Biomass roofing is the use of plant materials to build roofs. People from around the world have always used whatever vegetation was locally available and abundant to build their roofs. This cultural and environmental diversity has led to a wide range of roofing materials and styles, from the simple and ephemeral to the more durable and complex.
The Different Types of Biomass Roofing
Although hundreds of different plants have been used to roof houses, these can be classified into two main types: thatch and wood tiles.
Thatch is one of the oldest forms of roofing, dating back thousands of years. It is found in almost every country, from savannah grasses in Africa to coconut palm fronds in the Caribbean to banana leaves in the Amazon. It was the predominant roofing material in Britain up until the 19th Century and thatched cottages remain a hallmark of the English Countryside.
All sorts of plants have been used for thatching in Britain: oats, reeds, broom, heather, bracken and various grasses. But today only three main thatching materials are used: water reed, wheat reed and long straw.
Water Reed is the most popular thatching material. Both water reed and wheat reed (actually a straw but cut with a binder and combed to give the appearance of reed) give a compact and even texture when applied to a roof. This is in contrast with long straw (wheat straw that has been threshed so that the ears and butts are mixed up together), which gives a shaggy, rounded appearance. The lifespan of thatch is around 30 to 50 years, although this varies widely depending on the skill of the thatcher, the pitch of the roof, the local climate conditions and the quality of the materials.
The technique for thatching is basically the same for all materials: first the thatch is fastened together in bundles about 25 inches in diameter. Each bundle is then laid down with the butt end facing outwards, secured together to the roof beams, and pegged in place with wooden rods. Successive layers are added on top of each other, working from the bottom of the roof up towards the top, with a final layer used to reinforce the ridgeline.
Thatch roofs can withstand high winds and heavy rains, provide good thermal insulation and are easy to repair. Thatch is light and needs only a simple support structure, and is flexible so can be used for any roof shape. On the downside, thatching is labour intensive and a certain level of skill is required. The materials can be expensive as reeds are increasingly imported from Europe to keep up with demand. Like all biomass materials, thatch is flammable which means that building restrictions may apply and home insurance can be high.
Wood Tiles: Shingles and Shakes
Wood tiles have been used for roofs since medieval times in Britain. They are traditionally made by hand-splitting logs into small wedge shaped pieces, but today most are manufactured by machine. There are two basic types: shingles, which are sawn, and shakes, which are split. Shakes are thicker and have a more rustic, rough look, whilst shingles are thinner and smoother. Both come in a variety of lengths and are made from the heartwood of unseasoned wood. Hardwood is best, with cedar being the most popular, although any straight-grained wood can be used. Split bamboo can also be used to create Spanish-style tiles, and are popular in some countries, but bamboo has the disadvantage of decaying fast in wet conditions unless chemically treated.
Wood tiles are laid from the bottom of the roof to the top, with each row overlapping the previous one. A cap is placed at the roof ridge. Typically tiles are nailed onto wood strips spaced a few inches apart between the roof beams, to allow air to circulate and prevent decay.
Wood tiles last between 25 – 50 years. Like thatch, they give good insulation and are flexible so can cover any roof shape. They are highly resistant to wind, heavy snow and hail, but must be regularly cleaned of vegetative debris. They are also flammable, and building regulations may prohibit their use in urban areas.
Is Biomass Roofing Sustainable?
The recognised need to use renewable resources has led to a revival of traditional, natural building methods, along with a growing market for biomass roofing. Thatch and wood tiles are not only aesthetically appealing, but are durable and biodegradable. But their sustainability value is diminished if the materials have been imported or produced and treated with chemicals. Biomass roofing is only a true sustainable solution if the materials are obtained from a local, renewable source, and are grown, harvested and manufactured in an environmentally sensitive way.