What is a window sill support board?
A window sill support board is a board that supports, for example, an internal window sill from below. In Dutch it is called a vlagstuk. There is no fixed English term, so a descriptive name is used here. The board sits under the projecting part of the sill, against the wall, and carries it there. This stops a sill that overhangs the wall from sagging or tipping.
Construction
The board is usually visible and is therefore neatly finished. In a detail drawing by Peter van Teijlingen (published by Joost de Vree) it is made as a beaded board, combined with mouldings such as a cavetto. It is then both a structural and a decorative part of the window trim.
Application
- Timber window sills: especially sills with a large overhang, for example above a radiator or in a thick wall.
- Restoring interiors: in old houses you find these boards under the original sills. In repairs they are remade to the same profile.
- New build in a traditional style: for a classic window finish you can use a support board instead of hidden steel brackets.
Tips
- Fix the board first: set it level and fix it firmly to the wall with plugs and screws suited to the substrate. Only then lay the sill on top.
- Cover the joint: the junction between sill and board is a sight line. A small bead or moulding hides a gap and timber shrinkage.
- Prime before fixing: treat all faces of the timber. It will then absorb less moisture from the wall and distort less.
- Keep air moving: if there is a radiator below, the board must not completely block the rising warm air.
Related terms
- Window sill
- Overlay window sill
- Bead
- Cavetto
- Beaded matchboarding
- Door and window trim
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