Tuesday, 18 September 2012

Measuring openings

There are a number of measurements that are critical for a good design and a sound installation of window panels.

1. Tight Size: This is the full size of the glass opening with no allowances for expansion and contraction. In a wood or metal rebate frame one would measure from steel to opposing steel or wood to wood; in a stone groove installation, from the bottom of one groove to the bottom of the opposing groove. Depending on the size of the opening, this measurement should be checked in multiple areas; at a minimum at the top, bottom and middle horizontally and at the left and right jamb.

2. Sight Size is the daylight opening or the largest opening that allows light to pass through.

3. Rebate or groove details. With a rebate frame, the depth and the width of the rebate must be measured, as well as the interior return if round bars will be used (this dictates what size bar will fit and how long the bar should be). These dimensions are also necessary to determine the dimensions of the retaining moulding if one is to be used. If it is a groove, the depth of the groove and the width of the groove (measured from interior to exterior) are important.

4. Panel Size. This is the ideal size of a panel that will be installed into the opening in question. Typically, this will be a function of the tight size less 3mm in both width and height for a leaded glass panel, to allow for expansion and contraction. One must also recognize if the size varies throughout the frame and make allowance for this as well. With dalle de verre, you need a deep rebate or groove and allow at least 5mm in both directions for expansion.

5. The depth of the rebate or the width of the groove are also critical measurements. To allow for a proper installation, allow a minimum of 13mm to be added to the thickness of the panel to provide room for a proper putty fillet.

Based on comments from Art Femenella

When measuring older openings and especially doors, measure the diagonals in addition to all the other measurements. This provides a check of all your other measurements and also tells you whether the opening is a true rectangle or parallelogram.

Measuring a Rectangular Opening

Measure sight and tight sizes at top and bottom, and left and right. You should also measure the middle of the horizontals and verticals in addition to the details of the rebate.

Measure the diagonals to determine if the opening is “square” - all angles square. If these measurements are equal or +/- 5mm you can consider the opening to be a rectangle.

With bigger variations you may set out the cartoon using the measurements for the opening. Still, you need to know where the right angles are, if there are any, to be able to set out the cartoon to properly fit the opening. You can check for ”squareness” with a try square, although that is not completely accurate. If it is difficult to determine where the right angle(s) are, you need to take a template of the opening.


If it is not possible to tell where the right angles of the opening are, a template is called for. The material to be used for taking templates should be stiff, easy to cut, unaffected by moisture, and relatively inexpensive. This eliminates paper and some cardboards. If you can find stiff corrugated cardboard this works well. Mounting board works well too, but is expensive. Foam board is excellent, but also expensive. Hard board or other thin pressed board is inexpensive but difficult to cut with a knife. Thin plywood is also a good material for templates, especially if the opening is relatively regular. The more complicated the opening, the more cardboard, mounting board, or foam board becomes useful for its ease of shaping to the opening.

What ever material you use, you must mark which is the interior and exterior and for further checks, which is left and right. Fit this template into the opening to make sure it fits into the opening smoothly. This template will form the external extent of the built window when it is installed into an opening with a rebate.

Where the window is to be fitted into a channel, as in stone, you need to make the template of stiff material so you can determine the panel can be installed and that there will be enough of the panel within the stone channels to ensure the stability of the window in the future and still be able to manipulate the leaded panel into the opening.

Irregular rectangles

If you have found or can see that the opening is not a true rectangle and cannot determine where any right angles are, you need to take a template.

The objective is to make a piece that will fit into the opening without bending or being too small for the space. It will be the same size as the finished panel and so you will be able to put the finished panel into the opening without needing to trim or expand the panel.

  • First, trim the sheet of material you have chosen to use to a size a little larger than the measured size. Place the uncut side along one of the long sides of the opening. If the opening is a portrait format, place it on the right or left side as convenient to you.
  • Next, adjust the bottom by marking a line on the sheet. This is where a second person is very useful. One person can hold the sheet in place on outside of the opening and the other do the marking from the inside –in the case of the rebate being on the outside and vice versa if the rebate is on the inside. The marked line should be as close to the edge of the rebate as possible. The special case of an opening in stone will be dealt with separately.
  • Then take the sheet to a place where it can be safely cut. A long metal straight edge and craft or “Stanley” knife are often the best aids to cutting straight lines. Replace the sheet into the opening after cutting, and make any adjustments to the size and angles of the sheet at the bottom by marking and cutting as necessary.
  • When the side and bottom are adjusted, start on the other side. Proceed as for the bottom.
  • When the side is finished, start on the top.
  • Finally, present the whole sheet to the opening to make sure it slips into place with no snags, or bending of the sheet.

It may be that the opening is too large for a single sheet. In that case you will need to work with two or more sheets and try them together for the final fitting into the opening. You can put them together in the window. You can fasten them together with tape or other fasteners to make one sheet. You can also make two parallel lines both at angles and at intervals across the sheet so that when you get back to the studio you can exactly reproduce the full sheet by matching the marks and then firmly fastening them together. This makes transport of large templates much easier.

You will know that a panel made to a template made in this way will fit into the opening, no matter how irregular the opening may be.


Occasionally the window is circular and sometimes an oval. In both cases a template is important. The circle rarely is exact. Take the template in the normal way and then ensure you mark the verticals and horizontals for the opening. You often can use the jointing in the woodwork to help with these. Also mark any other reference points from the opening. Finally, mark which is the outside and which the inside.

This procedure will ensure that you will be able to fit the panel into the opening.

Round headed openings can be considered as a special case of a circle.

The horizontal you must find is the shoulder of the window. This is the place from which the curve springs on each side. The opening is generally vertical up to this point and then the curve begins. In stone, there is most often a joint at this point. This is often the case in wood too, if you can find the joint under the paint.

You need to make sure you have marked where this shoulder is on the template. You should indicate any reference points from the frame onto the template.

The join to the lower part of the window must be made obvious. Normally there will be an overlap between the lower rectangular template and this approximate half circle. You need to mark where this overlap occurs, if you do not fasten the two sheets together. This can be done by marking across the two sheets in a few places. This will enable you to join them exactly back at the studio.

Stone openings

When the opening is in stone, slight variations occur in the process of taking a template. The main difference is that the rebates are concealed. The rebates are slots into the stone. Thus, the template must slip into the slotted rebate. In these cases, the stiffer the material being used to take template, the better. Usually, thin plywood is the best material, as it has to be manipulated many times and in ways similar to the final panel.

Things are further complicated, as tracery is more common in stone than in timber framed openings. A complex opening shape may require two or more parts to enable the panel to be inserted. The taking of a template will help greatly in figuring out how the panel will be inserted into the opening.

Additionally, when the template is in position, you should mark the visible portion of the opening onto the template. Mark which is the inside and which the outside. Finally, mark on each template which side has the deeper slot as this will help in installation.


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