French Drain Installation in Austin, TX Subsurface Water Given an Outfall

Subsurface versus surface, and how to tell which one you need

These are two different systems and mixing them is the most common installation error in Austin yards. A surface system collects water already running across the ground and moves it in solid pipe to a discharge point. A subsurface system collects water traveling through the soil, and the Texas Section ASCE guidelines describe it as perforated pipe, with or without filter fabric, in an aggregate-filled trench, with a continuous minimum slope of half a percent to a surface outfall and cleanouts provided for maintenance. The same guidelines state flatly that downspouts should not be connected to perforated pipe subsurface drainage systems. Tie a roof into a french drain and it stops draining soil and starts injecting several hundred gallons directly into the ground next to the foundation. If the water you are chasing is sheet flow off a patio, the job is drainage and grading instead.

The construction detail the code actually specifies

Where a foundation retains earth and encloses habitable or usable space below grade, the residential code as adopted in Austin sets out the drain in Section R405.1, and the numbers are worth knowing because they are what separates a drain from a trench full of rock. The drainage system is installed at or below the top of the footing or below the bottom of the slab, and discharges by gravity or mechanical means into an approved drainage system. Gravel or crushed stone extends at least one foot beyond the outside edge of the footing and six inches above the top of the footing, and is covered with an approved filter membrane. Perforated pipe is placed on at least two inches of washed gravel and covered with at least six inches of the same material, with the washed rock at least one sieve size larger than the perforations so the openings do not plug. Those dimensions are why a properly built drain outlives a shallow one: the stone envelope, not the pipe, is what conducts the water.

The filter is what decides whether it lasts

On Austin's east-side clay the failure mode is silting, not crushing. Clay particles migrating into the stone envelope fill the voids, and a drain that silted up conducts nothing while looking perfectly intact from the surface. That is what the filter membrane in the code detail is for, and it is the layer most often left out to save a few dollars on a job nobody can inspect afterward. The second life-limiting decision is the outlet. Half a percent of slope is a fall of about six inches over a hundred feet, so a long run on flat ground east of MoPac has very little room for error, and a drain that ends in a buried sock or a low spot in the lawn instead of a daylighted outfall is a storage tank. Cleanouts, which the guidelines call for, are the only way to ever flush the line.

Where these go in Austin, and where they should not

The flat ground east of the escarpment produces the classic case: a yard with nowhere to fall, standing water along one elevation after rain, and clay that will not accept infiltration. On the hilly west side the problem inverts and a drain is more often used to intercept water moving down a slope before it reaches a lower-level wall or a garage cut into the hillside. Both need a real outfall, and on a small lot that is the constraint that kills otherwise sound designs. Austin's drainage rules do not allow a project to increase the flow or timing of water leaving the site, to block flow passing through it, or to discharge irresponsibly downstream, so a drain that solves one yard by loading the next is not a legitimate design.

The permit item specific to trenching here

Digging a trench near a large tree is regulated in Austin, and a french drain is a trench. The city requires a Tree Ordinance Review Application for any activity that may impact a regulated tree or disturb its critical root zone, and its plumbing permit form states that installing utilities or irrigation lines within the critical root zone of a regulated tree requires that application. Any tree nineteen inches or more in diameter is regulated, and within the inner quarter of the root zone no impacts are permitted. Where the trench cannot avoid the zone, hand excavation and a permit are the path. Root barrier installation runs into the same rules.

What the work involves and what sets the price

The route is laid out from the wet area to a viable outfall, and the fall between the ends is measured before anything is dug, because if the fall does not exist the design has to change. Depth is set from the footing or slab elevation rather than from the lawn. The trench is cut, lined, filled with washed stone to the specified envelope, piped, wrapped, covered and brought back to grade with a soil cap that does not seal the top. Cleanouts go in at the ends and at direction changes. Cost follows length, depth, whether the spoil can be driven out or has to be barrowed, whether rock is hit — a real possibility west of MoPac on shallow soils over limestone — and whether tree roots and the ordinance force hand digging.