Types of Thatch
There are three commonly used thatching materials:
- Water Reed
- Long Straw
- Combed Wheat Reed
- Others include Flax, Heather, Broom, Sods, Marram Grass etc.
Water Reed (Phragmites Australis) is the most durable thatching material.
Long Straw is a winter wheat straw that has not been combed (its name refers to a style of thatching rather than a material achieved by any singular process although generally for the last 90 years long straw thatching has used the processed straw from the threshing drum). It has the shortest life expectancy of the three commonly used materials.
Combed Wheat Reed is winter wheat straw which has had the leaf removed and is laid in a similar way to water reed. With modem farming methods tall strong straw has become less readily available.
Water reed, which is the most durable thatch, has been known to last up to 50 years. Maintenance will include re-ridging every 10 to 15 years and possible patch of the main coat and redressing works.
Combed wheat reed can have a life expectancy of approximately 20 to 30 years. Long straw will last from 15 to 25 years. Both these materials will require re-ridging at 10 to 15 year intervals.
These life expectancy figures can drop noticeably the further west the thatched property is situated. This appears to be due to climatic conditions. The warm, high humidity, clean air conditions experienced in the West Country are ideal for the microbes that begin the decomposition process.
Overhanging trees and bushes can also reduce the life expectancy of thatch because in many cases they will reduce the movement of air and light and can help to promote the growth of moss due to the damper conditions and falling debris.
Diagram 1: The pitch of the roof will relate directly to the pitch of the thatch and equally the thickness of the thatch will influence the pitch of the thatch. Thus, a 457mm coat of thatch will lie at a much slacker pitch than a 300mm coat and therefore a thicker coat will wear more quickly.
The thinner the thatch, the steeper the pitch, however there must be an adequate thickness of thatch over the fixings, thus a 100mm coat of thatch is steeper than a 300mm coat, but because the exposed stem length is longer (and therefore wears more quickly) and because there will be very little thatch over the fixings, the thatch will not last as long as a 300mm coat.
An optimum thickness for maximum longevity would be between 228mm and 381mm for water reed and 228mm and 300mm for combed wheat reed and long straw. Therefore, the point to remember is that there are an almost infinite number of specifications depending on pitch of roof and length of the thatching material.
Fixings (For All Types of Roof)
Hazel or steel runners can be applied and secured by steel thatching nails, fixing wires, twine or spars. The method will depend on the roof in question and the material used, however water reed on a new roof is usually fixed with thatching nails or stainless steel wires attached to rust-proof screws.
It is advisable to set the pitch at about 50 degrees. This is not due to the weight of the material which is in fact not over heavy, but rather to facilitate efficient drainage. Dormer roofs and eaves window-roofs should be at least at a 45 degree pitch, if possible.
Weight of Thatch
When calculating for a roof construction a weight of approximately 34kg/m² should be assumed for a typical single coat of thatch.