Mud Roof

A mud roof, or earth roof, is a roof whose weatherproof layer is compacted earth, clay or a clay-straw mix laid over a timber structure. Most are flat or very low-slope and are found in dry climates, from the American Southwest and North Africa through the Middle East and Iran to Central Asia and the Himalaya. Domes and vaults built of unfired mud brick are a related family, used where structural timber is scarce.

Typical build-up

Details vary by region, but the sequence is similar almost everywhere:

1. Main beams span from wall to wall. In the Pueblo and Hispanic tradition of the American Southwest these are vigas; in Iran they are called tir.
2. Secondary members lie across them: smaller poles (latillas in the Southwest) or boards.
3. A mat layer of brush, reeds or cane matting stops the earth falling through.
4. Earth layers: one or more coats of mud, often mixed with straw, then a thicker layer of earth. A Southwest roof of about 1850 had about 8 cm (3 in) of adobe mud over the latillas and about 45 cm (18 in) of dry earth on top.
5. Fall and drainage: the top is shaped to a slight slope toward a spout (canal or canale in the Southwest) that throws water clear of the wall.

When wetted, clay particles swell and close up, so the surface becomes relatively watertight. In Iran a stone roller (ḡaltabān) is used to compact the mud layers.

Maintenance

Mud roofs work only with regular care. In Iran, a new layer of clay-straw plaster (kāh-gel) about 2-3 cm (0.8-1.2 in) thick is traditionally added each year, and the roof is rolled or patched after rain. In the Southwest, weeds were pulled and the earth re-sloped annually. Neglect leads quickly to leaks, erosion of parapets and rot in the beams.

Strengths and limits

• Insulation: thick earth roofs moderate heat and are valued in hot, dry climates; flat roofs are often used for sleeping in summer.
• Fire: depending on materials, they can be highly fire resistant.
• Climate limits: they suit dry climates without heavy snow loads. Prolonged rain softens and erodes them.
• Weight: a thick earth layer is very heavy and needs large beams and sound bearings at the wall tops.

Domes and vaults

Where wood is rare, as on the central Iranian plateau, builders roofed rooms with barrel vaults and domes of unfired mud brick laid without centering, then plastered them inside and out. A second flat roof was often built above them for protection and summer use. Wood-framed pitched roofs in Iran are mostly confined to the wetter, forested Caspian provinces.

Regional traditions

Earth roofs appear in Najdi architecture of central Arabia, in the Sahel of West Africa, in Hausa vaults and domes of northern Nigeria, in the Nubian vault, and in Iranian village building. Earth over timber was also used in colder climates: Plains earth lodges in North America and some Jomon pit dwellings in Japan carried heavy earth roofs over timber frames.

Modern use

Contemporary earth roofs may use steel or engineered timber framing with mesh and earth spread like cob, and builders sometimes add cement to help the earth dry evenly without cracking, which can reduce its permeability and adds the carbon cost of cement.