Roof Structure & Covering
Roofs in climates with excessive amounts of sun and rain are best thought of as broad brimmed hats - extending way beyond the supporting walls to provide shade and discharge of what can amount to excessive amounts of rainwater.
Conventional classroom buildings require roof trusses that span seven to nine meters between side walls and are typically clad in roof sheeting which requires purlins at approximately 90cm centres.
The pitch of a sloping roof needs to be greater than ten degrees for roof sheeting. Tiled roofs require more supporting timberwork and should be steeper at around 25 degrees in order to shed driving rain. Thatch should ideally be over 40 degrees.
Roof structure should be efficient, functional and beautiful. Considered roofing design can add a lot of aesthetic character to a space.
Roof structure usually consists of prefabricated welded steel or bolted timber trusses, spanning between columns built into loadbearing walls. However, this is not always the case and roofs may be supported via a reciprocal structure or composite roof cassettes. Home grown timber is rarely more than three to four meters in straight lengths so shorter lengths or timbers are scarf jointed or lapped and bound in metal straps. Light steel angles and tubes welded together provide a more durable, though a more expensive structural solution.
Corrugated metal sheeting has become the default material for low cost roof covering for school buildings
Metal sheet roofing needs to be galvanised or powder coated to prevent rusting. A galvanised or light coloured finish will reflect solar energy and substantially reduce heat gain, although metal roofs often result is substantial overheating issues. A more sustainable and climate-appropriate alternative is bituminous corrugated sheeting or Onduline, which is made from recycled cellulose fibre and has better thermal and acoustic properties than metal sheeting. Sheet materials need to be fixed through the ridges using rubber or plastic washers to seal around the fixing. Proprietary bent sheets provide waterproofing for hips and ridges.
Mzuzu Health Centre 10.7m lengths of shiny galvanised 'alu-zinc' roofing sheets help to reflect solar radiation
Thatch and shingles are a traditional roofing material across many parts of East Africa but the sources of supply are getting scarce and less sustainable.
The best thatch is made from reeds, though straw and palm fronds can also be used. Bamboo or wood can be split into shakes or shingles and laid in courses like roof tiles. Traditionally natural roofing required a very thick layer of vegetative material to ensure water resistance; contemporary use of thatch or shingles often incorporates a layer of waterproof membrane instead. As with all 'harvested' building materials, thatch and shingles must be sourced from a responsible supply chain. As long as the material is from a renewable source, then natural roofing will contribute towards carbon sequestration and can reduce embodied energy in construciton.
Fired Clay Tiles
Fired clay roof tiles provide an attractive alternative to metal sheet roofing. If available locally, clay tiles are arguably more sustainable in importd steel.
Clay tiles require roofing battens at approximately 30cm centres, spanning between rafters at around 60cm centres so a lot more timber is required to construct the roof. The smaller module means there are more joints that could leak but clay tiles have better acoustic and thermal properties than metal sheets, helping to limit the sound of rain and reduce heat transfer to rooms below. As with fired clay bricks, clay tiles are usually produced locally so they contain less embodied energy from transportation and inject money directly into the local economy. Care must be taken to ensure the firing process is undertaken responsibly and sustainably.
ECD&F Centres The fired clay tiles are a sustainable and locally sourced roof covering system
Ceilings help reduce the radiation of solar heat from the roof to the space below and can also reduce the acoustic impact of rainfall on the roof covering
In educational buildings the noise of rain on metal roof sheeting can severely disrupt classroom teaching. A layer of woven glass matting or bamboo screens can dampen down the sound and also provide a layer of insulation to reduce the heat transmission from the roof sheeting. A ceiling fixed below the purlin structure with a continuous airspace of 10-15cm will further reduce heat transmission particularly if it can be ventilated top and bottom so that moving air can dissipate the heat.