Some advise to have just enough thickness of earth on the roof to maintain a green roof (approximately 6 inches / 15 cm), since this means less load on the structure. Increasing the amount of earth on the roof past this gives only modest increases in the benefits while increasing costs significantly. Despite being underground, drainage of water is still important. Therefore, earth shelters do not tend to have flat roofs. A flat roof is also less resistant to the weight of the earth. It is common for earth shelter designs to have arches and shallow domed roofs since this form resists vertical load well. One method uses finely meshed metal bent into the intended shape and welded to the supporting armature. Onto this mesh concrete is sprayed forming a roof. Terra-Dome (USA) is a company specializing in construction of earth-sheltered houses and sells a modular system of concrete domes intended to be covered by earth. Others advise the use of timber framed, gable roofs of pitch at least 1:12 to promote drainage. The roofs of Earthships tend to be mono-pitched, classically using vigas. Waterproofing On the outside of the concrete, a waterproofing system is applied. The most frequently used waterproofing system includes a layer of liquid asphalt onto which a heavy grade waterproof membrane is affixed, followed by a final liquid water sealant which may be sprayed on. It is very important to make sure that all of the seams are carefully sealed. It is very difficult to locate and repair leaks in the waterproofing system after the building is completed. Several layers are used for waterproofing in earth shelter construction. The first layer is meant to seal any cracks or pores in the structural materials, also working as an adhesive for the waterproof membrane. The membrane layer is often a thick flexible polyethylene sheeting called EPDM rubber. EPDM is the material usually used in a water garden, pond and swimming pool construction. This material also prevents roots from burrowing through the waterproofing. EPDM is very heavy to work with and can be chewed through by some common insects like fire ants. It is also made from petrochemicals, making it environmentally unsustainable. There are various cement-like coatings that can be used as waterproofing. The product is sprayed directly onto the unprotected surface. It dries and acts like a huge ceramic layer between the wall and earth. The problem with