Secondary Glazing Measurements Guild


To measure your windows before ordering secondary glazing or getting a FastQuote, you just need a tape measure. Our measuring guide will help you get the most accurate measurements and best results. See measuring for Horizontal Sliders, measuring for Vertical Sliders, measuring for Lift Outs, measuring for Hinge Units for product specific information. If you have a question about measuring a bay window, email details about the type of the bay. Our size guide shows the minimum and maximum sizes suggested for different window applications.

See the downloads on this page about tolerance deduction, splayed reveals and how to measure for splayed reveals.

Tolerance deductions simplify installation and improve the final appearance of your window. They accommodate out-of-square/distortions in the primary window frame (it may be necessary to increase this tolerance if the window frame is extremely distorted). Taking a lesser deduction is OK if you are confident about measuring and your primary window construction. However, additional work and adjustments to the secondary frame may be required. It’s better to take several width and height measurements, checking for variances. The smallest of these measurements can be used as a starting point. 10mm may not be sufficient in some cases. It’s better to have the unit made slightly too small, than too large. Any gaps can be covered by edging trim, and a few mm of inaccuracy is not easily spotted. If the unit is wedged in too tightly, it can distort the frame, reducing efficiency and operation. Also, a gap around the unit allows for seasonal expansion and contraction.

Tolerance: reducing the total measured width and height by 10mm and centre points/first panel points by 5mm. E.g. measuring a window 1195mm wide x 1245mm high would require a manufacture of 1185mm x 1235mm. Measuring details (internally facing the window) should be submitted by width x height x pane separation sizes. Centre points of horizontal sliders (sliding side to side) are taken from the left side to centre of mullion. Sash/vertical sliding windows (Sliding up/ down) the mid-rail measurement is always taken from the top to centre of mid rail. NB If the window panes are not equal in size (offset) still follow these instructions. Take your time, the measurements are important – we use them as our manufacturing instructions.

Take your measurements for secondary glazing when the primary window is in a finished state. Any painting, plastering, dry-lining, bulk heads or boxing out need to be complete before measuring. Need more help? Email with information about the primary window

Remember!
1 Measure left to right
2 Measure from inside looking out
3 When measuring a split in the height of a unit, measure from the top down
4 Horizontal sliders slider left-right, vertical sliders slider up-down
5 ALWAYS give your measurements in mm!