METAL AND FIBRE CEMENT SHEET ROOFING
Metal sheet and fibre cement sheets are reliable and economic as a roof cladding as well as for wall cladding (see Cladding). The minimum pitch for sheet roofing is generally considered around 7 degrees as, the lower the pitch, the slower the run-off and risks increase of leaking fixings as well as wind driven rain between sheet joints.
In practice, the latter effect can be eliminated by use of sealing strips between whole sheets running from ridge to eaves and the former by use of modern (neoprene washered) fixings that are easily inspected and replaced at hut scale. Note that the examples in Appendices one and two do not cover huts with flat or low pitch or mono-pitch roofs. It is hoped this omission will be remedied in the future.
TIMBER BOARDING AND SHINGLES
Timber boards or shingles can be used as a roof covering but there are risks attached that need to be understood and mitigated against. As with timber cladding, the main risk is decay through prolonged wetting so that choice of timber species and any treatment is particularly important. A roof will also be subject to more pronounced UV degradation and possibly stronger uplift wind forces.
The best design will be one that reduces wetting and increases speeds of drying so that steeper pitches will be preferred. All timber will need to be pressure treated or of durable species such as oak.
It may be acceptable at hut scale to apply traditional Scandinavian timber roof coatings such as “Stockholm tar” or “pine tar”. Contemporary modified timbers such as Accoya are used as roofing where budget permits and carry long guarantees.
Timber can be vertically orientated board on board with big overlaps - just like the wall cladding. Or horizontally overlapped just like weather-boarding on a wall.
Timber roofing may present a fire hazard where a solid fuel chimney is in use. (See Stoves, flues and chimneys)
Roofing membranes of a wide variety of types are available ranging from traditional tar felt with and without adhered grits to single ply membranes. The common PVC membranes are losing ground to synthetic rubber (eg EPDM), which has less environmental issues. Built up felt tends to be mechanically fixed but can be torched on with the more expensive systems. Single ply membranes can be mechanically fixed but penetration of the membrane can cause leaks and glueing is often preferable. Always refer to the manufacturer's instructions.
On flat roofs ballasting (e.g. with round beach or river stones) is possible with the stones helping to hold the membrane to the deck and protecting it from UV and frost damage. However the added load will have to be accounted for by an engineer. Unballasted EPDM sheeting has the advantage of being very light and at hut scale, the whole roof, or at least one pitch, may be covered by just one sheet radically reducing the number of joints to make. Whilst DIY is possible with EPDM roofs, it is good practice to have them installed by experienced contractors.
SOIL AND NATURAL MATERIALS
So called ‘green’ roofs, using turf or sedums are in fact largely dependent for their waterproofing on the underlay and their construction requires specialist knowledge if they are to give long and effective life.
Many proprietary underlay systems are available and if being considered by the hut designer/builder you should consult one of the many detailed guides. You can buy a number of proprietary systems with all the components supplied. If manufacturer’s instructions are closely followed there is no reason why huts cannot utilise them.
EPDM’s at low and flat pitches (see above) can be simply ballasted. If you gain sufficient understanding there is no reason not to pursue a low cost DIY approach too.
However, it is very important to understand, that many natural materials are very heavy, especially when saturated and weight in the roof has particular structural implications which cannot be ignored. For this reason if you wish to incorporate an organic roof type you must engage the services of a structural engineer to cover that element of your hut. Also the low and mono pitch roofs most suitable for green roofs are not currently covered by the two models in this guide.
SLATES AND TILES
Other than lightweight felt/synthetic fabric tiles, slates and masonry tiles come into the category of heavy weight roof coverings that require the involvement of a structural engineer to ensure that the roof and load transmitting structure is adequate.
Note carefully that the structurally certified models in this Guide do not include allowance for loads imposed by such roof coverings.
Rain can be allowed to simply fall off the edge of the roof and onto the ground from where it will resume its natural course for the site - though it is advisable to have a good roof overhang to avoid water blowing back onto wall cladding. If this approach is taken, it is also advised to provide a linear, clean stone drip area in the ground immediately below the roof edge to prevent mud splash and eventual soil erosion.
Vegetated (“green”) roofs absorb, evaporate and attenuate the water flows in a similar way to ground vegetation so can help to reduce the need for artificial systems.
While a minimal approach to rainwater run-off may be adequate for a single hut in a rural setting, a group of huts could collectively create an intensity of run off that merits a drainage system.
Rainwater collected in a gutter can be lead through a spout to a downpipe, which can be discharged onto the ground. At this point provision should be allowed for the water to drain away without excessive ponding or erosion. Experience will inform the hut owner whether there is a problem and he/she will be able to take action by the introduction of a very simple gravel or stone filled trench.
There is an opportunity to collect the rainwater and use it for other purposes. See Water.
Small Green Roofs by Nigel Dunnett, Dusty Gedge, John Little and Edmund G Snodgrass. Timber press 2011 ISBN -13: 978-1-60469-059-0