How to Measure a uPVC Bay or Bow Window
In the years that we have been supplying windows and doors to the trade, one question comes up time and again: how do I measure a bay or bow window? Here, we cover just how to go about it. Remember, though, that if you’re unsure we are here to help. We have surveyors who can help as part of our Survey Service.
If you’d like to learn more about best practice and give it ago, read on!
What is the difference between a bow window and a bay window?
Bow windows feature a shallow angle, resulting in a significantly reduced projection compared to bay windows. When measuring an existing bow window, follow the same process as for a bay window. However, for those converting a flat window into a bow window, provide external brickwork opening sizes and determine the required number of facets and projection. The purchase or construction of a canopy will dictate the bow's projection, requiring you to supply this dimension.
How to survey a bow window or a bay window
Surveying a bay or bow window demands a different approach compared to measuring for a replacement flat window. To ensure accuracy, follow our guide on how to measure these window types. As the complexity of bays increases with additional segments, obtaining precise measurements can be challenging. If uncertain, it's advisable to enlist the expertise of one of our surveyors. The cost of potential errors outweighs the expense of professional assistance.
Splayed Bay Windows
Length: A-B/C-D: 600mm
Length: B-C: 1200mm
Length: B-E: 300mm
Length: A-D: 2400mm
Height: 1250mm
Internal Facet Widths & Angles (Our Preferred Method)
These are the individual windows that make up your bay, each measuring 600mm in the diagram provided. Measure dimensions from inside to exclude any existing corner posts from your calculations. Use an angle finder to determine the angles of each weld or join in the cill, ensuring accurate projection of the cill/bay. Alternatively, the other method involves using projection to calculate the angle where the two windows meet, employing trigonometry.
For bays with more segments, follow the same guidelines, accounting for additional facets and projections. Don't hesitate to seek assistance with these measurements.
Length: A-B/E-F: 600mm
Length: B-C/D-E: 615mm
Length: C-D: 615mm
Length: B-G: 330mm
Length: C-H: 525mm
Length: A-F: 2550mm
Height: 1350mm
Determining the Back Span (Fig.2)
The back span (A-D) refers to the internal width of the bay, measured from inside frame to inside frame. In some cases, you might need to remove plaster to locate the frame edge accurately. Note that measurements are taken inside, except for height, which is measured from outside—considering outside brickwork sizes and inside plaster lines. Heights should include cills if necessary.
Measuring the Projection (Fig.2)
The projection (E-B) indicates the distance from the back span to the inside of the front window. In our example, it's 300mm to the inside of the bay. Utilize a string line to facilitate this measurement
Square Bay Windows
For square bays set at 90 degrees, simply measure the internal sizes of each facet. We'll incorporate the two 90-degree posts accordingly. Height is measured from the outside.
Length: A-B/C-D: 600mm
Length: A-D/B-C: 2400mm
Height: 1250mm
Bay Jacking Posts
When dealing with load-bearing bays, fitted with a bay window above, adhere to building regulations by installing load-bearing bay pole jacks. These jacks fit through the external welded sill and can be adjusted to compress the bay pole onto a top plate supplied and onto the head of the aperture. Verify with us regarding the load-bearing capacity of our bay pole jacks.
Please note:
This measuring guide serves to assist experienced tradespeople or competent DIY enthusiasts in conducting surveys. Alexander Windows Limited does not accept any liability for inaccurate surveys.