Installing Wooden Sash Windows (For New Builds)

Georgian or Rectory style designs are very popular for new build projects today. Classic designs remain timeless and homebuilders will continue to recreate these house types for years to come. If you plan to build a traditional house type there are some things you should think about before your builder commences.

Timber box sash windows normally go hand-in-hand with these designs and using them will affect block-laying. Adjusting internal block-work and head heights is not a problem but not all builders will be aware of the differences between standard wooden casement window frames and wooden sash window frames. It is important to speak to your sash window supplier and confirm unit dimensions and their requirements on site.

Inner leaf opening width will need to be 80 to 100mm wider than outer block leaf opening. This allows for sash boxes to be contained behind the outer block-work and leave a slender visible frame when viewed from outside. As a sash window head is constructed the same as the sides the inner leaf head will have to be raised the same 80-100mm. This will ensure an even visible margin externally as desired.

If you wish to finish your sash windows with traditional shutters, internal leaf will need to be angled in proportion to the windows width. The main shutter panel should be about one third of the windows width.  Depending on your construction method, the internal wall depth can be increased to allow extra room for the shutters to fold into or the architrave can be built up/out from the wall.

Georgian proportions are uniform; hence window sizes should be kept so. This will avoid discrepancies in shutter sizes and angles which may only become obvious when picture rails, dado rails or cornices are added later.

For more advice on fitting wooden sash windows contact the technical team at or

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  2. Safety Features of Double Glazed Sash Windows
  3. Timber Sash Windows
  4. Old Sash Windows
  5. Who Installs the Sash Windows ?