Measuring Guidance

Measuring windows or doors seems like a daunting task but it really does not need to be. The fundamentals of measuring / surveying are universal, once these practices are understood you will develop a method that suits your own process. This page outlines the basics to help you along the way, however please note that the information we provide is to be considered as guidance only.

Recommended deductions for the width and height of structural openings


Up to 1.5m

From 1.5 to 3.0m

From 3.0 to 4.5m

Over 4.5m






  • These deductions are from the TOTAL width and height, and not “per side”.
  • The gap required for effective polyurethane foam fixing at the head is 10 – 15mm.
  • NOTE: When the overall width or height exceeds 3.0m, intermediate expansion joints may be required.
  • NOTE: BS7412 limits window sizes up to 3.0m only

Measurements as viewed from the outside.

Three measurements of width and height should be taken across the structural opening, it is also good practice to measure how square the opening is. Your deduction should be taken from your smallest measurement.

Measuring & Ordering from Zoda Homes | Weymouth | Dorset

Note: Always consult your designer before surveying if your glazing is going into a new build or extension.

How do I know what size external cill I need?

If an external cill is required, it should overhang the face of your external skin by 25mm. On a new build or extension, the backside (internal frame side) of a window or door should overlap a cavity closure by a minimum of 30mm. Once you know your required overlap distance, mark a line and measure to the face of the building and add 25mm.

Door thresholds

With doors an extra consideration must be observed. This is called a threshold. Sliding, front & bi-fold doors will have different styles of threshold, what needs to be determined is how high this will be above your finished floor covering. In most cases the preferred options are to have minimal interference between your finished floor covering and your transfer outside. View our downloads section to see what threshold we use on our doors.

Measuring & Ordering from Zoda Homes | Weymouth | Dorset

This section detail represents a sliding door flush threshold creating a seamless transition between your interior and exterior. Its good practice to decide how you want to position threshold before working out the overall height.

One way to work out this threshold detail is to measure from your finished floor covering to the lintel. Deduct the adequate head tolerance and add on the depth of the threshold. This will give you the required height of the door system as shown in our section drawing. Make sure you have adequate room between your DPC and finished floor covering. In many cases the recommended tolerance and threshold depth will create sufficient clearance.

Feel free to contact call us on 0300 3035 405 or via our contact form we can support you with any queries you may have on threshold setup.

Points to remember

  • The overlapping measurement of the thermal barrier (cavity closure) can vary depending on the property design.
  • If a property features render or cladding, additional deductions maybe required.
  • Stone or tilled cills may not require an external aluminium cill.
  • Always think about door threshold heights.

Building regulations

It is always important to make sure you are complying with building regulations. To help you along the way, we have added the relevant approved documents to our website. View our Approved Documents for further details on UK building regulations.