Do you know about Thatch?
Author: Lulu Yang
Thatch is a common roofing material in many traditional Lesotho houses. Used as a building material, thatch is functional, economic, biodegradable, renewable, locally available, and requires minimal to zero artificial energy input in production. Its sustainable properties and characteristics provide massive potential for expanded building material usage.
Although it is barely recognized by construction experts, thatch is the one of the most commonly used roof covers in the world, particularly in the past. It is easily available, almost any vegetable material, from the bark of trees to finely-tapering water reeds, can be used. Amongst many, grasses, reeds and palms are the most common.
Thatch can come from 3 different sources:
- naturally occurring indigenous vegetation
- as a byproduct of food or cash-crop agriculture
- through the cultivation of a plant grown specifically for thatching
Water reed is most durable, but cereal straw (mainly wheat, but also rye, barley and rice) is more widely available. The less artificial fertilizer is used, the better it is.
- Performance properties
Traditional types of thatch have short durability and performance, but in certain regions (North West Europe, Southern Africa, Japan) skilled workmanship produces good quality functional roofing, with life expectancies between 25 and 70 years.
The main drawback is combustibility, but this is significantly reduced through good quality workmanship,common-sense precautions and fireproofing. Thatch is also susceptible to biological decay and weathering.
Thatched roof fire protection is important. There are three kinds of materials that can be used to fireproof thatch: thatch batts, aluminum barrier foil, and fire retardant spray.
Thatch batts are fire-resistant slabs installed to the underside of a thatched roof to provide both fire resistance and thermal/acoustic insulation. Used alone, they slow down the progress of a fire in order to give occupants more time to get out. Used in conjunction with aluminum barrier foil, they provide total fireproof protection.
Aluminum barrier foil acts as a reflective insulator against excessive heat. It is essentially heavy gauge aluminum foil that conducts neither heat nor electricity. It is applied to a thatched roof in the same way felt is applied to a conventional roof.
Lastly is fire retardant spray. This spray, when applied to the exterior, provides both fire protection and an additional layer of water resistance. Fire Retardant spray is non-toxic, biodegradable, and can last for as long as ten years. It can also be used on the interior of the structure to protect the underside of the roof.
Thatch material properties and characteristics
|Excellent thermal and sound insulation
|Good, depends on material and workmanship
|Special training and experience
|Locally made thatching tools
|Resistance to rain
|Medium to good
|Resistance to insects
|All zones where the material is available
|Highly sustainable, low environmental impact
- Techniques (roof)
• Almost any shape of the roof with a minimum pitch of 45° can be thatched. Thatch will mould itself to any curve except a convex-shaped roof.
• Pole timbers and split battens may be used, and simple configurations work best, which means valleys and other changes of roof pitch are not recommended.
• The structure should be capable of supporting up to 40 kg/m2, which is the weight of the heaviest material – reed.
• A tilting board, 35 mm thicker than subsequent battens, fixed along all the eaves and barges at eave level, is essential to force the first course into tension, making the rest of the thatch more tightly compacted.
- Accessibility & cost:
Depending on the location, thatch can be collected from natural crops, or can be harvested. The source of the material has a direct impact on the quality of the material and labor cost, hence, it needs to be carefully analyzed.
The usage of thatch can be expanded beyond just the traditional roof. Think of thatch as a material that can be utilized and transformed into different forms and functions, its sustainable properties can have expanded influences.
Recent studies and projects have pioneered this school of thought. Some examples are:
- Thatch used as a cladding system
- Case study: University of East Anglia Enterprise Center
In this project, thatch is used as a rain screen cladding panel. The material is developed from local straw for external facade cassettes applied by local thatchers. Varieties include Foster Special, Maris Huntsman, and Yeoman Wheat.
Other examples are:
- Modern seaweed thatch panels
New Addition for Wadden Sea visitor center in Denmark
- Thatch used as roof
- Case study: The tent by a21 studio shades guests from Vietnamese heat
Although thatch is used in a traditional function as a roof, this project is one of the representations of the reimagination of utilizing thatch to create contemporary architecture.
“Commissioned by a hot spring and mineral resort, Vietnamese practice a21studio has completed a small spa perched upon the incline of a hill near Nha Trang city. Oriented to face an adjacent river, ‘the tent’ offers a place for relaxation, isolated from the site’s other, more physical activities.”
- Thatch as part of a material
- Case study: Paks Moabi’s house in Lesotho
Thatch as a material that can also be used as a part or mixture to create other materials. This case study illustrates the possibility of using thatch as insulation.
Deep-rooted in the Lesotho context, Paks as a “barefoot architect” is building an energy-efficient house by utilizing all available indigenous materials. Thatch is one of the main insulation materials that has been applied in the project. Not only is it readily available and economic, it also provides sound isolation properties. Moreover, the naturistic aesthetic matches perfectly with the surrounding environment.
Readily available, high sound insulation, water, and UV proof made thatch a desirable economically sustainable material. The potential of using thatch beyond roofing is endless. With more creativity, testing, and practicing, thatch can be one of the gems for architects and designers to innovate and create.
Author of article: Lulu Yang
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