What is a closed thatched roof?
A closed thatched roof (Dutch: schroefdak, literally “screw roof”) is a thatched roof in which the reed is laid without a cavity onto solid roof boarding or onto (insulated) roof panels. The Dutch name comes from the fixing method: the reed is held down with wire or fixings that are screwed into the solid substrate, rather than tied with binding wire to open battens as on a traditional open thatched roof.
On an open thatched roof the reed rests on battens and is visible from below, inside the roof space. On a closed thatched roof there is a solid layer beneath the reed, which makes insulating and finishing the inside easier.
Application
Closed thatched roofs are widely used on new-build houses with thatched roofs, because a closed substructure makes it easier to meet insulation and fire-safety requirements.
Advantages:
- the insulation sits in or under the roof boarding or roof panel, and the inside can be finished directly;
- no reed dust or debris falling down inside;
- the solid substrate slows fire from breaking through to the inside.
Points to watch:
- Airtightness is crucial. If warm, moist indoor air can get into the construction through gaps, it can condense against the cold underside of the reed, causing the reed to rot. Design the airtight layer in advance and finish joints and penetrations carefully.
- The reed only dries outwards. Make sure the roof pitch is steep enough and the workmanship is good, so the reed can dry out quickly.
- Follow the trade guidelines. The Dutch thatchers’ federation (Vakfederatie Rietdekkers) has drawn up quality requirements and installation guidelines for the closed thatched roof and its substructure; ask the thatcher whether these are followed.
- Insurance: check with the insurer in advance; the premium for thatched roofs often depends on the construction and fire-protection measures.
Related Terms
- Thatching reed
- Thatching
- Roof sheathing
- Airtightness
- Airtight construction
- Condensation
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