Sash windows are one of the most specification-sensitive products in the trade windows market. Get the order wrong and you’re looking at remakes, programme delays, and unhappy clients. Whether you’re replacing existing timber sash windows or supplying new builds with a period-appropriate aesthetic, understanding exactly what to specify — and in what order to gather that information — will save you significant time and cost.
Understanding Sash Window Types
Before placing an order, confirm which type of sash window is required:
- Vertical sliding sash (VSS) — the traditional style with two sashes that slide up and down within the frame. Available in PVC-u with authentic-looking horns, deep bottom rails, and run-through sash joints that closely replicate timber originals
- Mock sash / tilt and turn sash — a casement window styled to look like a sash from the outside, often used where planning does not require a functional sliding mechanism
- Sliding sash with tilt facility — the most common trade option; both sashes tilt inward for cleaning access, while maintaining the external appearance of a traditional sash
In conservation areas or listed buildings, you may need to confirm with your local planning authority which type is acceptable before ordering.
Measuring for Sash Windows
Measuring sash windows — particularly replacement sashes — requires more care than standard casements. The critical dimensions are:
- Overall frame width and height — measure at three points (top, middle, bottom for width; left, centre, right for height) and use the smallest dimension
- Reveal depth — the depth of the recess the frame sits in, which affects fit and finishing
- Existing box frame dimensions — if replacing into an existing timber box, confirm whether the new PVC-u frame is going inside the old box or replacing it entirely
- Check for square — measure both diagonals; if they differ by more than 5mm, the opening may need remediation before a new frame is installed
Allow adequate clearance for fixing and sealing. Your supplier should advise on the tolerances built into their frames.
Glazing Specification for Sash Windows
Glazing options for sash windows are more constrained than for casements because the sash weight limits how heavy the sealed unit can be. Confirm the following with your supplier:
- Unit thickness — a 24mm or 28mm unit is standard; heavier units may require spring weight adjustment
- Obscure glazing — specify which panes require privacy glass and confirm which pattern is required
- Georgian bars — confirm whether bars are internal (between panes) or external (applied), and the bar width
- Energy rating — ensure the glazing unit meets or exceeds the required U-value for the project
Colour and Finish
Standard sash windows are available in white, cream, and a range of foil finishes that replicate painted timber. For replacement work, matching the existing frames on the building often takes priority. Confirm:
- Frame colour (inside and outside — dual-colour options are available)
- Sash colour (may differ from frame in some designs)
- Horn profile — traditional horns significantly change the visual character of the window
Hardware Selection
Sash window hardware is functional and aesthetic. Key choices include:
- Sash lift handles — confirm style (bar lift, recessed lift) and finish (chrome, antique brass, white)
- Sash lock — confirms both sashes are secured when closed; confirm style and finish
- Egress restrictors — required in rooms used for sleeping in residential properties; confirm which sash requires restriction
Trade Sash Window Order Checklist
Use this checklist before confirming your order:
- ☐ Window type confirmed (vertical sliding sash / tilt-in sash / mock sash)
- ☐ Overall frame dimensions measured at three points — smallest used
- ☐ Opening is square (diagonals checked)
- ☐ Reveal depth confirmed
- ☐ Fitting method confirmed (into existing box / full frame replacement)
- ☐ Glazing unit thickness specified
- ☐ Obscure glass panes identified and pattern confirmed
- ☐ Georgian bars — internal or external, width confirmed
- ☐ Energy rating / U-value requirement confirmed
- ☐ Frame colour confirmed (inside and outside)
- ☐ Horn profile required — yes / no
- ☐ Hardware style and finish confirmed
- ☐ Egress restrictors required — yes / no / which sash
- ☐ Quantity confirmed per opening
- ☐ Delivery address and access confirmed with supplier
- ☐ Lead time confirmed and fits site programme
- ☐ Compliance documentation requested (U-value cert, energy rating)
Communicating with Your Supplier
For guidance on measuring window openings correctly before ordering, see our guide to how to measure windows for replacement. If you’re working in a conservation area, read our article on conservation area windows in Scotland before specifying.
Always confirm your order in writing, including the full specification. A reputable trade supplier will issue an order acknowledgement that lists every item — review it carefully before the order goes into production. Errors are far easier to correct before manufacture begins than after delivery.
If you’re new to ordering sash windows, ask your supplier for a technical consultation before you finalise measurements. A good trade supplier will have experienced staff who can walk you through the options and flag potential issues before they become problems on site.