Roofing: MATERIALS ENCYCLOPEDIA
Applications for this system
Roof sheathing for roofs with a minimum pitch of 10:12
Long-stemmed reeds or straw
Twine or wire to fasten thatch to strapping
Ratings Chart for Thatched Roofing
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- RAINWATER COLLECTION CAPABILITY
- FUTURE DEVELOPMENT
thatched roof System
While it may seem strange for modern builders to think that a bunch of dried grass stems can provide a thoroughly water-resistant and long-lasting roof, thatch roofs have a long and successful history across a wide range of climatic zones. Thatching techniques have been developed worldwide, adapting the basic principle to suit available materials and to work in specific climates. Modern thatched roofs are installed in almost every region of the world, though in relatively small numbers.
The system of thatching used in many wet and/or cold climates involves fastening bundles of long, thick reeds or straw to the roof strapping in successive courses, each overlapping the preceding course. The thatch is laid at a thickness (which can range from 8–20 cm / 3–8 inch) that prevents water from working its way through the layers. Thatched roofs have very steep pitches to aid in this drainage.
Traditional thatch was hand-tied to the roof strapping using twine or rope. Modern thatchers often use screws and wire to provide attachment. Regardless of regional variations in material and technique, the thatch is held in place by securing a horizontal member across the thatch and tying that member back to the strapping through the thatch. The next course of thatch then covers the tie point as the roof is built upward. At the edges of the roof, the thatch is laid at a slight angle to encourage runoff to leave the edge of the roof and to provide a consistent appearance.
Thatching on flat sections of roof is relatively straightforward, but the same cannot be said for ridge, hip and valley sections. These areas take considerable knowledge and experience to execute in a weather-tight and long-lasting manner.
Many modern installations use a fire-resistant (often fiberglass) membrane under the roof strapping to prevent the spread of a fire from inside the building to the roof. Eavestroughing is not used with thatched roofs, making them incompatible with rainwater harvesting.
Environmental Impact Rating
Manufacturing — Negligible to Low
Thatch requires little to no processing other than cutting, cleaning and bundling. These processes are done on a small scale and with minimal machinery and fossil fuel input. There are no toxic by-products created.
At the most intensive, a thatch roof will use a small amount of metal wire and screws and a layer of fiberglass matting that has high energy input and some toxic by-products. At the least intensive, round wood strapping and natural fiber twine is used.
Transportation — Negligible to High
Sample building uses 2,250 kg of thatch:
3.4 MJ per km by 15 ton truck
0.9 MJ per km by 35 ton truck
0.36 MJ per km by ocean freight
Some thatch projects in North America are completed using thatch imported from Europe, because there are no commercial suppliers on this continent. This adds high transportation impacts to an otherwise low-impact roof. Many thatch roofs are made with locally, manually harvested material, keeping impacts very low.
Installation — Negligible
Thatch is largely installed without the use of power tools and does not create any problematic waste or by-products.
Compostable — All reed or straw thatching, natural fiber twine.
Recyclable — Polypropylene twine, metal wire.
Landfill — Fiberglass matt offcuts, if used. Quantities can be negligible to low.
Chart of Embodied energy & carbon
Energy Efficiency: n/a
Historically, thatched buildings relied on the fairly significant amount of air trapped in the thatch to insulate the roof of the building. However, thatch allows for a lot of air infiltration and would not be considered adequate insulation or airtight enough to meet codes or modern comfort levels on its own. Modern buildings with thatched roofs rely on an insulation layer independent from the roof sheathing.
A thatch roof can have some beneficial effects by reducing summertime warming of the attic space quite significantly. Thatch roofs will also eliminate the issue of condensation on the back side of the roof sheathing as the material will not have the low surface temperatures of more dense sheathing and is able to adsorb and absorb moisture without condensation.
Material costs: very low to high
Suitable material for thatching can be harvested locally in many parts of North America for no cost, but material is often harvested and processed far from the building site in low quantities as a “boutique” material with high costs.
Labour Input: High
Thatch roofing is unique in that, for most North American builders, harvesting the material is likely to be a necessary preliminary step. While suitable materials are widely available, harvesting and preparing them can be a very labor-intensive process, easily requiring more hours than the installation itself. In areas of the world where thatch material is harvested commercially and available for delivery to a job site, the labor input is obviously much lower.
Thatching is the most labor-intensive form of roofing. An experienced thatch crew can move at a speed that approaches that of an experienced cedar shingle crew. Beginners will move a great deal slower, as the process of laying thatch is very particular and must be done accurately and correctly.
Working at heights to install roofing has inherent dangers. Proper setup and safety precautions should always be taken when working on a roof.
Skill level required for homeowners: moderate to high
Thatching requires a good deal of skill. In European countries, it takes many years of apprenticeship and experience to obtain the title of “Master Thatcher.” Beginners are advised to start with a very small roof, such as a small shed, and to keep roof shapes to simple gables or sheds. Hips and valleys add a lot of complexity to the thatching process, and should be left to those with plenty of experience.
Sourcing & availability: Difficult
Both the material and the expertise to build a thatch roof can be difficult to source in North America. A few master thatchers practice in the U.S. and they tend to import their thatch material from Europe.
A homeowner wishing to attempt a thatch roof may have to resort to harvesting thatch material locally and learn from books or by taking workshops with experienced thatchers and bringing the skill back home.
Durability: moderate to High
Thatch roofs are surprisingly durable. In northern European climates, they can last for forty to eighty years. Depending on the style of ridge cap used, the ridge may need repair or replacing every ten to twenty years. A thatched roof at the end of its lifespan is not typically replaced; rather new thatch is built over top of the existing thatch.
No building codes in North America address the use of thatch roofs. Proposing a thatch roof will likely require a fair bit of documentation and persuasion, as there are few examples of thatched roofs on which a code official can base an assessment. The historical and modern use of thatch in Europe means that a lot of code-related testing and documentation exists to support it. A building department may be willing to consider a thatch roof with the right amount of documentation and some assurance that the installation is being done properly. The few master thatchers working in North America have been able to have their work approved, as have a small number of owner-builders.
RAINWATER COLLECTION CAPABILITY: LOW TO MODERATE
It is possible to filter rain water from a thatch roof, but not without issues. Thatch roofs can be difficult to mount with gutters suitable for collecting rain water. Even with functioning gutters, the thatch is capable of holding onto a considerable amount of biological material that can come out in the rain water, requiring additional filtration.
Resources for further research
Billett, Michael. The Complete Guide to Living with Thatch. London: Robert Hale, 2003. Print.
Fearn, Jacqueline. Thatch and Thatching. Aylesbury, UK: Shire Publications, 1976. Print.
Sanders, Marjorie, and Roger Angold. Thatches and Thatching: A Handbook for Owners, Thatchers and Conservators. Ramsbury, UK: Crowood, 2012. Print.
There is no reason for thatch to be disregarded in North America, as it is a viable, durable roofing option that is remarkably environmentally friendly. As the costs of conventional roofing materials rise with the price of fuel to make them, thatch will start to look better and better. The machinery required to mechanically harvest and bundle thatch is not complicated or expensive, and viable thatch material grows in many places on the continent. There will always be limitations to the use of thatch roofing in urban areas, as fire safety concerns would limit the density of thatched roofs. But there are many locations where thatched roofs are feasible, appropriate and the best possible environmental choice. It will take many dedicated homeowners willing to push the boundaries and create a market in which thatch may start to take the kind of foothold where it creates a viable niche market, similar to cedar shingles.
Tips for a successful Thatched roof
1. Thatching methods vary widely with the type of thatch material being used and the tradition of thatching used in the region. Ensure that the reed or straw being used is compatible with the climate and the installation technique.
2. Be sure you are able to obtain the material and expertise required to create a thatched roof. It is a rare type of roofing in North America, and must be well researched before deciding to proceed.
3. Plans for a thatched roof must be properly detailed before construction. The uncommon thickness of the roofing, the steep pitch required and the particular details at hips and valleys must be incorporated into the drawings to ensure the roof will work when built.