8.5 Subsoil Drains, Areaway Drains, Storm Sumps & Building Storm Sewers

Subsoil Drains, Areaway Drains, Storm Sumps & Building Storm Sewers

1. Subsoil drains

A subsoil drain intercepts groundwater before it reaches a foundation. In Iowa it is not an optional refinement: the state's glacial till and loess soils hold water, spring thaw saturates them, and a basement wall with no drainage system behind it carries the full hydrostatic head of whatever the soil holds.

Construction

• Piping is perforated or laid with open joints, so water can enter along the run rather than only at an inlet.
• The pipe is bedded in and surrounded by filter material — washed gravel or crushed stone — that lets water pass and keeps fines out.
• A filter fabric wrap over the aggregate is standard practice, because the failure mode of a footing drain is silting, not crushing.
• The drain is placed at or below the level of the footing it protects, so it lowers the water table below the slab rather than merely intercepting what has already arrived.

Where the water goes

A subsoil drain discharges to an approved point: a storm sewer, an approved outfall to grade, a drywell, or a sump from which it is pumped. What it may never do is discharge into the sanitary drainage system. A footing drain tied into a sanitary line is the single largest source of clear-water infiltration in older housing stock, and it is precisely the illegal connection that municipal inflow-and-infiltration programs spend money to find and sever.

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2. Building subdrains and storm sumps

Any storm drainage that originates below the level of the point of disposal cannot leave by gravity. That includes:

• Footing and foundation drains on a deep basement.
• Areaway drains in below-grade window wells, stairwells, and light wells.
• Ramp and pit drains at a below-grade parking entrance or loading dock.

The code answer is a building subdrain collected in a sump and lifted by a pump into the gravity storm system.

How a storm sump differs from a sewage ejector sump

This comparison is worth memorizing, because the exam likes to test whether you will over-apply the sanitary rules:

The reason for every difference in that table is the same: a storm sump has no connection to the sanitary system and therefore no sewer gas to seal in. If a question describes a sealed, vented sump receiving footing drainage, it is describing something the code does not require — and if it describes an open sump receiving water closet discharge, it is describing a violation.

Pump discharge discipline

• The discharge piping rises to a point above the flood level of the receiving system, then falls to it, so a surcharged storm sewer cannot run backward into the sump.
• A check valve prevents the vertical column from draining back and short-cycling the pump.
• An accessible shutoff valve downstream of the check valve lets the check valve be serviced.
• A high-water alarm is cheap insurance on any sump whose failure floods finished space.

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3. Areaway, window well, and stairwell drains

A below-grade areaway is a bucket cut into the ground beside a building, and the door or window at the bottom of it is the reason it must drain reliably.

• Size the drain for the areaway's own catchment, including any wall area that sheds into it — the same Section 1103.4 logic applies in miniature.
• Trap the drain where sewer gas could otherwise reach the building through it, and protect the trap seal. An exterior trap that freezes or evaporates is a trap that is not working.
• Protect against freezing. In Iowa an areaway drain that ices at the inlet is a flooded stairwell in the February thaw. Keep the inlet clear, and where the drain runs shallow, insulate or increase cover.
• Use a strainer sized for leaf load, and select a grate rated for the traffic it will see — a pedestrian grate in a drive lane is a failure waiting for a delivery truck.
• Areaway drains discharge to the storm system, never the sanitary system.

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4. The building storm sewer

The building storm sewer is the horizontal piping from outside the building wall to the point of disposal. Chapter 11 does not reinvent the rules for it; it borrows them.

• Cleanouts. Cleanouts for building storm drains comply with the same cleanout requirements as the sanitary building sewer — upper terminal, junction of drain and sewer, aggregate changes of direction exceeding 135 degrees, maximum spacing, direction of flow, and rodding clearance.
• Materials and joints. Approved storm piping materials and jointing methods parallel those for sanitary building sewers, and Iowa's thermoplastic trenching amendment applies here too: trench width at the base limited to six times the pipe outside diameter, not less than 4 inches of aggregate bedding, and 3/8-inch pea gravel or 1-inch clean class-one bedding as initial backfill (rule 481—425.4(2)"c").
• Slope. Horizontal storm piping is sized at the slope it is actually installed at, from Table 1103.2 — 1/8, 1/4, or 1/2 inch per foot. Choose the slope first, then size, and do not mix blocks.
• Never reduce in the direction of flow. As with sanitary piping, a storm drain may increase downstream but never decrease.
• Testing. Storm drainage piping is tested for tightness before it is concealed, in the same manner as other drainage piping.

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5. Pulling Chapter 11 together

You now have the full chapter: scope and prohibited connections (Section 8.1), vertical sizing from Table 1103.1 (Section 8.2), horizontal and gutter sizing from Tables 1103.2 and 1103.3 (Section 8.3), wall allowances, overflow drainage, and controlled flow (Section 8.4), and below-grade collection and the building storm sewer (this section).

Two habits will convert that knowledge into points. First, tab the three sizing tables physically — 1103.1, 1103.2, and 1103.3 — because at roughly two and a half minutes per question you cannot afford to hunt. Second, when a storm question appears, write down the area, the rainfall rate, and the slope before you open the book. Nearly every wrong answer in this subject area comes from reading the right table in the wrong column.