How to DIY Install a Sash Window | Colin’s Sash Windows Review

Being broke-ass DIYers, we never thought we would be able to afford a sash window. Not in a million years. A quick google will tell you even uPVC ones start between £600-800 – a figure well out of our price bracket!

Then I discovered Colin’s Sash Windows (uPVC) which start at an amazing £299 for almost any size. Yes – you read that right! £299 – almost ANY SIZE.

Is this a con, I thought? What about quality? Are they going to look like those dodgy windows on that house three streets away? Oh believe me – I was highly suspicious about sash windows at that price. But the videos shown on his YouTube and website made the window look fab – and even the reviews on Facebook were faultless too.

So, we decided to take the plunge and go for it – so we ordered a window! Yes just one, ‘cos did you read the bit where I said we were broke-ass DIYers? 😉 But anyway – I’ll be sharing all the details with you, from how we measured to how we installed to a little close-up review of the window at the end of this post too.

Oh, and if you’d like to see the video-journey of us installing the window, check out my Instagram highlights under ‘DIY Sash Window’.

My first piece of advice for this step is to think really carefully about how you plan on fitting the new window! We actually removed some of the plastic trims around our existing window so we could see how that had been installed to help us figure out how we’d be installing this new one.

There are two options for installing a uPVC sash window, and which one you choose will depend on how your house is built.

Option 1: Fitting Brick-to-Brick
This essentially means your window will be installed sandwiched between the brickwork either side of the opening. Windows installed this way may be slightly more proud – so if you have a stone sill, it may sit further forward, giving you less of a sill.

Option 2: Fitting within the Reveal
With this option, your window will sit behind the brickwork into the reveal either side of the opening. If you have a period house that originally had sash windows, this space may have been where the weights and pulley would have been. With this option, there’s more room on the inside of the house than the outside. I also recommend checking the depth of space you have available here against the depth of the window too – to make sure this option is totally viable.

Here’s a snapshot of Colin’s Measuring Guide:

Measure the external opening of your window, taking a minimum of 3 measurements across both the height and the width to check for any disparity. If there is, you should always use the smallest measurement.

If you’re fitting brick-to-brick, you should deduct 10mm to both the height and width. If you’re fitting within the reveal, you should add 40mm to the width and 20mm to the height. Once you have these measurements, re-measure and check the window will fit.

You can find all this info along with Colin’s Installation Guide in this handy PDF.

This is just a guide sharing how we installed our window. We’re DIYers and not builders or window-fitters, so please do not solely rely on this guide. We always recommend doing your own research and if in doubt, always seek professional advice.

If it wasn’t already obvious, you’re gonna have to remove the old window first. How your window is attached really depends on the type of window you have and how it opens. For our window which only has a very small top section which opens, we needed to remove the glazing to get at the screws. However, if your window fully opens, you may be able to see the screws when the window is in its open position and you won’t need to remove the glass. It is, however, often a good idea to remove the glass so the window weighs a lot less and will be easier to manage!

To do this, you’ll need a filling knife and a glazing hammer. Hit the beading around the edge of the glazing to pry it away from the glass.

Once you’ve removed all four pieces, you’ll be able to simply take the glass out and you can then access the screws hidden behind the glass. If you have expanding foam around your window, it may be necessary to remove this as well. We had lots of plastic trim on the outside which also needed removing too. Again, a filling knife works great for this!

Once you’ve removed any excess trim, you can untighten those hidden screws and simply remove the whole frame from the window opening.

If the original sash frame is still hidden behind your window, then this will need removing too. Sadly does indeed removing the original weights too, and yes, I was sad about this – but the way I see it is that the original window is long gone, I can’t preserve what’s now been left, so the best thing I can do, is installing a new period window to put back some of those original features.

To remove the old sash frame, you’ll need to cut away the wooden sides and pull out the weights (don’t worry, we plan on using ours as decoration elsewhere in the house!). You’ll also need to remove the old wooden sill, leaving the new window to sit directly on the stone sill.

Step 2.5 – Packing the Sides with Wood
If you’re fitting your new sash window within the reveal, then you won’t need to do this. But, as we have an original shutter casement around the window, we aren’t able to push a window into that space as the casement protrudes too much. So instead we’re using the brick-to-brick method, but packing the sides with wood and fitting it flush against that.

We selected some wood that was the perfect depth to match the brickwork. We then cut this to size and screwed in place, filling the gap of the frame we just removed.

You’ll need to sit the new uPVC sash window onto a level base, so the recommended approach for this is either to use shims place across your sill or sitting the window on a bed of mortar. We felt the shims approach was much easier.

Lay several of them across the bottom of the opening and use a spirit level to make sure it’s perfectly level. If it isn’t, you may need to use different sized shims to even it out. Make sure you use a shim in each corner below the jambs as well.

Sash windows are much heavier than usual windows, so you’ll need a minimum of two strong people to lift the window. If you’re lifting the window particularly high, it may be necessary to use a platform. We got SUPER lucky by finding a neighbour who had one, otherwise, we planned on building one at the last minute (we had MDF and wood at the ready!).

Lift your window onto the shims or bed of mortar you laid in the step above, and position it centrally, so there’s an even gap either side. If you’re using the brick-to-brick method like us and measured the opening correctly, there will be about a 5mm gap either side. You may have a larger gap if you’re fitting your window within the reveal, but you want to make sure the window looks central from the outside of the house.

Your window must be absolutely level in order to work properly, so although we’ve levelled the base, you’ll now need to make sure it doesn’t have any lean inwards or outwards. Use a spirit level to check this, as you continue through this step!

Providing your window is level, you should then pack the sides of the windows using the appropriately sized shim. This should be the perfect size for the gap, so when slotted in, it’s holding itself in position. You may have to use a couple of shims together if you have a larger gap and don’t have the correct size shim.

The packing shims will stop the frame from being pulled into the bricking (bowing it) as you screw the window to the wall. If the frame is pulled in, the sides of the frame will push out towards the brickwork and you’ll end up with gaps either side of the actual sash window. Make sure to pack the sides out really well and ensure you use shims in the locations where you’ll be adding the screws too.

The recommended approach to fitting a uPVC sash window is to using fixing brackets. There’s a video that demonstrates this here, however, I do think there are a lot of limitations to be able to use these brackets. I can think of many situations, but as an example, we have a shutter casement in front of our window – so clearly a bracket would have covered over this, preventing it from working and it would also have been visible.

If you can use the brackets, they do look incredibly easy to use – buttt, I reckon there’s a lot of period properties out there who won’t be able to use them. As an alternative, you can screw through the frame, however, unlike other windows, a sash window has spring either side concealed within the frame, so you’ll find there are very few suitable spots available to place your screws. So, here’s what we did:

Screws at the Top
These were the easiest as there were no obstructions – we just went straight through the frame.

Screws at the Sides
At the very bottom, we placed screws behind the stop caps, which are on the outside of the window. These stop the window from hitting the sill. You can simply remove the covers of the caps, and the whole thing will slide up, allowing you to add a screw behind it. You can then slide it back down and voila, you’ll never know it’s there.

The middle screws went in behind the springs. This was super tight and rather awkward to do and we needed to make sure the head on the screw was small enough not to interfere with the springs or the balance channels which fit further up.

We tried lots of different ways to gain better access, including taking the sash off altogether, which did help a little. We also removed a channel balance to get a screw in higher up too. I’m not sure if there was an easier way which we’d overlooked (do let me know if you have a better way!), but this is just how we did it and thankfully, the window does seem secured solid!

Once your screws or brackets have been secured, you need to check everything is sliding correctly and works. You should also check the locking mechanism on the window and ensure there are no gaps between the draught brushes on the window and the actual frame.

If there is a gap or your window isn’t sliding correctly, you may not have packed the frame with shims correctly, or your window also not be level. It’s a bit of “trial and error” in problem-solving any of these issues – we personally had a bit of a gap problem caused by not packing the frame enough on one side. To fix this, we removed the screws and added more shims. When we screwed back in the second time, there were no problems!

Providing your window is working correctly, you can then seal the frame externally so no water can get into any of those gaps around the frame! We used a white sealant for this, but you can also buy sealant in brown or red to match the brickwork if you have a coloured window.

To get the best finish from your sealant, I recommend investing in a Fugi Professional Sealant Kit, which saves you using your finger to smooth out the sealant and typically provides a better finish!

So that’s it – you should hopefully now have a lovely new sliding sash window that’s operating perfectly! But what about FENSA you ask? Yes, all new windows and doors do come under building regs which means any new installation should indeed be certified.

If you use an approved fitter, they can supply their own certificates. If you DIY install, you can have your installation certified by your local building control for a fee. I would recommend installing all new windows/doors first before having your work inspected, so you only need to pay once.

Alternatively, some people over on Instagram recommended indemnity insurance as another option – this is up to you and isn’t something I’ve fully researched yet.

So I’ve shown you how we fitted the window, but you probably want to see what it actually looks like as well, and how it works!

So as I mentioned earlier, Colin’s Sash Windows start at £299 for virtually ANY size (up to 2300mm x 1250mm) which is absolutely bonkers! This is their ‘base rate’ and then any extra or additional feature is charged as extra. We added 4 extra features – which I’ll show you individually and also give a breakdown of what each feature cost.

Our window is finished in smooth white (this is included in the £299) although Colin’s Sash Windows can offer any bespoke RAL for extra, or they also have a few set colours in their ‘woodgrain’ selection to choose from too. We decided smooth white was fine for us because it would at match our fairly new french door and also the windows at the back of the house too – which, at the moment, we don’t plan on changing.

The bars on the window are known as astragal bars. These are a decorative feature which can be added to a variety of different styles. So for example – you may want a grid of astragal bars in the “Georgian style” or a single one down the centre of the window. Ours is the “offset three light” style, which matches the neighbouring properties on the street – which you can compare below. Our bars are definitely spaced a little further apart, but I think it’s a pretty decent match otherwise!

These bars make your window appear to have separate panes of glass – but they are in fact, planted on the outside with a filler panel in the middle. It looks pretty authentic if I’m honest! These astragal bars are charged as extra and the cost depends on which style you chose. The style we have was an additional £48.

Another feature we opted for was the chrome window furniture (the standard is a white finish) which was an extra £10. A really small price actually, considering it makes the window (in my opinion) feel more expensive in design. This furniture includes a lock, a pair of travel restrictors, tilting knob, lift hooks and pole-eyes. The hooks and pole-eyes come unattached so you can choose whether or not you want these added. We’re still undecided!

The travel restrictors are really clever and basically, prevent the window from opening too far as both a safety and security feature. When left in their open position, as the window slides up, it hits on the restrictor stopping it going any higher. This stops small children from being able to fully open the window and it also stops the bad guys externally being able to fully open the window.

If you want to open the window further, you simply need to push the restrictors in and then the window will open the whole way. These are out-of-reach for children and give you peace of mind for both situations.

I love the fact that both sash windows slide and they also both tilt, allowing you to clean the windows from the inside. I think this is my favourite feature actually as it means no more window cleaners!! Both of these features are standard with all Colin’s uPVC sash windows.

There’s also a brush draught excluding seal around the window – which feels really high quality and so far, is working like a dream. No draughts with this window! Which is a huge improvement from the old window, I have to say. This knob shown below allows the top window to tilt.

A distinctive feature of sash windows are the horns on the external of the window. You can pick from two types of horns; plant-on horns or run-through horns. The later is an additional £25, but I think makes a huge difference visually to the window. Plant-on horns are just as it sounds, made attached separately and planted on. Run-through, by comparison, is made continuous with the frame. I think the second look more authentic, but both add a lovely decorative feature on the outside!

The last feature we opted for, was a deep bottom rail, which also includes an upstand. This basically means the bottom part of our window is deeper, therefore has less glass. The reason we decided to add this on was that our internal window is quite a bit higher than the external sill and when we measured everything up, we were concerned the bottom would look unbalanced visually as part of the frame would be ‘eaten’ by the internal sill. This comes with an upstand (an internal decorative feature you can also add-on) and was £48 extra. This is probably something you wouldn’t need, but apparently, people also choose to add this on as it’s considered more authentic to original sash-window designs.

So in total, our window would have cost £430 with the four additional features we opted for; astragal bars, chrome furniture, run-through horns and a deep bottom rail. Without these features, it would have been £299 – so I hope that gives you some idea as to what you’d get for that price!

Colin’s Sash Windows all come with A+ rated glass and the bottom sash is also toughened glass. For the price we paid, I think this is seriously amazing! I’m absolutely over the moon with the window and literally couldn’t be more pleased. We’ll definitely be planning to replace the other two windows at the front as well when we get a bit more dollar together.

DIY fitting the sash wasn’t super hard, it was, however, a bit fiddly and we spent most of our time just figuring out the best way to go about installing it. Definitely DIYable though – but you’ll need some strong persons around to help to lift!!

I’d love to know what you think – and please do check out Colin’s Sash Windows if you’re looking for uPVC sash’s, as I’m sure you won’t be disappointed!

*I received a discount on this window in return for sharing this blog post on its installation. Thanks for supporting this blog!

**Post may also contain affiliate links.