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Drain Tile Location Filter paper and properly oriented perforations, however, will not guarantee that drain tile will work. The pipe must also be installed carefully and in the right location with respect to the footing and any interior slab. From a pure engineering point of view, the ideal place to lay exterior drain tile is alongside the footing, because water from a rising water table enters the pipe sooner (Figure 5).

Pipe Even with Top of Footing

Figure 5. The best location for rigid drain tile is alongside the footing. Minimum requirements for stone cover depend on whether the tile is flush with the top of the footing (top) or the bottom (middle). In either case, the top of the interior slab should be at least 6 inches above the top of the drain tile. The pipe can be laid level or pitched slightly.Where drain tile must be located lower than the bottom of the footing (bottom), avoid undermining the footing by keeping the pipe outside of a 60-degree angle measured from the corner of the footing. This location also requires more stone cover for the pipe.

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Pipe at Bottom of Footing

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Pipe Below Footing

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The drain tile does not need to be sloped, although a slight pitch helps keep the pipe clear of silt and clay (particularly when the pipe has just two rows of holes on the bottom). Avoid trying to slope flexible drain tile, however, because you can inadvertently create dips and sags that will eventually collect silt and clog the pipe (Figure 6).

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Figure 6. Regardless of the type of pipe used or its shape, unfiltered drain tile can easily be plugged with silt and clay (left). Water-seeking roots from trees growing too near the foundation can also completely clog perimeter drains (right). In fact, undulating drain tile can result in premature failure of the drainage system. This problem is more pronounced when trees are growing close to the foundation, because wet silt and clay accumulating in low spots become targets for water-seeking tree roots in dry periods or in dry climates. In a relatively short period of time, tree roots can completely plug drain tile. Some contractors create an even lower elevation for the tile by digging a small trench next to the footing. To avoid undermining the foundation, however, most codes require that the tile be placed outside a 60-degree angle from the footing. Drain tile can also be placed on top of the footing. The advantage here is that the tile will be as level as the footing — a good strategy when using flexible pipe (Figure 7). Figure 7. To keep flexible drain tile from developing low spots that will collect silt, place it on top of the footings, making sure that the top of the pipe is not higher than the top of the interior slab. But this higher placement doesn’t control a rising ground water table as effectively, and may require raising the elevation of the interior slab.