Does an old thatched roof always have to be stripped to the rafters?

No — revetment exists precisely to avoid that. The weathered upper layer is replaced with fresh straw or reed while the sound thatch underneath stays in place, and the strip-back usually removes only 300-400 mm from each slope.

How can I tell whether the thatch underneath is still sound?

By the depth of the decay. If the top 300-400 mm comes away and the material below is firm, the roof is a candidate for revetment; if the decay continues below that depth, the job moves towards full reroofing.

Why has the ridge worn out when the slopes still look fine?

Because the ridge is a wearing part. It is renewed every 8-12 years on reed and every 5-8 on straw, while the covering itself lasts 40-60 years on reed or 15-25 years on straw.

Can the new thatch be laid over bitumen felt to keep the rain out?

No. Thatch is ventilated at the eaves and the ridge rather than closed with felt, and sealing the covering stops it drying, which shortens its service life.

Revetment, not rafters

Revach5 — Revetment and repair of thatched roofs | revach5.com

An old thatched roof rarely needs stripping back to the rafters. In most cases the decay sits in the top 300-400 mm: strip that layer away, lay fresh straw or water reed over the sound thatch, and rebuild the ridge.

Assess before you strip

The first decision is scope: a new ridge, a 300-400 mm strip-back and revetment, or full reroofing. The answer comes from the condition of the top courses against the sound thatch underneath.

Strip, lay, fix

Each slope is stripped to sound material, then new courses go up at 200-250 mm spacing. Every course is fixed at 150-200 mm centres with 1.6 mm wire or 13 mm hazel rods.

Ridge and renewal cycle

The ridge is twisted into cops 400-500 mm wide and 300-400 mm deep. It becomes the roof's maintenance clock: renewal every 8-12 years on reed, every 5-8 on straw.

The decay lives in the top 300-400 mm. Strip that off, and the roof underneath is usually still sound.

A ridge is a wearing part: it will be renewed several times before the covering itself needs full work.

The revetment procedure, step by step

Strip the weathered layer, lay new courses over the sound thatch, rebuild the ridge.

Revetment works because decay in a thatched roof concentrates in the weather-exposed upper layer. The standard repair removes 300-400 mm of thatch from each slope, down to the ridge, until sound material is exposed; what comes off is re-thatched with fresh material rather than reused.

Material supply has its own calendar. Water reed is cut between November and March while the plant is dormant, and straw arrives in bales of about 300-400 mm, so the work is usually planned around the dormant-season harvest.

Assess the slopes: confirm the decay is confined to the upper 300-400 mm.
Strip back each slope down to the ridge until sound thatch is exposed.
Lay the new courses up the slope, spaced 200-250 mm apart.
Fix every course with 1.6 mm (16 gauge) galvanised wire or 13 mm hazel rods at 150-200 mm centres.
Rebuild the ridge: 400-500 mm at the base, 300-400 mm deep, bundles twisted into cops.
Set the maintenance cycle: the new ridge will need renewal in 8-12 years on reed, 5-8 on straw.

What comes off, what stays, what goes on

A revetment in three layers: the weathered top, the sound thatch, the new courses.

The key terms of this guide, drawn to one scale

Working dimensions and cycles

Working dimensions, materials and cycles for a revetment
What is measured	Value	Notes
Strip-back depth per slope	300-400 mm	Exposes sound thatch; the removed layer is re-thatched, not reused
Course spacing up the slope	200-250 mm	Applies to straw and reed alike
Fixing centres across a course	150-200 mm	Wire or hazel rods driven at these centres
Galvanised wire gauge	1.6 mm (16 gauge)	Standard course fixing
Hazel rod thickness	about 13 mm	Traditional alternative to wire
Ridge base width / depth	400-500 / 300-400 mm	Built from bundles twisted into cops
Minimum roof pitch	45-50 degrees	Usually laid at 50-55 degrees
Commercial water reed length	1.5-2.5 m	Cut November to March, while dormant

Working dimensions, materials and cycles for a revetment

Questions that come up before the work

Does an old thatched roof always have to be stripped to the rafters?: No — revetment exists precisely to avoid that. The weathered upper layer is replaced with fresh straw or reed while the sound thatch underneath stays in place, and the strip-back usually removes only 300-400 mm from each slope.
How can I tell whether the thatch underneath is still sound?: By the depth of the decay. If the top 300-400 mm comes away and the material below is firm, the roof is a candidate for revetment; if the decay continues below that depth, the job moves towards full reroofing.
Why has the ridge worn out when the slopes still look fine?: Because the ridge is a wearing part. It is renewed every 8-12 years on reed and every 5-8 on straw, while the covering itself lasts 40-60 years on reed or 15-25 years on straw.
Can the new thatch be laid over bitumen felt to keep the rain out?: No. Thatch is ventilated at the eaves and the ridge rather than closed with felt, and sealing the covering stops it drying, which shortens its service life.