Period properties have a charm and authenticity rarely found in modern construction, but they also have certain complexities. The openings of the windows in older properties are not so perfectly uniform, and years of natural settlement mean the dimensions can vary even within the same frame. That complexity is exactly why getting the measurements of timber sash windows right matters more than the homeowners realise.

Getting the measurements right will result in a clean installation of the new window, while incorrect measurements will create problems from the very beginning. Understanding the standard sizes of timber sash windows, how age affects dimensions, and the common pitfalls to avoid will put you in a much better position before ordering anything.

Measured with care, timber sash windows fit well, perform reliably, and maintain their character for decades. Here’s a guide on how to measure timber sash windows London correctly.

Why are timber sash windows standard sizes not universal

Timber sash windows are unique, and there is no universal size chart for all properties. This is one thing to understand initially. The right size of timber sash windows is determined by the time a building was constructed, the architectural tradition to which it belongs, and the way its original frames were made.

In the UK, timber sash windows standard sizes are usually within these ranges:

  • Width: 500mm to 1,200 mm
  • Height: 900mm to 2,100mm
  • Frame depth: 68mm to 100mm, which is quite deeper than uPVC frames

These figures are the most common openings on Georgian, Victorian, and Edwardian properties, providing a solid foundation for a standard or bespoke timber sash window.

How the architectural period influences timber sash window dimensions

The year your property was constructed provides significant information about the window openings. Timber sliding sash windows were designed to reflect the proportions of their time, and those proportions change with the era.

1. Georgian sash windows (1714 to 1830)

The windows in Georgian properties were tall and narrow, built vertically and symmetrically.

2. Victorian sash windows (1837 to 1901)

Victorian windows featured slightly wider openings, with sash widths of about 630mm and heights of about 1500mm. Bay window configurations were also popular during this era.

3. Edwardian (1901 to 1910)

Edwardian properties then moved toward wider, squarer proportions, allowing more natural light into the rooms.

How to measure your window opening correctly

Measuring from the exterior of the property is always the right place to start. The external face shows the actual size of the structural opening into which a new timber sash window would fit, and, if working from the inside, you can miss distortions that will lead to difficulties during fitting.

The following steps will help you get a good result:

1. Width

Measure at three levels, i.e. top, middle, and bottom. Record the smallest of the three figures, as this is the narrowest point the frame must pass through.

2. Height

Measure at three points, left, centre, and right, and again work with the smallest figure.

3. Squareness

Measure diagonally from one corner to another corner in both directions. A significant change is enough to indicate that the structure has changed.

4. Frame depth

Check the depth, as the timber window frame is much deeper than the uPVC one.

Taking all four measurements before ordering removes the most common source of sizing errors at the point of installation.

The role of fitting tolerance in getting the right fit

Fitting tolerance is the deliberate gap between the ordered window size and the actual structural opening, and it is a step that is far too easy to overlook. Without it, an installer cannot level, align, or secure the frame properly once it is in position.

For timber sash window replacements, the standard guidance is to deduct 5mm to 10mm from both the recorded width and height before placing the order. It is a small adjustment, but an important one. For timber sliding sash windows, this matters even more, as both sashes must move freely within the box frame, without binding or sticking.

Speaking with your timber window company about the correct tolerance for your specific opening, particularly in older properties where walls may have shifted, is always a worthwhile step before confirming your order.

When are bespoke timber sash windows the right choice

Standard dimensions work well for many properties, but there are circumstances where only bespoke timber sash windows work.

1. Listed buildings and conservation areas

In these areas, the planning authorities may insist on replacement windows of the same size, with both glazing bar positions and sightlines matching.

2. Non-standard openings

Bespoke windows are needed when measurements are not within the standard sizes of timber sash windows, and no stock product will fit the space appropriately.

3. Timber sash window repairs on significant properties

Where matching an existing profile, moulding detail, or glass type is central to the brief, only a window built to those exact specifications will give the right result.

4. Upgrade projects

Where custom finish or draught-proofing has to be installed in the frame during construction.

Conclusion

Measuring timber sash windows is a discipline built on three components: accuracy, preparation, and understanding of your property. From creating the correct size baseline of your property to three-point measuring, checking squareness, and fitting tolerance, every step of the process is interrelated. Remember, the measuring stage is always where problems start. Once you get it right at the beginning, then everything that follows becomes easy.

For a project like this, the quality of the outcome depends just as much on who you work with as it does on the measurements themselves. SJB Sash Windows is a London-based company with a long-established reputation for designing, manufacturing, and installing timber sash windows across all types of property. The company offers the same quality and accuracy on every project, whether it is a simple replacement of a sash or a complete custom timber sash window.