A particular factor that strongly influences the cost of an earth shelter is the amount of earth that covers it. The more earth covering the structure, the greater the expense is needed in having a structure capable of withstanding the load (see also: Roof). Another important cost factor that tends to be unique to earth shelters is site excavation and backfilling. The amount of waterproofing is also more costly. On the other hand, earth shelters should have lower maintenance costs since they are mostly covered with little exposed exterior.