What is a window subframe?
A window subframe is an auxiliary frame built into the wall opening while the walls go up; the finished window or door frame is fitted into it later, once the wet trades are complete. In North America a similar frame is often called a buck. The Dutch term is stelkozijn, and in German it is a Montagezarge or Blindstock. Subframes are usually timber, sometimes plastic, and are common with aluminium and PVC windows.
The big advantage is protection. The expensive, finished frame arrives after bricklaying, plastering and screeding, so it is not scratched or splashed. Wet mortar and plaster can mark aluminium, so fitting it late matters. Meanwhile the opening can be closed with clear, waterproof sheeting so the building is already weathertight.
How it fits in a cavity wall
In a Dutch cavity wall the subframe sits between the inner and outer leaf, often combined with a spouwlat (cavity cleat) that closes the cavity at the opening. A sealing strip (EPDM or foam tape) between subframe and inner leaf provides airtightness. Some systems skip the subframe and fit the window directly against the outer leaf.
Application
- Measure the real opening. The window is made to the inside size of the installed subframe, not the drawing size.
- Set it plumb and square. A twisted subframe gives a twisted window or uneven sealant joints.
- Plan the seals. Fit airtight tape or sealing strip when installing; the joint is often hard to reach later.
- Watch for thermal bridges. A steel subframe can create a cold bridge; timber or plastic reduces that risk.
- Check local rules. Requirements for airtightness and window installation differ between countries (for example the Dutch Bbl and UK Building Regulations).
Common mistake: ordering windows from the drawing before the subframe has been measured on site. A few millimetres out means a window that will not fit or joints that are too wide.
Related terms
- Window frame
- Frame anchor
- Cavity cleat
- Airtightness
- Airtight tape
- Thermal bridge
- Door frame spreader
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.