Technical Joinery Guide

Measuring Historic Window Rebates

Precision measurement is the foundation of successful heritage glazing. Enter finished sealed unit sizes in millimetres, verify rebate depth, and understand sightline concealment.

1. Finished Unit Dimensions vs Opening Sizes

In our configurator and enquiry schedules, entered width and height are FINISHED SEALED UNIT SIZES in millimetres.

Fitting Clearances (Expansion & Setting Blocks):

Do not enter the raw tight timber opening. A standard glazing clearance (typically 2mm to 3mm per side, or 4mm to 6mm overall deduction from tight timber daylight opening) must be allowed for setting blocks, perimeter sealant, and timber movement.

* Note: The owner-supplied ±1mm tolerance applies to OVERALL UNIT THICKNESS ONLY. It is not an allowance for width/height or rebate clearance.

2. Measuring Rebate Depth & Width

Rebate Depth (Front to Back):Measure the distance from the front face of the timber sash to the internal rebate upstand. For example, a 16mm deep rebate can accommodate a 12mm or 14mm slim unit with a 2–4mm face putty line or timber bead.

Rebate Upstand (Height):Measure the height of the rebate shoulder. This must exceed your chosen spacer sightline (5.5mm, 7mm, or 8.5mm) so the spacer bar remains hidden behind the frame moulding.

3. Supplier Sizing Boundaries & Review Guides

Due to physical deflection and edge-seal stresses in narrow 4mm and 6mm gas cavities, our engine enforces the authoritative supplier size pairs:

4mm Cavity Maximums:

600 × 600mm OR 500 × 1000mm

Orientation matters: 1000 × 500mm does not fit the 500 × 1000mm guide by rotating. 600 × 1000mm is outside guide.

6mm Cavity Maximums:

1000 × 1000mm OR 800 × 1600mm

Must fit within the same listed pair in entered orientation.

Toughened Minimum Size Guide:

Width ≥ 150mm AND Height ≥ 250mm

Applies when either pane is safety toughened. Units below this threshold require specialist supplier furnace review.

Start Your Schedule

Input your measured finished sizes into our multi-window schedule builder.

Open Configurator