According to the US EPA, many of the features, which make a house radon-resistant, are already used by builders around the country. First, a layer of gas-permeable material (such as gravel) is placed where the house’s slab will be poured. This layer is then covered with plastic sheeting. The concrete slab is poured to minimize cracking, and all floor assemblies in contact with the soil are sealed. Any other places where radon could enter the home are also sealed. The plastic sheeting and sealant block radon’s main entry routes in the house. A length of perforated pipe is installed horizontally beneath the plastic sheeting prior to pouring the slab. This is connected to a vertical, unperforated pipe that extends through the slab and through all floors of the house to vent above the roof. This pipe removes radon from the soil and vents it safely above the dwelling. The gravel beneath the foundation makes it easier for the ventilation system to remove radon, even from the far corners of the foundation. Electrical junction boxes are installed during construction in case a fan is needed to achieve further radon reductions. Normally, suction on the pipe is provided by natural pressure differences within the house. A fan is needed only if the pressure differences cannot lower radon concentrations to acceptable levels.