A radon sump is a void formed in the granular fill beneath a building's floor slab, connected by a pipe to the outside. If radon levels prove high, a fan is fitted to the pipe to draw soil gas from under the slab and vent it above roof level, preventing it entering the house.
The sump itself is a simple void, often a proprietary plastic unit or a stone-filled space, formed in the hardcore below the floor. A pipe runs from it to an accessible point outside or in the roof space. In standby form the pipe is capped; if radon testing shows a problem, a small extract fan is connected and the system becomes active.
Active sump systems are the most common and effective radon remediation method for houses with ground floors on the ground. They work by creating a slight negative pressure beneath the slab so soil gas is drawn away rather than being pulled into the building.