Some surveyors don’t like surveying Bay Windows. Over the years I have seen quite a few surveyors struggle with the measuring of replacement like for like bay windows and just recently a surveyor friend of mine asked me for some advice on surveying bays, which prompted me to tell you about the methods that I have used successfully and in particular one ingenious, very cheap piece of equipment that has been invaluable to me successfully surveying bay windows, along with many other uses for this simple piece of kit in the surveying and inspection process.

Taking into context bay windows are measured from inside, the two main methods of measuring of measuring bays are “facet widths and angles” or my preferred method “facet widths and throws”

Facet widths and Angles


Personally I don’t like this method as you only have to be a degree or so out on one angle and it can throw the bay all over the place. The only bonus point about this method is that if you are replacing like for like thickness frames (i.e. 65mm for 65mm) you could measure the frame widths from outside if you wanted.

Obviously for the facet widths and angles method you will need a Winkelfix or angle finder and they don’t come cheap at around £130 for the 600mmm version. I have seen one that has had a bump and was 4˚ out so make sure you check them regularly.


Facet widths and Throws


This is my preferred method where you can accurately measure the widths and projections of a string line across the internal width off the internal corner points of the frames. This method is by far more accurate

Facet widths and Throws

Now for the ingenious, ultra cheap piece of equipment I told you about, you’re going to love this !


All it is , is a one metre piece of fishing pole elastic with a loop either and two cork board drawing pins. A piece about a metre long will stretch to about five metres and makes a superb string line. All you need to do is stick a pin in the internal window board at the corner point of the frame either side of the bay and hook the elastic loops  over the pins and “hey presto” a perfect string line across the internal width to measure the projections of each internal corner point of the bay.

You can also use this method to measure the bows and facets on Victorian faceted conservatories. This is a really handy piece of kit to keep in your pocket you can check for straightness, check for bows and bends in conservatory cills and ring beams. You can check warped door sashes by hooking it over the handle up over the top of the door sash and back down to the bottom the other side. It has a multitude of uses. No need to carry a six foot straight edge all the time.

Total cost of this high-tech equipment is about £3.50 for five metres, that’s about 70p per string line, a pretty good investment for surveyors I think.


Seeing as how we are measuring bays I thought I would give few pointers on things you need to check to produce an accurate bay survey


  1. Check the levels plumbs and squares of the existing bay and opening and calculate any width deductions or packers required. Leave a 5mm tolerance to either side of the bay.
  2. If necessary remove a small piece of plaster off the internal corners to get to the exact corner of the side units of the bay to measure the internal width. Ensure you do not bury to much of the outer frame behind any internal plaster or external render.
  3. Check any mullion breaks.
  4. Ensure you measure all projections at 90˚ to the internal width string line.
  5. Make sure you comply with any fire escape regulations that are in force in regards to minimum clear opening widths and heights for openers


  1. Check the levels plumbs and squares of the existing bay and opening and calculate any height deductions or packers required. Leave a 10mm tolerance to the height of the bay.
  2. Check the height difference between outside cill level and inside window board level to ensure the window frame is not buried below the internal window board. Put a datum point on the glass with a felt pen to measure down from inside and out, to calculate the difference in heights accordingly. The same methods are used to calculate any differences between internal plaster heads and external heads, this could mean fitting frame packers or add ons.
  3. Determine what width of external cill is required i.e. No cill, Stub cill, 150mm cill, 180mm cill, 225mm cill
  4. Check any fascia overhangs that might interfere with any side opening vents, and make allowances to suit. Again this could mean fitting frame packers or add ons to the heads.
  5. Check any transom heights. ( Check customers can reach any top opening vent handles)
  6. Check the height of the glazing area above floor level if it is below 800mm it will require toughened glass by law.
  7. If trickle or gas vents are required in the sash, or if fitted in the head of frame will add ons, or packers be required to the head to accommodate vents
  8. Check if the bay is structurally supporting. If it is you will need to use structural bay poles and make any allowances to suit the jacking pole packing plates below the cill or above the pole. (see general assembly design below)

Load bearing bay pole jacking assembly


Obviously you will need to confirm all the colours, styles, furniture and glazing specifications to complete the survey

Let me know if the tips in this article will help you accurately survey existing like for like Bay Windows, It’s a general outline of a method that has served me well for years.
  1. Dave Blakeman says:

    Hi Steve,

    That’s great!

    Not sure if you can do I on standard WordPress, but might be a good idea to add a Facebook and twitter “like” button – I think this could become a great blog! Let me know if I can help…

    (See for an example!)


    Dave Blakeman Managing Director RPS RoofWright

  2. Shorefields Windows says:

    Any tips on surveying an orangery

  3. Rupert says:

    This is such a great idea! So simple. Thank you very much

  4. Thanks for your comments Rupert

    I found it invaluable for surveying and checking installations, for example, roofs, ring beams and frames being straight without any bows in them. I even encouraged installers to use it.



  5. Isabella says:

    This sounds like a great way of being able to check structures. Thank you for posting. It’s great to hear you are sharing it with other installers. Thank you.

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