Base Slope, Drain Tile Pitch, and the Outlet

The IRC base slope under the slab, the 1/8 in per foot pitch on the drain tile, the gravel and fabric quantities, the pipe size, and the daylight or sump decision, with the dampproof and below-grade insulation list.

IRC R401.3 asks for a base that extends 10 ft beyond the footing and slopes at 1/4 in per 10 ft. Drain tile runs at 1/8 in per foot to a daylight outlet, a sump, a dry well, or a storm sewer, with 12 in of gravel over a pipe and 2 ft of cover.

—Base slope under the slab
—Drain tile pitch and drop
—Gravel volume and fabric
—Pipe size, cleanouts, and cover
—Outlet, sump, and power
—Dampproof, insulation, and water
How the math works
  • Base (IRC R401.3): a base under a slab-on-grade and below a footing extends a minimum of 10 ft beyond the footing, and the base is sloped at a grade of 1/4 in per 10 ft, a 1:400 slope, to a daylight outlet, a sump, a dry well, or an under-slab drainage system. A 10 ft strip at 1/4 in per 10 ft is a 1/4 in drop.
  • Drain tile (IRC R401.3.1): a perforated pipe at the footing with the top of the pipe below the bottom of the footing, pitched not less than 1/8 in per foot (1:96) to a daylight outlet, a sump pit, a dry well, or a storm sewer, with a cleanout at the building and every 50 ft, a gravel bed, and 2 ft of cover.
  • Site grade (IRC R401.4 and R408): the finished grade slopes away from the wall with at least 6 in of fall over the first 10 ft, with a swale, a berm, and a 2 percent slope on a lot that will not carry the 6 in, the downspout discharged 6 ft from the wall with an above-grade outlet, and 18 in of clearance over the exposed ground in a crawl (IRC R408.1) with 12 in over the joists.
  • Dampproof and insulation (IRC R401.1, R401.2, table R402.2.11): the wall below grade is dampproofed on the exterior face, a soil that wicks water to the wall gets the tanking, and a conditioned basement wall gets the below-grade continuous insulation by climate zone in the table: 1 in of rigid in the mild zones and 2 in in the cold zones, with the vapor barrier under the slab over a 4 in gravel base.
  • Sump and discharge: a sump pit at the low point with a solid cover, a pump with a float, a check valve, a battery backup, an alarm, an air gap at the discharge, a 6 in weep hole at the pit, a discharge line above the frost line pitched 1/8 in per foot to daylight, and a storm tie-in with a cleanout and an air gap. A sump on a power loss is a wet basement, and a battery backup is part of the system rather than an accessory.
Planning tool on the International Residential Code sections R401 and R408 and table R402.2.11 as adopted locally, with the standard field practice for footings, drain tile, and sump discharge. Local code edition, the wet and dry soil classification, the soil report, the groundwater map, the flood zone, the storm sewer connection rule, the dry well rule and its size table, the sump discharge rule with the air gap requirement, the frost line depth, the permit, and the inspection are jurisdiction calls. A perched water table, an artesian condition, a clay shell over sand, a sinkhole and karst, a fill over a former pond, a hillside with an upslope water source, a basement with exterior drain tile at 6 ft deep, a system on a slope with a 40 ft run, a tile tied to a combined sewer, a tile tied to a storm sewer, a basement with an interior channel drain, a basement with a radon system under the slab, a basement with a radiant floor, and a basement with a 3 in foam layer under the slab are all site-design calls, and a geotechnical report is the document a wet site starts with.

What slope does a foundation drain need?

The IRC asks for 1/8 in per foot (1:96) on the perforated pipe at the footing, and 1/4 in per 10 ft (1:400) on the base under the slab with the base extending 10 ft beyond the footing. A 60 ft run at 1/8 in per foot is a 7.5 in drop, and a 10 ft base strip at 1/4 in per 10 ft is a 1/4 in drop.

Does drain tile need gravel?

The field build is a 12 in wide trench with the pipe at the footing, 6 in of clean 3/4 in gravel over the pipe, geotextile fabric over the stone and down the trench walls, and 2 ft of cover. A gravel trench with no fabric plugs in a few years, and clean stone with no fines is the void that drains.

Does my basement need a sump pump?

A sump is one of the four outlets the IRC lists, with a daylight outlet, a dry well, and a storm sewer, and a sump is the answer on a flat lot with no daylight run. The sump build: a pit at the low point with a solid cover, a pump with a float, a check valve, a battery backup, an alarm, an air gap at the discharge, a weep hole at the pit, and a discharge line above the frost line pitched to daylight.

How far away do I slope the grade?

6 in of fall over the first 10 ft from the wall, with a swale, a berm, and a 2 percent slope on a lot that will not carry 6 in of fall, and the downspout discharged 6 ft from the wall with an above-grade outlet (IRC R401.4). A grade that stops at 3 ft of fall is the grade that fills the gravel trench on every rain.

Related tools

Footing concrete and rebarThe footing the tile lands beside Excavation spoil and shrinkThe dig, the spoil, and the backfill Gutters and downspoutsThe roof water that stays out of the tile Crawlspace encapsulationThe 18 in clearance and the membrane Gutter capacity by rainfallWhere the downspout discharges Sump pump sizingDry pit and discharge line