A thatch clamping board — Dutch knelplank — is the member at the eaves of a thatched roof against which the bottom course of reed is clamped. It does two jobs: it stops the wind getting under the thatch and lifting the lowest course, and it makes water run off the reed and drip clear instead of tracking down the wall.
The clamping board can be a wall plate projecting past the rafters, a purpose-made board, or a roll of reed or straw.
Dimensions and build-up
- Clamping depth — the reed is compressed by roughly 40 to 60 mm, depending on wind load (guidance of the Dutch thatchers’ federation)
- Reed overhang — the reed projects about 15 cm beyond the clamping board, measured on the underside
- Clamping roll — a roll of reed or straw at least about 60 mm in diameter
- Ventilation — the board must have ventilation openings; modern versions are perforated copper or aluminium strips
Where it is used
- The eaves of thatched roofs on houses, farm buildings and outbuildings
- Both on closed thatch over a deck and on open, ventilated thatched roofs
Practical tips
- Ventilation is not a detail, it decides service life — an eaves that cannot breathe traps moisture and rots from below; sealing the board with mastic or foam is a classic mistake
- Keep the clamping face straight — a bowed board clamps unevenly and the thatch works loose in patches
- Inspect the eaves every year — together with the ridge this is where thatch damage starts, and where birds and martens pull out loose stalks
- Let the water drip away freely — a gutter or paving under the drip line catches the run-off, otherwise dirt splashes back onto thatch and wall
Related terms
- Eaves
- Wall plate
- Roof covering
- Rafter
- Tile batten
- Thatched roof
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