Spoon Drains (Full Guide)

Spoon drains are drain lines with an interior trap that protrude into the pipe enough to create a “spoon” shape. The term was coined early in the 20th century when cast-iron soil stacks were round and traps had flat bottoms (more like spoons than today’s traps). Today, spoon drains typically have interior bends rather than flat bottoms. Spoon drains usually exist as part of a wet venting system that vents multiple fixtures through one opening in the drain line. In this way, they prevent public sewer gases from entering dwelling units through open stand-pipe or other vents serving more than one fixture.

Outside ground spoon drains are often used to vent mains and lateral lines below the sewer level, rather than at the gutters and sewer level. They are also used in some special drainage applications, such as when exterior vents may be located at a height where they cannot be vented through the soil stack.

Concrete Spoon Drains

This type of spoon drain is popular in new construction, especially when the entire wet vent is being installed at the same time as the rest of the drain stack. The trap for a concrete spoon drain is formed during installation by placing concrete into the pipe and allowing it to cure.

Cast iron pipe spoon drains are also used where building height does not allow for wet vents above each fixture. As always, check with your local code authority before specifying this system as some jurisdictions do not allow cast-iron soil stacks and require PVC drainage systems instead.

Surface Water Problems

Where surface water is a problem, concrete spoon drains are the best choice. The concrete traps at each fixture prevent sewer gases from entering through open standpipes or vents serving more than one fixture. In this situation, cast-iron traps would allow sewer gas to enter dwelling units through these openings.

False Drains or Dry Vented Floor Drains

Spoon drains, which are also called false floor or dry vents, can cause problems where there is no water in the trap. Dry venting traps allows odours to enter through open standpipes or vents serving more than one fixture. If wet vented fixtures are frequently dry-vented, fumes could build up in the system and become a health hazard.

Note that jurisdictions differ on whether it is permissible to dry vent fixtures in certain circumstances, particularly where the building’s drainage system does not necessarily serve any specific fixture. If in doubt, consult your local code authority for guidance.

When using cast iron pipe, be sure to use a material compatible with waste piping such as ATSM F 125 or ASTM A 74 type E cement mortar (instead of standard Portland cement mortar). Unlike some other materials, a compatible mortar may not swell and cause problems in the spoon open drain.

What is a Spoon Drain?

An outside spoon drain is a drain that extends from an opening in the wall of a building, through the exterior wall to form a “spoon” shape. This type of spoon drain is usually only found on buildings with a continuous foundation or in areas where it is too impractical to run a conventional piping system under the foundation. In Australia, this type of drain is referred to as a “channel drain”.

A channel drain is normally fitted to invert in a drain sump, in which case there is no internal pipework. The channel drain is typically made of concrete and has one end in the foundation wall, often with an access hatch. It may be used by fitting a suitable flexible hose between the external spout and the internal pipework of adjacent drainage systems.

Retaining walls, however, may not have sufficient height to accommodate a drain sump. This can be solved by using an inverted bowl type insert that is made of plastic or concrete. Surface water flows into the bowl and because of its inverted position, it does not siphon. Inverted inserts are practical for use with a dry well sump or in combination with a soil stack.

spoon drain
Spoon Drain

Spoon Drain Definition

A shallow ground surface water drainage system, in the shape of a spade, spoon or shovel, that provides low-cost installation, but is not suited to rainwater drainage.

A spoon drain system consists of an open channel with a shallow S-shaped cross-section and no trapped air pocket at the invert. It may be formed by casting concrete along an open trench or bedding mortar in pre-formed corrugated metal sheets, which are then covered with earth backfill. The spoons are about 2.5 cm deep. The width of the channel depends on soil conditions and whether there is any anticipated settlement over time. There could be enough flow so that land stormwater may not pond in flat areas after moderate rains.

Spoon Drain Type

The term “spoon drain” originates from a time when cast iron soil stacks were shaped like spoons.

Spoon drains are varieties of compartmentalised drain systems, where there is a trap at each fixture to prevent sewer gas from travelling back up the stack and entering buildings. The traps are formed by making an S-bend in the pipework, rather than by using an integrated design as is more usual for modern pipework.

Copper Spoon Drain

The copper spoon drain is made out of copper or any other material that can withstand corrosion, such as stainless steel / galvanized steel etc. It has two openings on one end which leads water into its interior section. One opening leads to the “T” trap while the second lead leads to the building’s waist–line. To this end, it is best to install a 5cm copper or galvanized pipe for the soil stack that way it has more support and is less likely to sag (and add on additional costs).

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