How to measure for your new PVCu sash windows?

stanley powerlock 5m tape measure

If you would like to order your first PVCu sash window or you are in the trade looking for some expert tips, our comprehensive pro-guide “How to measure for your new PVCu sash windows” will help you measure and order with confidence.

In this article:

  • Tools to measure for sash windows like a pro
  • Measuring Victorian sash window openings
  • On-site pro-tips to measure PVCu sash windows

We are here to help you achieve perfect installations of our Evolve PVCu sash windows every time.

You can email us or use our Website Chat to ask any questions.

Important. This guide is for a tradesperson or competent DIY person to carry out a survey for an Evolve PVCu sash window. We do not accept any liability for inaccurate surveys. If you require NVQ training click here

Pro tip – Stay focused!

Tools to do the job?

In order to measure for PVCu sash windows like a pro, you must use high-quality tools which provide reliability and ease of use.

Using cheap or worn out tools will almost certainly let you down sooner or later, costing you time and money.

We recommend the following tools because they are affordable, tough on-site and do the job:

  • Stanley 5m Powerlock tape measure
  • Stabila Medium level (610mm)
  • Stabila Long level (1200mm)
  • Youngs Ladder and step ladder
  • Stanley clawhammer
  • Any 1-inch cold chisel
  • Any 1/2-inch wood chisel
  • Makita battery drill
  • HSS 6mm long reach drill bit
  • Tool belt

Remember the old quote
Measure twicecut once,” Obviously, it is a great rule for a carpenter and can be applied to measuring windows.
Measure twicefit once,”

Measuring Victorian sash window openings fitted with an original timber sash window

The Victorian sash window opening in its original format has a number of factors to be considered; subsidence, alterations, damp, dry-rot and hidden wall plates.

Subsidence to the brickwork

The overall condition of the surrounding exterior sash window brickwork will impact your window installation. Look out for any cement pointing, cracked or lose brickwork on arches/lintels and cracked stone sills. Any severe problems will require attention before you take your survey measurements.

Many Victorian period openings suffer from moderate subsidence, this is not considered a problem unless it’s a recent occurrence. If you are unsure, then the best practice would be to call in a structural surveyor.

Finally make a note of phone, TV aerial or alarm cables that have been attached or passed though the timber sill, as these will need to be removed prior to installation.


Has the box frame been backfilled? ie: bricks and concrete

This may not seem to be an important check as the whole window will be removed during installation, however, finding the timber box frame full of bricks or even concrete will present you with some challenges. Make a quick check to see if the window is still operating, at least in part.

You can also try tugging on the sash cords to feel for the weights. If there is no sign of the weights then remove the weights access panel and look inside. You may not be able to get access to this panel if a PVC or aluminum window has been fitted into the original frame – you could drill into the box section to check for infill.

Damp and dry rot

Look for damp or dry rot as this can significantly affect your new window installation. Original sash window openings have internal timber lintels and timber wall plates at the bottom of the opening. Be aware, if either of these are rotten, they will require replacement. If you would like to learn how to deal with this problem, subscribe to our newsletter and receive pro tips directly into your inbox.

Hidden timber lintels and wall plates

Many timber sash windows are built into the opening during construction, with timber lintels and wall plates running through the head of the box frame. In this case, these internal timbers can be lower than the external brickwork.

As a result, if you measured the external brickwork as normal your new window will not fit below the wall plate, it’s too tall!

Pro tip – Hidden obstructions 

Take a battery drill with a long reach 6mm drill bit. Open the top sash and drill a series of holes through the head of the box frame, which is about 15mm thick. If your drill shoots into an empty space, you are good to go, however, if you continue to drill into timber after 15mm you know you have a wall plate in the way. Take your measurement from the bottom of this wall plate to the stone sill, allow at least 20mm tolerance for the height of your new PVCu sash window.


The following illustrations are from a real photo with the original sash window digitally removed, to demonstrate surveying a typical Victorian period sash window opening. In this case study, you will see a small amount of subsidence to one side, which is not immediately obvious.

Let’s start measuring/surveying

Always keep in mind that PVCu sash windows need to be installed plumb, level and square to operate correctly. One benefit of the Victorian openings is that part of the sash window frame fits behind the front brickwork, allowing for a little extra tolerance whilst fitting. However, the sculptured parts of the frame should not run behind the brickwork because it will appear unsightly.

typical sash window opening measuring guide
Fig. 1. Typical measuring guide

Many PVCu sash window survey instructions will tell you to simply measure three vertical and three horizontal points using the smallest as your window size. Then adding or subtracting tolerances.

That’s great and will probably be fine, certainly for small windows. Well, hopefully!

Now, let’s have look at how to measure PVCu sash windows like a pro. Learning these simple steps will take you to the next level.

How to measure the width for your PVCu sash window

How to check the plump of a pvcu sash window

Fig. 2. Plumb readings

Start by checking the plumb readings as shown in Fig. 2. In the diagram the left side reveal is perfectly plumb while the right side is slightly off plumb.

The gap between the level and the brickwork shows the reveal has moved approximately 15 mm making the top wider.

This indicates the bottom area is the narrowest point from which to take your width measurement.

You could have got the same result by measuring in 3 places as shown in Fig. 1. However, if the reveals were bowed or the left side was out of plumb, you might have run into trouble, also by using the spirit level there is only one measurement point to concentrate on.

In this case we will assume the width measurement is 900 mm, then we add the amount of frame thickness that goes behind the brickwork.

We would recommend half of frame face, which is 25 mm on each side.

Width : 900 + 25 + 25 = 950mm .

Fig. 3.

How to measure the height for your PVCu sash window

Take a close look at the stone sill surface, it may have a render coating to cover up cracking or subsidence, to get the true measurement this should be removed.

In Fig 4, you can see the stone sill is out of level by approximately 15mm. So the highest point of the stone sill, in this case, is on the left.

The arched head is checked by placing the level on the shoulder. You can see the right side is lower by about another 15mm. Lower the spirit level to that point, re-level and make a mark on the left side with a pencil, as indicated by the green arrow shown in Fig. 5.

how to check the sill level of a victorian window opening with stone sill
Fig. 4. Stone sill level check
how to check the level of a victorian sash window opening
Fig. 5. Arched head level check


How to measure the height of a pvcu sash window
Fig. 6.

Now measure from your pencil mark to the highest point of the stone sill. In this case we will assume the height measurement is 1700 mm.

Next we add the amount of frame thickness that is needed to go behind the arch brickwork.

We recommend 35 mm for a arched head

Height: 1700 + 35 = 1735 mm

Note, If the head was a straight, then you only need to add 25 mm. Remember to check for hidden wall plates or lintels.

Final checks

How a PVCu sash window fits to the brickwork
Fig. 7.
  • Check sill, head and plumb readings
  • Mark out the brickwork as needed
  • Measure the smallest points for height and width
  • Add 50 mm to the width
  • Add 25/35 mm to the height subject to variations.
  • Check all opening measurements again and add the tolerances

In Fig. 7, you can see a gap between the top of the PVCu sash window and the arch. This will require a 30 or 50 mm frame extension. The extension will be included in the total sash window height. For example, your sash window height is 1735 mm and to fill the gap you add a 30 mm frame extension. This brings the total sash window height 1765 mm. This may seem obvious but can cause some confusion.

Evolve sash window outer frame cross section

pvcu sash window cross section outer frame
Fig. 8. Illustrates the 50mm external frame face.

We hope this guide helps you understand the basic rules of surveying. Please add any comments below.

Measuring New build sash window openings

Coming soon!

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