Below we will briefly explain the most important subject areas of the „Fachregel für Dachdeckung mit Reet” (Rules for Reed Thatching) and of the data sheet on reed. A complete version of the Rules for Reed Thatching and of the data sheet on reed can be ordered from the Rudolf Müller publishing company (www.rudolf-mueller.de) (ISBN 3-481-02033-3).
Different types of construction:
A thatched roof can be tied, sewn or screw-fixed.
Different types of construction can result from regional distinctions.
In the case of a tied roof the upper layer of the roof is secured to the lathings with steel wire and steel rods are placed on top of the bound reed bundles. In the case of a sewn roof the upper layer is secured to the lathing with steel wire but without the use of steel rods. And, finally, with a screw-fixed thatched roof the upper layer is fixed with steel wire which in turn is screwed to the lathings (see section 1.1 and section 3.2 Rules for Reed Thatching).
Requirements of Thatching Reed:
The eaves projection of a thatched roof should be at least 0.5 m all-round. The precipitation moisture must not penetrate deeper than one third of the thickness of the roof above the bond layer. The life span of a thatched roof depends on the roof pitch. As a rule, the roof should have a slope of at least 45° whereas smaller areas (e.g. the area around the dormers) can, as an exception, have a minimum slope of 40°.
In such a case, however, longer reed should be used and a shorter lifespan can be expected.
If your house features a converted attic, a heat-insulation has to be installed below the thatched roof (see section 1.2 and 1.3 Rule of Reed Thatching).
Material Requirements for Thatching:
The reed itself has to comply with the standards of the data sheet on reed of the body of rules and regulations applying to thatching.
In general the reed should be 1.4 – 2 m long, 3 – 9 mm thick and straightly grown. At joints and terminations also other lengths can be employed.
Up to an interspace of one meter between the rafters the substructure of a thatched roof can be built using laths with a cross section of 40/60 mm. All fixing material (screws and wire) should be made of rustless materials (e.g. steel wire, plastic coated wire or copper wire) with a minimum thickness of 1 mm (see section 2 Rules for Reed Thatching).
Construction of a Thatched Roof:
With a roof pitch of up to 60° the interspace between the laths should be a maximum of 0.35 m. If the roof pitch is more than 60° the distance between the laths should not fall below 0.3 m.
The thatched roof should form an even surface from the eaves to the ridge. The individual layers are to be applied horizontally. The thickness of a thatched roof, measured vertically to the slope of the roof, should be 0.3 m. The sewn thatched roof should have a maximal stitch width of 0.25m, the screw-fixed roof should not exceed a distance of 0.2 m between the screws (see section 3.2 Rules for Reed Thatching)
The visible front end of the eaves should be at least 30 cm thick. The thickness of the roof at the first lath should be between 0.35 and 0.45 m (measured vertically). The eaves projection (measured from the masonry) should measure between 0.15 m and 0.30 m. The projection at the verge, however, should only lie between 0.15 m and 0.25 m.
Thatched houses with a reed ridge are provided with a so called „Kehrband”. The butt ends of the ridge bundles – contrary to the usual orientation of the reed in the main roof area – point at the roof ridge. Measured at the penultimate lath below the ridge, the thickness should not fall below 0.25 m (in relation to the batten level).
Thatched houses with a heather ridge need, depending on the length of the rafters, a length of flange of up to 1 m on both sides. From the roof crest the heather ridge narrows down to approx. 0.1 m at its lower edge. Traditionally the heather is fixed using wooden dowels (0.3-06 m in length and with a diameter of 15-20 mm). Per meter of heather ridge 100 dowels should be used. It is possible to cover the ridge with e.g. plastic net. The use of mesh wire is not allowed in the combination with lightning protection.
Thatched roofs with a sod ridge need a rounded under layer of e.g. bitumen sheeting. The sod ridge has to be laid facing the prevailing weather and is fixed with hard wood dowels.
Valleys in a thatched roof should be 1.5 times thicker than the main area of the roof. The pitch of the valley rafters should not fall below 40°. If the roofing in the valley comes in contact with a differently covered roof area the valley has to be overlapped by at least 0.15 m, measured vertically to the valley line.
In a thatched roof flashings can be either concealed or overlying. When using the concealed version the soaker flashing lies inside the reed layer and you do not necessarily have to follow the „Fachregeln für Metallarbeit im Dachdeckerhandwerk” („Rules for Metal Work in the Roofing Trade”). If the overlying version is used, the flashing plate lies on top of the roofing and has to be fitted in accordance with the „Rules for Metal Work in the Roofing Trade”.
The connection of the main roof area and the dormers should be rounded. The transition between the dormer and the main roof area should measure 0.6 m – that is why a sufficient distance to hips, valleys, verges and other dormers is necessary.
The distance between the lower edge of the dormer and the roof battens should measure 10 cm less than the thickness of the reed layer.
The design of the side gutters of dormers should help to lead off rain water as far as possible and in direction of the main area of the roof (see section 4 Rule for Reed Thatching).
Care and Maintenance:
In order to durably preserve the functions of a roof, periodic maintenance has to be performed. That is how more serious damages on the building can be avoided and the conditions precedent to the liability of the insurer can be fulfilled.
This falls into the area of responsibility of the house owner. Comprehensive maintenance checks on the roof covering, the flashings, terminations, roof permeability, roof components, safety devices (steps that allow to safely access the roof) and on the protection against the wind uplift force.
If damages in the roof covering have to be repaired the relevant areas are re-thatched and new material of similar length is applied and dressed.
Data Sheet on Reed
Requirements of Reed
The reed should be free from grass, old and bent canes and other impurities. The stalks should be strong and flexible at the same time and feature a colour between yellow and brown.
A bundle of reed should have a diameter of at least 0.6 m. Short reed must not be longer than 1.5 m (and have a stalk diameter smaller than 6 mm). Medium reed should have a length of up to approx. 1.8 m (and a stalk diameter of 3-9 mm) and long reed should have a length of up to 2.3 m (and a stalk diameter of 6-12 mm).
The reed bundles should not contain mould and have to be processed and packaged under dry conditions. Superficial humidity does not harm the reed, but the inside of the bundle has to remain dry.