The cheapest construction material in the wold is 'earth'.
As shown in the photo below, one can make hand-sized 'sun-dried bricks' by putting kneaded clay with fibers such as straw in a wooden bottomless frame, which is pulled up and off after a while, and drying it in the sun (photoed at New Gourna village).

However, in order to make an arch with stone or brick, expensive timber is required for centering and for a support system as shown in the photo below (repair work in the White Monastery in Asyut, using steel in the lower part).

Hassan Fathy came to perceive that the B.C.E. mud-brick warehouses of Ramesseum in Luxor (photo below) must have been built without centering and wooden support systems.

Barrel vaults can be made by applying mud-bricks onto slanting arches againsst a vertical wall continuously without centering.
Fathy found workers handing down such techniques through successive generations in the Aswan regions, and he called them to Cairo.

The photo below shows a market building (now unused) erected by this method based on his design in New Gourna. Since it is half demolished, the building method can be seen.

Moreover, earthen buildidngs are warmer in winter and cooler in summer. Fathy resolved to construct houses by this cheapest method 'for the poor' throughout his life.