How to reglaze a sash window

How To Reglaze A Sash Window

If like me you are a do it yourself kind of person then at some point you will come across a window that either needs reglazing or the putty replacing at the very least. The window I’m working on today is a sash but to be honest the principle of glazing is the same whatever type of window it is. One of the panes of glass in my back sitting was cracked. Not too badly thankfully and I’ve managed with it for years. But the other day whilst doing up my outhouses the window frame took a bang. And what was once a crack fractured even more and a piece of glass fell out. What wasn’t on my list all of a sudden became high priority so I thought I’d drag you along with me for anyone wanting to know how to reglaze a sash window.


But before we get to how to reglaze a window let’s take a look at what you will need for the job


First up let’s protect ourselves. Remember it’s all about safety girls! So first and foremost don a pair of sturdy gloves and a pair of goggles. Glass rather obviously is sharp and brittle so we don’t want any trips to A&E. You will also need a hammer, chisel, sandpaper, putty and a putty knife or similar (I use a narrow wall scraper). Putty wise please go for the good old fashioned linseed version. If it comes in a tube don’t touch it with a bargepole (more on that later). Honestly it’s rubbish!


How To Reglaze A Sash Window


Removing the broken glass before reglazing your sash window


So now we are equipped and suitably attired it’s time to remove the broken glass, like duh. But to remove the glass first we have to remove the old putty that’s holding it in using your chisel and hammer. This is my preferred method but some people like to heat the old putty up with a hot air gun and scrape it out. The only issue with this method though is that heat and glass do not a great partnership make. So best be careful not to catch any of the unbroken glass with intense heat as you will end up with more than one pane needing replacement.


How To Reglaze A Sash Window


Tidying up a window frame prior to putting in new glass


Once all the old putty is removed its time to take out the glass. Carefully but firmly hold it with you hands and give it a wiggle and a tug. Remember to wear your gloves and goggles for this part too. You may find some tiny nails holding the glass in place at this point though. If so remove them with snipe nose pliers (pointy ones). These would have been used to hold the original glass in place whilst the putty was applied.


When the glass is all finally out you will now need to remove the old putty that sits behind it in the return. Once again carefully use the hammer and chisel to remove it. You will find if the putty is particular old that it’s quite brittle and will knock off in chunks with a well aimed chisel. The important thing here though is to make sure your chisel is sharp. A blunt chisel will literally make a mess! It’s a this point that I give the rest of the window a check over too. Removing any loose paint, other loose bits of putty and give the whole frame a good sand down.


how to fit the glass when you reglaze a sash window


So now we have a window that is thoroughly prepped and ready for its new glass. At this point especially with newer windows give any bare wood a coat of primer. This part is important as the new wood can absorb the oil from the putty causing it to shrink and crack. Once the paint is dry it’s time to apply our first bit of putty but before then let’s take a look at what putty actually is and the best way to use it.


what is window putty?


Traditional window putty is a mixture of chalk dust and linseed oil. It has a wonderful texture, is very therapeutic to roll in your hands and has a smell that takes you back to easier and less complicated times. Oh, and it leaves your hands incredibly soft too. BTW if you see window putty in a caulking tube do not buy it. It is not the same as traditional putty which comes in a tub and costs only a couple of quid. There are now a couple of things you need know though before the putty is ready to use.


Firstly, traditionally putty when stored should be turned regularly from top to bottom. The reason for this is that the linseed oil in it cannot battle gravity and makes it’s way to the bottom of the tub. This causes the bottom to be too gooey (technical term) to use. And conversely the top to dry out and become lumpy and not pliable enough to use. More often that not when you purchase a new tub it will never have been turned. And so you need to give it a really good mix. I tend to dig some out with a scraper and then keep working it in my hands until I’ve mixed the whole pot. It will be a bit ickie at first but eventually it will become smooth and will no longer stick to your fingers.


Secondly, putty does not like cold weather. It goes hard and stiff (phanah, coughs and then giggles like a school girl). And so reglazing a window is always better done in warm weather if you can. Having said that I’m doing mine in early spring due to the fact that I have a smashed pane of glass and no choice other than to do it now. I just tried to work quickly and kept warming the putty up my massaging it in my hands.  OK so we know a bit more about putty, let’s get on and get the glass put back into the window frame.


Measuring up and ordering your new glass


What depth of glass should I use when replacing glass in a sash window?


Before we fit the new glass of course we first need to order it. Measuring up to be honest is my least favourite part and by far the hardest step too. I always worry that I will end up with a pane either too small or too large! Not only that but with a sash window in particular you will also need to think about how thick the glass is. Sash windows are such an amazing design but they are delicately balanced. When you lift a sash the counterweights in the side boxes that attach via sash cords to your window help to lift it and keep it open at the exact height of your choice.

This balance is very delicate. So if you replace your glass with a thicker version than the original you risk your sashes no longer working properly. They will be harder to open and due to the weight of the glass will not stay open either. They will either close themselves very slowly or at lightening speed with a deafening thud which can result in yet more broken panes. As a rule of thumb though 3mm glass is usually what you want when it comes to sashes. Oh, and for those that have broken sash cords (goes with the territory I’m afraid), I’ve also done a blog post on how to replace those too.


measuring up the length and width of glass for reglazing a sash window


So now we know that more often than not we need glass that is 3mm in depth. You can also double check this by measuring the glass you have removed. But what size of glass do we need width and length wise. This is the measurement that you have to get 100% right. Too small and you’re buggered! Too large and although not completely buggered you will be making another trip back to your glass merchants to have it resized.


So measure once, then twice and then three times! Firstly the width and then the length. You need to be very accurate at this point so if you can please do measure in millimetres. This is called your tight size. As in this is the size of the frame but….. And there is a but! If your order this size you will not be able to get it into your frame and add putty too. So you take the tight size and deduct 5mm from both the width and the length. This then allows for the use of putty, a rather important aspect of reglazing. So just as a for instance. If your window frame measures 200mm x 400mm you will need to order a piece of glass 195mm x 395mm.


How To Reglaze A Sash Window


preparing your window frame with putty


Every window with single glazing actually has two lots of putty applied. One lot for the inside of the glass and one for the outside. It’s a bit of a glass sandwich! So first up we need to apply the putty to the inside of the frame. This will create a seal for the glass with the return on your window frame. Pick up some putty from your tub. Not too small a piece that you will constantly be picking more up. And not so large that you can’t manage it in one hand. Then knead it in the palm of your hand until it’s warm, smooth and pliable. And then create a sort of teardrop shape with the top of your teardrop level with the tip of your thumb.



Now squidge the putty into the return of the window frame. Push it in with your thumb from the back of the frame towards the return. And then use the edge of the return to scrape your thumb against leaving a triangle shaped wedge of putty in the frame. It takes a bit of practice to speed up at this point. But you will eventually be able to keep feeding putty up towards your thumb in a continual motion rather than doing a bit and then another etc, etc.


How To Reglaze A Sash Window


fitting the new glass into your sash window frame


Once the putty is applied around the whole frame it’s time to offer up your glass. Now a glazier will probably have suction cups that they can utilise for this step but I don’t have any and nor do I need them. Though it is a little trickier without this aid. Remember your new glass is 5mm smaller in both directions that your actual frame. And so if we just offer the glass up by leaning it on the bottom rail of the window chances are it won’t be tall enough and you will have a gap at the top.


So before we offer up the glass place the blade of your stripping knife on the bottom rail. This will raise the glass up that precious couple of millimetres that you require. Stripping knife in place it’s time to put those gloves back on and to get hold of your glass. Place one hand around each side around half way up the pane. And then angling the lower half towards the window place it centrally onto your stripping knife. Now gently and slowly push the top towards the frame too. And once in place, gently but firmly press the glass back into the frame. You will see the putty that you applied gradually spread on the other side of the window creating a seal with the inside of the frame.


How To Reglaze A Sash Window


Applying putty to the outside of the window


With your new pane of glass in place you can now remove your stripping knife. The putty will also have pushed through the small gap between the glass and the frame and it will now stay in place without the help of the stripping knife. At this point you can now also add in some small panel nails to hold the glass in place. One small nail close to the glass in the centre of each pane directly into the frame is sufficient. I haven’t done it here though as the pane of glass I’m fitting is quite small and should stay in place without using nails as extra security. Be careful though as at this point it is easy to break your new pane of glass. A hole punch will help to keep the hammer away from the glass


How To Reglaze A Sash Window


And now to add more putty. Using the same principle as the first time around apply a bead of putty with your thumb. Working from the inside/glass out towards the outside frame and using this to scrape the putty off your thumb. Again creating that wedge of cheese effect. If the weather is colder or you are new to glazing then work on one side at a time as the putty will remain more pliable and easy to work with. Once applied it’s time to use your stripping or putty knife to create a smooth bead of putty. Insert it at an angle into of the corner of the frame.


How To Reglaze A Sash Window


And then draw  your knife along the putty. Keeping one side of the knife on the frame draw it down whilst trying to keep the other side of the knife where it’s touching the glass at a parallel distance. I rule of thumb here is to use the side return of the window as a guide for how deep your outside bead of putty should be. When you get to the other opposite corner it takes a bit of practice to make the same angle but you kind of slide the knife out at a angle towards the corner. A trick with this is also to keep your pressure nice and even. And don’t worry if it doesn’t go right as first. Just collect up your bits of putty, knead it and use it again. There is hardly any waste with this job.


How To Reglaze A Sash Window


You will at this point most likely be left with putty on your glass which is so easy to rectify. Do not be tempted to pick at it or you could undo your amazing work thus far. Instead grab a small piece of unused putty and gently dab it against the putty needing removal. You will find it sticks to it very easy and as you lift your hand away the unwanted putty will come away with it. And as mentioned earlier too, pop this putty back in your tub as it can still be used.


How To Reglaze A Sash Window


Finally, using the softest of paint brushes gently sweep it over any imperfections in your finish. To be honest you are supposed to dip your paintbrush if chalk powder at this point but I have never found any to buy at my local DIY store and as long as I am careful with the paintbrush I haven’t found this to be a problem.


Tidying up the putty on the inside of the glass


How To Reglaze A Sash Window


Remember when we first popped in the glass? Remember how pushing the glass inwards seals it to the frame causing the putty to squeeze out the other side? Good, it’s best not to forget this last step. So grab your stripping/putty knife along with your paint brush and head inside. It’s time to tidy up this side too.


How To Reglaze A Sash Window


Using your knife draw it down level with the frame to remove the excess putty. remeber to save this too for another project too. Finally give it a brush to neaten it up. And then stand back and admire not only your work. But also the fact that you took on the task and smashed it! All you need to do now is leave it a few days before giving the whole window a paint. First give it a coat of undercoat and then two top coats of your chosen top coat colour and finish. The putty will remain quite soft for some time, that’s just how it works. But it does form a skin at some point within the first week (weather dependant) and it’s at this point that paint can be applied.



And so there you have it. Should you ever need to replace glass, that’s my ‘How To’ reglaze a sash window ( or any window for that matter) all complete and done with. What do you think? Would you give it a go? Here’s another guide that I quite liked too with a slightly different technique.


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