Thatch Maintenance and Decay

A thatched roof wears differently from most other coverings. Rather than failing at joints, the whole surface slowly erodes as the exposed ends of the straw or reed decay and are washed away. The roof stays watertight as long as there is enough thickness of sound material over the fixings. Maintenance therefore follows a predictable cycle of ridge work, patching and, eventually, a new top coat or full rethatch.

The maintenance cycle

• Ridges. The ridge is the most exposed and most worked part of a thatched roof. UK thatching advice commonly cites ridge replacement roughly every 10 to 15 years, regardless of the main coat's condition.
• Coatwork. The main body of thatch lasts much longer, but its life varies with material (water reed, combed wheat reed, long straw, sedge and others), the skill of the thatcher, roof pitch, local climate, shading and how well the roof can dry. Repairs become poor value once the surrounding material has little life left.
• Rethatching. Traditionally, many roofs are re-coated by fixing a new layer over sound base coats; in other cases the roof is stripped back. Heritage authorities may require that the original material type and local style be retained on listed buildings.

Signs of decay

• Wire netting standing proud and fixings (spars, liggers, crooks) exposed on the ridge or coat.
• Lines, hollows and dips across the coatwork.
• Dark, wet-looking areas at the eaves that may indicate water penetration.
• Cracked or detached flashings at chimneys.
• Heavy moss and lichen, which tend to accompany well-decayed roofs and keep the thatch wet.
• Bird and rodent damage, with holes and loose material.

Care

Thatchers advise keeping the roof able to dry by cutting back trees and vegetation, avoiding ladders and foot traffic on the thatch, and wiring ridges or whole roofs against vermin where appropriate. Isolated bird damage can be patched in matching material. Fire safety is a specific concern: chimney maintenance, spark arrestors, fire-retardant barriers and suitable insurance are widely recommended. Thatch is used in many regions, including the UK, the Netherlands, Denmark, Japan and southern Africa, each with its own materials and conventions.