Or set on hollow blocks
Charcoal may be used to keep your chicks warm. Burning rice hull or ipa, may also be used as alternative source of heat. Compost material is also a good idea for keeping your chicks warm and close to their natural brooding temperature. Just make sure that the chicks can't be burned :)
Doc Rey suggests that if you are using electric bulbs or gas brooders, have on standby, alternative means to brood. The time spent thinking, planning and preparing may mean that they start crowding and piling up one on top of the other. Wanting to get heat and pushing against one another, extols a lot of pressure on them.