A mortar rebate (Dutch: kalksponning) is a groove cut into the jamb of a window or door frame, at the point where the masonry meets the frame. While the wall is being built, this groove is filled with mortar, creating a firm, sealed connection between the wooden frame and the stone-based material. In Dutch it is also called a kloostersponning.
Why a mortar rebate?
Wood and masonry are very different materials that behave and shrink in different ways. If they simply butt against each other, a continuous, dead-straight joint quickly forms, through which moisture and draughts can penetrate. By milling a groove into the frame jamb and filling it solid with mortar, the masonry effectively locks into the frame. This prevents:
- A continuous joint — the mortar is anchored inside the rebate
- Draught penetration along the edge of the frame
- Shifting of the frame relative to the wall
Application
The mortar rebate is used mainly on internal walls and in traditional masonry around frames. The groove is typically about 8 mm deep and around 65 mm wide; the exact size depends on the tool used and the thickness of the frame jamb.
In practice, the mortar rebate, combined with careful detailing of the junction (such as an airtight sealant or membrane finish), is a detail that helps prevent moisture problems and gaps.
Related terms
- Rebate
- Frame
- Masonry
- Reveal
- Junction
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