Chiltern Open Air Museum carried out a ground breaking thatching project in the early part of the 2005 season using medieval techniques to thatch an original late medieval barn in the Museum’s collection.

Arborfield Barn is a three bay cruck barn, believed to date from c1500 and originally located at Arborfield, just south of Reading, in Berkshire. It was re-erected at the Museum in 1980, the repair work being done by Peter McCurdy.

Although thatched professionally in 1981, we now know that the type of thatch and method of fixing was incorrect. This was revealed by research done by James Moir and John Letts, on behalf of English Heritage.

The roof had been in need of re-thatching for some time, so the Museum put together a project to re-thatch in an authentic manner. In addition, the aim of the project was to carry out the necessary repairs to the split oak wattle infill panels, to construct and fit replica medieval doors, and to create a lifelong learning centre with an interactive exhibition interpreting the medieval thatching techniques.

Historic thatching consultants John Letts and Keith Quantrill devised the thatching method based on research into existing medieval thatched roofs and their experience of traditional thatching methods. At the time of its original construction the barn would have been thatched using a two-step method:

Firstly, a base coat of poor quality material was tied to the roof structure, normally the rafters. The method of tying was to use ‘greenwood’ ties – e.g.. brambles; old man’s beard; thin hazel; osiers; straw or hay rope and other similar materials. The tying would leave horizontal wooden sways exposed on the surface, so the base coat would need to be covered with a ‘weathering’ coat.

The base coat was not necessarily straw and many other materials were used, such as heather, bracken and gorse. The base coat would nominally be 8” – 10” thick. As the fixing is to the rafters, the battens are only there for support and do not need to be as strong as modern battens.

Secondly, the weathering coat would be added, made of good quality long straw 8” – 10” thick. This would be sparred to the base coat using spars and horizontal sways. The weather coat would be maintained, and other coats added to it. Providing this was done regularly, it would never be necessary to renew or even repair the base coat.

In order to achieve maximum authenticity, a special crop of medieval species wheat had been grown by the Davis family at Prestwood in Buckinghamshire and stored for several years inside the barn. It consists of more than thirty species of wheat and rye, known as a ‘maislin’ mix as this was the farming technique used at the time to ensure some crops survived for harvest.

To show the possible variety of base coat materials, one side of the roof was thatched with different materials: heather, gorse, bracken, water reed, broom, birch twigs, rye straw, rushes, and marsh grass. At the barn’s original location, all of these materials would have been readily available. The rest of the roof has straw as the base coat. One side of one bay has woven reed mats laid over the battens. This demonstrates a method known as ‘fleaking’, used to present a tidy appearance from the inside. As the mats would provide the necessary support for the thatch, there would be no need to use battens.

Although we knew what materials were used for natural ties and base coats, nothing was known about the appropriate time of year to cut the materials to ensure they were fit for use in thatching. The bracken was too dry to take a spar, for example, and many of the other materials such as the blackberry stems were cut too early and the high sap content left the material liable to break easily. Some experimentation with tying methods, including twisting the materials and soaking them, produced excellent results.

Most of the thatching was done by Roger Evans, a professional thatcher based at Northampton, who also teaches thatching at Knuston Hall to NVQ 3 level. He was assisted by Museum staff and volunteers, and by his apprentice Nick Surridge.

It is hoped that this project can be taken as an example for future restoration work on thatched buildings. Where every attempt is made to ensure accuracy in constructional details, it is now possible to use the authentic thatching materials and methods with similar costs and timescales to the use of modern thatching techniques. The work took much less time than had originally been anticipated and was completed in around 9 weeks from March to May 2005.

Apart from a recent similar project on reconstructed buildings at West Stow Anglo-Saxon Village, it is thought that no one else has thatched a roof this way since at least the 18th century. The process of experimenting with these techniques will provide the building conservation field with valuable information about the practice of early thatching work in this country.

The renovation of Arborfield Barn was generously supported by funding from Lafarge Aggregates Ltd through Groundwork Hertfordshire and Brett Environment Trust Ltd as part of the voluntary Landfill Tax Credit Scheme.

 Click here to see a full photographic account of the project.