Garden Paths and Patio Designs – Drainage
Garden Paths and Patio Designs
Water is the life-blood of any garden, yet it can also undermine foundations and the entire landscaping framework, washing away plants and paving and causing substantial damage at times. Even if the soil is stable and the ground absolutely level, patios and paths must be constructed so that excess rainwater drains away from buildings and does not accumulate in pools on the surface.
Subsurface drainage must not be overlooked. Regardless of the materials used, the durability of garden paving and the structures that go with it will depend to a large extent on the stability of the ground beneath it. If the site is level and the soil drains naturally, you can lay most materials on a bed of sand without any additional subsurface drainage. If, however, there is a high clay content or marshy areas are evident in fine weather, it is advisable to lay a sub-base of well-compacted hardcore or crushed stone under the surface. Perforated plastic pipe buried in this sub-base will help to draw off water to a surface channel or a soakaway. Alternatively, porous geosynthetic pipes (made from high-density polyethylene) wrapped in filter fabric (permeable polyester geotextiles) may be used. These may also be set in narrow trenches along the edges of paths, driveways, patios and so on to create a very effective drainage system.
On sloping properties or where substantial drainage is necessary (behind a retaining wall, for instance), a vertical drain may be required. The most common type is made by setting pipework into a French drain filled with rubble and stone. This can involve substantial earthworks, and instead, a pre-fabricated fin drain may be installed if the materials are available. Lightweight and simple to install, this kind of drain is made by wrapping geotextile material (kept rigid by a layer of synthetic geonet) around geosynthetic pipework (see above), so that the fabric extends above the pipe to form a fin. Water is attracted to the highly absorbent fin, and flows down to the pipe, which directs it away from the base of the slope. It is ideal for the owner-builder to install as it obviates the need for cumbersome hardcore and stone and cuts down on both transport and labour costs.
Some form of drainage will also be needed behind most steps, unless they are very gradual and sited away from buildings and paved areas. In most instances it is sufficient simply to backfill behind each riser with well-compacted crushed stone or gravel; if the steps are steep or involve a retaining wall, it may also be necessary to lead a pipe from a drain behind the structure to divert the water, or to use a fin drain.
Surface drainage is necessary to channel water away from buildings, and there should be a fall of about 1:40 or 1:50 across any patio, path, driveway or other paved area. With practice you will soon learn to judge just how off-centre the bubble in the spirit-level vial should be to achieve this run-off. To establish the correct gradient, attach a small block of wood under one end of a straight-edged piece of timber and place the spirit level on top; the bubble in the vial should be exactly centred. To achieve a gradient of 1:40, use a 25mm block under a 1m long straightedge; for 1:50, use a 20 mm block. It is also good building practice to set up a line as a guide following the angle of the straightedge. The finished surface of the patio should be below the inside floor level, and if there is a damp-proof course (DPC) in the walls, the patio floor should be at least 150 mm below the top of this. In cases where the interior floors are made of timber, the upper surface of paving, concrete and so on should be below the bottom of the floorboards.
If you want to divert rainwater underground, you will have to build a gully or lay a precast channel above the ground and direct this into a subsurface drainage system. You will not usually be permitted to drain surface water into an existing sewerage system, but you may be able to link up with a stormwater drain.
Often drainage is not an issue with garden paths; however, solid pathways (including those which are brick-paved or made from concrete laid in situ) should be fractionally lower on one side than the other to allow for a runoff, while the treads of steps should slope down to the front very slightly (no more than 1:100) to prevent puddles from forming.