establish the position of the gutter outlet.

Working back from this point with a line at a fall of lin in 50ft the position of the supporting brackets can be determined. Brackets must be fitted in close proximity to and on either side of the gutter outlet. They must also be fitted closely to both internal and external angles of the roof. For straight runs of guttering, brackets should be fitted at 1 metre centres with Marley heavy gauge guttering and at no greater than 900mm centres for the lighter ‘System 2’ guttering. Brackets should be fixed to the fascia board with lin X No.8 gauge zinc plated or sherardised round head screws.

Marley ‘heavy grade’ roof guttering is provided with one socketed end and one notched spigot end. ‘System 2’ guttering has notched spigot ends only and separate Marley gutter joints must be used for connecting lengths of gutter. In both cases however the method of connection is the same.

The flexible plastic gutter strap is clipped round the socket of the gutter or of the separate gutter joint. The spigot of the other length of gutter is then placed in the socket and turned so that the retaining projection of the back edge of the gutter strap fits into the notch of the spigot. The front edge of the spigot is then eased down into the socket until its notch snaps under the front projection of the gutter strap. This has the effect of compressing the spigot against the prefixed synthetic rubber seal to give a watertight joint.

It will be seen that the notch in the spigot of the gutter is an essential feature of the patent Marley joint. Where it is necessary to cut a length of gutter a similar notch must be made in the cut end. The gutter should be cut absolutely squarely with a fine toothed saw and a notch 40mm wide and 3 mm deep made 10mm from the end. A special notching tool is made for this purpose but it is possible to make the necessary notch with a file.

The first task to be undertaken in connection with the fixing of the downpipe is assembly of the offset. Marley offsets have three components-an offset end socket for connection to the gutter outlet, an offset end for connection to the downpipe and a length of offcut rain water pipe to connect the two. The offset is made into one unit by means of solvent welding. This technique, which is used for connecting p.v.c. water supply and waste pipes, is described fully in a preceding post2. The instructions set out below relate solely to rain water offsets.

The offcut length of rain water downpipe must be cut squarely to length and all rough edges removed. Wipe internal surfaces of the solvent weld sockets and the outside of the pipe end perfectly clean with a dry cloth. Next assemble the offset dry and draw a pencil line along pipe and offset ends to ensure correct alignment. Withdraw pipe from offset ends and apply solvent cement evenly round the spigot ends of the offcut and the inside of the offset sockets. Press pipe and offset sockets quickly and firmly together taking care to line up with the pencil guide lines that have already been made. Leave the completed offset for several minutes before fitting into position to ensure that the joints set properly.

Various offset components are available to overcome the problems that may arise from, for instance, a deep fascia or a corbel projection. Rainwater downpipe components are provided with ring seal sockets but the ring seal is not normally inserted in external rainwater work. Since the downpipe should never run full a ring seal is normally unnecessary and the absence of such a seal makes it possible to locate quickly any blockage that may occur.

However, where an eaves projects more than 600mm there will be a tendency for the weight of the offset to pull the offset away from the gutter outlet. To counteract this tendency a ring seal should be provided for large eaves projections.

The offset spigot for connection to the rainwater downpipe is 112mm long. This allows for adjustment of the position of the first pipe clip. Offer up the socket of the first downpipe to the spigot of the offset. If the holes of the pipe bracket back plate do not align with a mortar joint, measure the amount of spigot that needs to be cut off in order that the pipe clip can be fixed to the next joint up.

Two kinds of pipe clip are available. A one-piece clip and a two piece unit in which the plastic strap that supports the pipe socket is bolted to a back plate secured to the wall. In both cases the mortar joint should be drilled and plugged with purpose made fibre or plastic plugs and the back plate-or the clip itself in the case of the one-piece unit—secured to the wall with two l ½ in or l 1/1in by No.lOg zinc plated or sherar-dised round head steel screws. A clip should be located at the socket of each length of rainwater downpipe and an intermediate clip provided for any length of downpipe exceeding 2 metres. An important point to note is that an expansion gap of 10mm should be left between the end of each pipe and the bottom of the socket into which it is fitted. This is done by inserting the pipe to its fullest extent into the socket, withdrawing 10mm and marking the point to which it has been withdrawn.

A trapped gully should always be used where the rainwater is disposed of into a sewerage system. Where discharge is into a soakaway or ditch an untrapped connection is permissible.

Ultimate disposal of the rainwater will depend upon the policy of the sewerage authority. In some areas some, or all, rainwater from roofs is permitted to flow into the normal household drainage system and thence to the public sewer. This results in the sewerage authority having to provide unnecessarily large sewers to cope with the surge that occurs at times of intense rainfall. It means too that the sewage arriving at the treatment works will be highly concentrated in times of drought and very dilute during periods of heavy rainfall.

These factors have led many authorities to require separate provision for rainwater drainage. In some areas a separate surface water sewer is provided. In these areas it is very important that the builder should make the ultimate connection, of both the foul and the surface water drain, to the right sewer. Connection of the foul drain to the surface water sewer can result in untreated sewage flowing into a stream or ditch. In other areas the householder is required to provide a soakaway for the reception of rainwater drainage from roofs. Typically a soakaway consists of a rectangular pit about 5ft deep and 5ft square in plan. This is filled with brick rubble to within about 1ft of the surface and the top soil is then replaced. The snag about a soakaway of this kind is that, after a few years, the interstices between the rubble will become full of silt and the soakaway will then have to be dug out and remade. This eventuality can be delayed by laying a sheet of polythene over the brick rubble before back-filling with top soil.

There are nowadays precast concrete soakaways available into which rainwater can be discharged. These resemble cesspools but have holes in the sides from which water can escape into the surrounding soil. A manhole gives access to enable the silt to be dug out when required.

It should perhaps be added that soakaways are rarely very successful except where the soil is light and friable and there is a low subsoil ‘water table’. Few soakaways will cope adequately with long periods of continuous, heavy rain.