Check foundation drainage by testing it: pour a bucket of water into the high end of the French drain and time its arrival at the outlet, confirm the ground falls 2% away from the wall, and look for standing water after rain. A 100 mm drain at a 1% fall can carry about 5 L/s, roughly 9 times the 0.56 L/s that a 100 m² roof sheds in 20 mm of rain an hour, so drains fail from silt, roots, a blocked outlet or no fall, not from lack of pipe size.
Part of Building Maintenance Schedules
Foundation drainage checks are the tests and inspections that show whether surface water and groundwater are leaving the base of a building or collecting against it.
A Drain Is Judged By Where The Water Goes
The maintenance guide asks in spring for French drains that flow freely and show no blockage, a stem wall free of cracks, settlement and erosion, and no standing water near the foundation, a sign of drainage failure; it also asks for ground that falls 2% or more away from every building. The French drain guide sets the drain: perforated pipe in gravel at footing level, falling at least 1% to an outlet at daylight or a dry well lower than the pipe, with the gravel under filter fabric (French Drains & Perimeter Drainage). Its test is to pour water in at the high end and watch it emerge.
Worked Example: Capacity, Rain And The Pour Test
Inputs: pipe diameter 100 mm, running full; fall 1%; Manning roughness n = 0.013 for smooth pipe (a standard handbook value, recalled and not read from this corpus); roof 100 m² in plan; rain 20 mm/h (an illustrative heavy shower); drain run 20 m.
- Pipe area: π × 0.1² ÷ 4 = 0.00785 m². Hydraulic radius: 0.1 ÷ 4 = 0.025 m.
- Manning: Q = (1 ÷ n) × A × R^(2/3) × √S = (1 ÷ 0.013) × 0.00785 × 0.0855 × 0.1 = 0.00517 m³/s = 5.2 L/s. Speed = 0.00517 ÷ 0.00785 = 0.66 m/s.
- Roof water: 100 × 20 = 2,000 L/h = 0.56 L/s, which is 0.56 ÷ 5.2 = 11% of the pipe’s capacity; the pipe could carry 5.2 ÷ 0.56 = 9.3 times that flow, about 9 times.
- The fall over 20 m: 20 × 0.01 = 0.20 m, so the outlet must be at least 20 cm below the pipe’s start (French drain guide).
- Pour test: water entering at the high end runs the 20 m at up to 0.66 m/s in 20 ÷ 0.66 = 30 s if the pipe is flowing full; in a part-full pipe the speed is lower. First water at the outlet within a minute or two is a pass. No water after several minutes is a blockage, a crushed pipe, or an outlet that is blocked or higher than the pipe.
The pipe is rarely the limit. The limits are silt, roots, a blocked or buried outlet and a pipe laid with no fall, and all of them are found by the pour test and by walking the outlet.
For a soakaway taking roof water, the drainage design guide shows how fast a pit of rock with an assumed 40% voids fills: a 1 m³ pit has 0.4 m³ of voids, and at 0.56 L/s it fills in 400 ÷ 0.56 = 710 s, about 12 minutes, if no water soaks away (Drainage Design). Check that it empties between storms.
What You Need
- A bucket and a watch
- A string and line level, or an A-frame level, for the ground fall (A-Frame Level)
- A rod for clearing the outlet
- A spade, for opening an inspection point
- The log, with the dates and times of each test
Method: Test After The Rains Begin And Before They Peak
Step 1: Walk the outlets in the autumn dry and clear leaves, silt, roots and animal nests.
Step 2: Pour 10 L into the high end and time the first water at the outlet. Record the time.
Step 3: Check the ground fall: 2% is 6 cm over 3 m.
Step 4: After heavy rain, walk the building looking for standing water, wet ground at the foot of the wall, and damp at the wall base.
Step 5: Check where roof downpipes discharge, and carry that water away from the wall, not into the drain without checking capacity (French drain guide).
Step 6: Compare the times with last year’s. A slower test is an early warning of silting.
Common Mistakes
- Sizing the cure by the pipe. It is nearly always the outlet.
- Roof water dumped at the foot of the wall. It undermines the base.
- Trees near the drain. Roots find the joints.
- No record. A drain that slows over three years gives no warning.
Safety
Deep trenches collapse, and water drowns
A drain trench at footing level is often deeper than about 1.2 m, especially at the low end where the fall adds depth. Slope the sides or shore them, keep spoil at least 0.6 m from the edge, never dig alone and stay out of a trench after heavy rain. Cover open inspection pits, because a child can drown in 30 cm of water.
What’s Next
- Learn next: Pest Inspection (Termites, Rodents) — what damp ground and timber attract
- Combine with: Waterproofing and Drainage — the design side of keeping water out