1. Wood: Wood was the most widely used material for building roundhouses. Oak, ash, and hazel were commonly used for the structural framework, while wattle and daub were used for the walls.
2. Stone: Stone was used in regions where it was abundant, particularly in areas with rocky terrain. Roundhouses made of stone were usually drystone constructions, meaning they were built without mortar.
3. Earth and Clay: In areas where wood and stone were scarce, roundhouses were often constructed using earth and clay. The earth was mixed with straw or other organic materials to create a mixture called cob, which was then used to build the walls.
4. Thatch: Thatch was commonly used to cover the roofs of roundhouses. Materials such as straw, reeds, and rushes were bundled together and tied to a wooden framework to form a thick, insulating layer.
It's important to note that Celtic roundhouses were not standardized, and the construction techniques and materials varied across different regions and time periods.