Maltese Deffun Roof

The deffun roof is the traditional flat roof of Malta and Gozo. It spans rooms with slabs of local limestone and seals them with lime-based layers, the top one being deffun, a waterproof mortar of hydrated lime and crushed pottery. Roofs of this kind were the norm on the islands for hundreds of years, notably in buildings of the Knights of St John (1530-1798), and University of Malta researchers note that similar layered roofs are found in many neighbouring Mediterranean countries.

Build-up

From the bottom up:

1. Structural slabs of limestone resting on stone arches, or in later buildings on timber or iron beams.
2. Fill of limestone chippings mixed with stone dust and lime.
3. Deffun surface of lime mixed with crushed pottery. Water is sprinkled on it and it is beaten and compacted, by hand in the past, then covered with straw to slow evaporation and help it cure.

A slight fall carries rain quickly to the cistern that every traditional house had. The roof therefore doubled as the household's water catchment.

Performance

The deffun surface is porous. It absorbs moisture, including heavy dew on hot summer nights, and the evaporation of that water during the day cools the surface through latent heat. A University of Malta project, EO4HBCS (2020-2022), funded by the Malta Council for Science and Technology, compared traditional, "hybrid" and modern roofs using satellite data, drones and sensors. It found that traditional roofs offered the best balance between dispersing heat and insulating, and described them as the cooler type, although more research is needed. Very high resolution satellite imagery could also identify traditional roofs from their colour.

Maintenance and change

Deffun roofs need yearly maintenance after the summer heat and before the first September rains. In some recent interventions the deffun has been replaced by a concrete slab and screed on the original arches and fill, or covered with a bituminous membrane. The researchers are developing guidelines to protect remaining traditional roofs and exploring a modern version for new buildings, with possible benefits for energy use and the urban heat island.

Related Maltese terms

• torba - a covering layer, used with gravel on the corbelled huts called giren
• ċagħqija - a pebbly material sometimes used on girna roofs