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Replacing a broken glass pane in a leaded light window.

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How to replace a broken or cracked pane of glass in a leaded light window.


If you live in an old listed building, be it a grade 1 listed building or a grade 2 listed building here is a simple easy way to renovate a leaded light window and replace a broken glass pane without causing any damage to existing glazed units, frames or surrounds and without needing to pay a fortune to get them done professionally.

Secondary glazed leaded light windows

This method is exactly the same as the professionals do it as once having been in the building trade and having renovated many properties I have seen it done many times at great cost to the customer.
It works, as I have used it myself on my own windows shown in the picture above, as I live in a grade II listed building and during the renovation project needed to replace several broken glazing units which would have cost nearly £750.00 to have them all replaced professionally.

Materials needed.

For this simple project you will need:

Water proof Clear/grey silicone or glazing putty.
Measure around the edges of all your broken glazing units to work out how much silicone you will need. Depending on how many glazing units you are replacing. An average 310ml cartridge with a bead depth of 4mm and bead width of 3mm will give approximately 25 linear metres bead of silicone so I would think 1 silicone cartridge would be more than enough.
Silicone gun. If you do not already own one you will also need a silicone gun to dispense the silicone. You can usually obtain a fairly cheap one where you buy your silicone cartridges.

Replacement glass units.
You will need to measure the size of each glazed panel and its thickness as depending on the age of your property it will vary.
Important:Do not assume the glass panel sizes will be all the same, old leaded light windows can and do vary slightly and once you have all the glass panes cut you will not be able to trim them easily if they are a little large, also you will not be able to fit them securely and make them water tight if they are a little small.

Glass choices:
My property is early 1800c and the closest I could find to match the thickness without paying a fortune for period glass was to use picture glass, the other alternative is to visit a glass supplier, or as I did a local glass artist who may be able to order authentic glass to match the age of your property and cut the units for you, (Listed buildings will expect this option but it will come at a price.) Your supplier will first have to order a full sheet of period glass, regardless of how much you require. You might get lucky and find they have a few suitable offcuts laying around their studio which will work out cheaper but it's really up to you and your budget.
Note: Modern requirements are that you use safety glass when replacing glazing units so be aware that most glass suppliers will not let you use picture glass. However, with my supplier, I explained my situation and told him that I was renovating a leaded light window with less than 3mm glass so he was happy to cut my units for me. I needed 8 panes of glass at approx 185mm x 135mm which cost me £5-00 in total including cutting. I also later needed 2 more units for my upstairs windows which he gave me for free as he had 2 off cuts that were just the right size for the job.

Tools required.


A sharp woodwork chisel. I used a 22mm wide chisel which was ideal for the job but you can use whatever you have to hand, try to use the widest blade you have as this will make the job much easier. If you do not have a chisel then perhaps you can use just a Stanley knife blade that has been removed from the handle.
A putty or filling knife/paint scraper
This is instead of using a woodwork chisel as not all of us have chisel's but most have a filling knife hanging around somewhere. Also if you do not have a filling knife or putty knife one can be purchased quite cheaply.
A sharp Stanley knife.
A flat bladed screwdriver.
Paint or dusting brush.
A damp cloth.
Goggles/eye protection.
Gloves for glass removal.

Stage 1:Cutting the lead.

The first thing to do is to remove the broken glass panel This will need to be done from the outside as the steel/brass internal saddle bars can cause problems.

Cutting marks.
1) Put on your eye protection then take your sharp knife and carefully cut a 45-degree angle in the lead on the top corners of the unit (you must do this at the top to prevent the risk of leaking). This will also need to be done from the outside as the saddle bars on the inside of the leaded light window usually get in the way. Make sure that you only cut to the edge of the existing glass panel which you should be able to locate with the tip of the knife blade. Make sure you cut all the way through the lead and as I have already said only cut enough to get to the edge of the existing glass panel which can also sometimes be located by a slight ridge in the lead as shown below.This requires patients and care.
Leaded light cutting marks

Stage 2: Lifting the lead

With that done take your sharp chisel/filling knife or even a Stanley knife blade and carefully lift the edge of the lead by sliding your chisel or knife blade between the glass and the lead making sure that you only lift enough to free the glass panel. Do this down each side also until you feel the panel is loose enough to be removed.

Stage 3: Removing the broken glass panel

Tip: Apply some tape over the broken glass pane to help prevent the broken glass shards from falling all over the floor.
The next thing to do, with gloves on, is to carefully remove the broken glass pane making sure that you do not over bend the lead as it will be very fragile. This again requires patients and care. Try to lift the panel up toward the top sliding it out of the lead surround. You may need to ease the bottom corners slightly with your chisel but try not to over distort, split or tear the lead.

Stage 4: Cleaning the lead rebate.

Next with the glass removed you will see that there is a slight rebate between the inside and outside lead surrounds. Take the flat bladed screwdriver and carefully remove any old putty/cement from around all the edges and faces of the lead where the glazing panel was seated, again take care not to damage or distort the lead.
Tip:If the lead distorts, use a narrow flat piece of timber that you can easily place along the length of the lead and gently ease the lead back into shape by carefully tapping or pressing it back into shape.

5) With this done now take your dusting brush and remove any dust, dirt or debris from the lead.

Stage 5: Checking the fit of the replacement glass pane.

With all dust, dirt and debris removed you are now ready to offer your new piece of glass into the lead frame. Do not apply the putty or silicone yet. This is when you will know if you have got your measurements correct.

Stage 6: Applying the silicone or putty seal.

If your glass panel fits correctly, remove it, as it is now time to apply the silicone or glazing putty to the lead.
Tip:I always use silicone as putty dries hard and holds the units firm allowing any vibration, especially if like me you live near to the main road to potentially re-crack the glass, whereas silicone is more flexible and acts like a shock absorber reducing the risk of re-cracking the glass.
This has been proven in my case as both my next door neighbour and I have restored our windows. He had his repaired professionally using putty and mine where done by myself using silicone. Most of his have re-cracked whereas none of mine have.

Apply a thin bead of silicone or putty to the back edge of the lead first, making sure you apply a continuous bead. You can apply the putty with the flat of the chisel, your filling knife/putty knife or knife blade.
Tip:If you are using glazing putty make sure you knead it well before applying it as the softer and more malleable it is the easier it will be to fit the replacement glass panel.

Stage 7: Refitting the new glass pane.

Take your glass pane and carefully refit it into place.
You may need to offer the glass in from the top and gently ease it down to the bottom of the lead and into the rebate.
From the top down apply even pressure around all the edges to get a good seal. A good way to check is if you can see a thin line of putty or silicone around all the edges of the lead.

Tip. If you see any slight gaps around the edges fill them by carefully pushing the excess putty or silicone into them with the flat of the chisel. filling/putty knife or knife blade until you have a continuous bead around all the edges.

Stage 8: Resealing the glass and dressing the lead surround.

With that done apply another thin bead of silicone or putty under the raised lead on the outer edge of the glass and then using the top of the chisel handle or the screwdriver handle carefully push the lead back into place onto the glass, applying gentle even pressure until the lead is seated evenly onto the glass and there is a continuous fine line of silicone or putty visible around all its edges.
Tip. If you see any slight gaps around the edges fill them by carefully pushing the excess putty or silicone into them with the flat of the chisel, putty/filling knife or knife blade until you have a continuous bead around all the edges.

Any excess putty can be removed by carefully running your chisel blade around the edges of the glass and then wiping them with a damp cloth. You will need to repeat this on the inside of the glass pane also.
Tip:If you are using silicone and find it does not come off satisfactorily by running a chisel blade around the edges of the lead you can remove the excess with a drop of white spirit or turpentine, or by letting the silicone dry first and then with the knife blade carefully run it around all the edges of the lead to free it up, then carefully slide the blade around the unit until all the silicone is removed. DO NOT press hard or scrape the glass as it may scratch.

Stage 9: Blending and resealing the cut edges of the lead.

The next thing to do is to blend the cuts in the corners.
To do this you will need to again use the handle of your chisel or screwdriver and gently rub it across the cut until the lead blends into itself, hiding the cut, work from the outside edge to the inside. You may find some silicone or putty spews from the joint but this is ok as it means the joint is sealed and any excess can be removed with the cloth, or you can wait for it to dry and then carefull remove it with a chisel or sharp knife blade.


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