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How to Fit a uPVC Window

Updated on January 21, 2014


If you are good with your hands than why pay other people a lot of money to fit a window. Most people would be able to to do this and with a little know how you will be fitting your own uPVC windows.

Types Of Fitting

There are two main types of fitting
1: Pin in - This is were you don't have to take out an old window. All you are doing is standing the new window in and fixing it to the wall and someone else is plastering up. This happens on new builds
2 : Full fit : This is were you are taken out old windows and fitting a new PVC window and finishing up the walls if damage was done.

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How to Measure for a new uPVC Window

First thing you must do before you can order a window is measure it. There is a few different ways and below is a guide to help you measure your windows

Measure on a new build
Width : measure in three places top - centre - bottom and use the smallest measurement than minus 20mm from your smallest measurement. This is your width
Height : measure from the head to the top of cill again check in three places and use the smallest measurement than minus 10mm from the smallest measurement this is your height.
Note : you are reducing your measurements because this will allow your PVC window to expand in warm weather. Plus if the walls are not 100% level you will still be able to but your new window in level as you will now have room to move your window around a piece.

Measure for full fit with old window coming out
If you are going to replace a window the way to measure it is different from a new build.
Width : measure from reveal to reveal in three place top - bottom - width use the smallest measurement than plus 14mm to it this is your width
When you are fitting your new window it will be 7mm behind the plaster reveal on each side. But it will need to go into one side by 14mm so you can get window into place. So if need put a chisel in one side to check how far the old window goes behind the reveal.
Height : measure from the top of window to the cill in three places than minus 25mm you minus here because you want your measurement to be to the top of cill.
Double check if needed - If you think the top of cill is more than 25mm up it is best to chisel away a bit of the window. You will be tearing out the window soon, so it don't matter about doing damage to it.

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Different Types of Fitting

Fitting your new window (pin in)
Pin in : first check the cill and make sure it is level. Now put a run of silicone across the top of cill. Stand your new window in and make sure it is level. Than all that left is to fix the window to the wall with the fixing brackets.
If the cill is not level than pack under one side of the window so that it is level otherwise your sash openings may not work properly. Plus if you do have to lift one side make sure you fill in the gap. Once you have something solid to take the weight of the window silicone will be good enough to fill the rest of the gap. If the gap is bigger than 5mm than Maybe you should get the cill redone, as it may lead to trouble later on.

Full fit & taken out old window
There are two ways you can do this. The first one is were you don't need the window that is coming out. So you can break it up and do very little damage to the surrounding plaster. The second is were you want to save the old window this requires a lot more work as explained below.
First type breaking up the old window
First take of all the sashes. Than remove any other glass that is in the window. Sometimes it is just easier to break the old glass out depending on how it was put in. If you are going breaking the glass, get someone to hold a blanket up and than break the glass into it.
Once all sashes and glass is removed. You now cut out any mullions or transoms.
Now on the bottom of window cut in the center and prise upwards until it breaks. Once you have the bottom of the window out it now time to take out the sides.
First run a sharp knife around the window on the inside. This will help separate the plaster from the window. Than when you are taken out the side the first thing you do is get a nail bar in behind it and prise the bottom out slowly. When you are prising keep an eye on the plaster on the inside. If you see a crack starting to come stop straight away. This is more than likely were the fixing bracket is that is holding your window in place. Just get a chisel and drive it in between the window and plaster to break the fixing bracket. Once done prise the window frame some more and keep an eye on the top because in most windows there will be four brackets. Two on ether side, plus sometimes you might get one on the head. Remember every window is different it all depends on who put them in. So always take it slowly when prising out the window frame and watch for cracks.
Fitting your new uPVC window (Full fit)
To fit your new window just slide it into the cavity at one side. Than get your window all the way in on the cill. Once you have the window in on the cill just slide it so that it is behind both reveals. Once this is done you will than need to lift your window so that it goes up behind the reveal on the head, 5mm should be enough to lift the window. Once done you will now need to fill the cap between the bottom of the window and the cill. Once the window is sitting on something solid in 3 or 4 places you can just fill the rest with silicone. If you had to lift the window up more you might need some cement to pack under the window. Once you have filled under the window, clip on the cill adapter. This is a strip of PVC which clips onto the bottom of the window and steps around the cill to keep the weather out. Now just put a run of silicone around the outside and depending on now much damage is done on the inside a small bit of patching plaster should do and a run of caulk around the window.
To hold your window in place you can cut away some of the plaster and put fixing brackets there. Or what most people do is pump expanding foam around the window this will hold the window and seal the window right around.

Taken out old window in one piece.
Some people may want to save the old window for a shed or something. If this is the case you will need to take the plaster of around the window until there is enough room for the window to fall inwards. Once you have the window out. Just stand the new one in, but just put a run of silicone on the cill first.
If you are planning on saving your old window. You should first way up the cost. You have extra work in taken it out plus you have a lot of extra work in finishing off around the inside. It may work out cheaper to just buy a second hand window somewhere for that shed.

Gear needed to fit a uPVC Window

Silicone Gun
Patching Plaster
Expanding Foam
Nail Bar
Screw driver
A bit of Know How


Reading this should give you some ideas on now a uPVC window is fitted. It does not cover everything buy it will hive you a general idea on how it is done. Depending on the part of the world that you live in. There is different types of methods of fitting windows. But if you are anyway handy with DIY you should have no problems at all.

If you have any comments or helpful hints just leave them in the comment box below. It may just help someone out.

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