How to Implement the Installation of Sash Windows?


Sash windows bring timeless charm, but getting them right requires a bit of know-how. Fear not! We’ll explore the process step by step, from measuring your window jamb to securing the frame and ensuring a snug fit. 

Whether you’re a DIY enthusiast or just starting out, this direction aims to make installing sash windows an accessible and enjoyable project for your home.

Measuring Your Window Jamb

To begin the installation of sash windows, start by measuring the window jamb.

  1. Measure the Height: Start fitting your sash windows by measuring the height of your window jamb. This ensures the window fits right and stays put.
  2. Find the Width: Next, determine how wide your window jamb is. This is key to choosing or making sash windows that fit the space.
  3. Check Diagonals: Make sure your jamb’s diagonal measurements match. This keeps your window square and well-balanced, making it last longer.
  4. Measure Depth: Get the depth right by measuring your window jamb carefully. This helps choose windows that sit securely and work well.
  5. Check Outside Sill Angle: If using a kit, measure the outside sill’s angle for a good fit. This step ensures your sash windows line up nicely with the outside parts, making the setup look great.

How to Install a DIY Sash Window

Here’s a DIY process for installing a sash window. It’s crucial to research before you start. If any issues arise, reach out to a professional for assistance.

Things You Will Need:

  • Glazing Hammer
  • Glazing Shims
  • Frame fixing screws
  • Drill 
  • Wood (if needed)
  • Sealant and Caulk Gun

Step 1 – Removing the Old Window

Firstly, removing the old window is essential, with the method depending on the window type. For windows with a small top section, glazing removal may be needed to access screws. If your window fully opens, screws might be visible without glass removal. However, taking out the glass lightens the window for easier handling. 

To remove it, use a filling knife and a glazing hammer to pry the beading around the glazing. After removing all pieces, access hidden screws, and if there’s expanding foam or additional trim, remove those too. Finally, untighten the screws and take out the entire frame from the window opening.

Step 2 – Removing the Old Sash Frame

The original sash frame must be removed if it remains concealed behind your window. Unfortunately, this also entails removing the original weights, a sentiment that can evoke sadness. However, recognising that the original window is no longer preserved, installing a new period window becomes a way to reintroduce some of the original features. To remove the old sash frame, cut away the wooden sides and extract the weights, possibly repurposing them as decoration. 

Additionally, eliminate the old wooden sill, allowing the new window to rest directly on the stone sill. In cases where the new sash window is fitted within the reveal, this step may be unnecessary. Using the brick-to-brick method with wooden packing ensures a flush fit against the original casement. The selected wood, cut to match the brickwork’s depth, is secured, filling the gap left by the removed frame.

Step 3 – Levelling the Base

You need to position the new uPVC sash window on a level base, and the suggested method is to use shims across your sill or place the window on a mortar bed. Lay several shims across the bottom of the opening and use a spirit level to ensure it’s perfectly level. If it’s not, you might need different-sized shims to level it. Make sure to use a shim in each corner below the jambs.

Step 4 – Lifting the Window into Position

Sash windows are notably heavier, requiring at least two strong individuals for lifting, potentially necessitating a platform for added height. Lift the window onto the shims or mortar bed, ensuring it’s centred with an even gap on each side. If the measurements are accurate, a 5mm gap should be present for a brick-to-brick installation. Those fitting windows within the reveal may have a larger gap but should prioritise exterior symmetry for an aesthetically pleasing result.

Step 5 – Packing the Sides with Shims

Ensuring your window is absolutely level is crucial for proper functionality. After levelling the base, use a spirit level to check for any inward or outward lean. If the window is level, pack the sides with appropriately sized shims, ensuring a snug fit to prevent bowing during screwing to the wall. Properly packed shims prevent the frame from being pulled into the bricking, preventing gaps on either side of the sash window. Use multiple shims if necessary for larger gaps, and pack the sides thoroughly, especially in areas where screws will be added. This meticulous approach ensures a secure and aesthetically pleasing installation.

Step 6 – Screwing the Window Into Place

The recommended method for installing a uPVC sash window involves fixing brackets, but limitations arise in certain situations, such as when there’s a shutter casement in front. In such cases, screwing through the frame becomes an alternative, although challenging due to concealed springs on either side of the frame. 

Top screws pose fewer obstructions, going directly through the frame. For side screws, placement behind stop caps at the bottom allows easy sliding for access. Middle screws go behind springs, requiring careful manoeuvring. Attempts to improve access included removing the sash and a channel balance. Although challenging, the process ultimately secured the window firmly. While fixing brackets is easy where applicable, alternative screw placements accommodate diverse installation scenarios.

Step 7 – Making Sure Everything Works

After securing screws or brackets, ensure smooth sliding and functionality. Check the window’s locking mechanism and verify no gaps between the draught brushes and the frame. If issues arise, like improper sliding or gaps, it may be due to inadequate frame packing or uneven levelling. Troubleshoot by adjusting shims – a bit of trial and error. In a personal experience, a gap issue was resolved by removing screws, adding more shims, and re-screwing without complications. This step-by-step approach ensures a well-functioning and properly sealed uPVC sash window installation.

Step 8 – Sealing around the Frame

Seal the frame externally once your window operates smoothly to prevent water ingress. Opt for a sealant matching your window colour or brickwork, such as white, brown, or red. This final step enhances both the aesthetic appeal and protective qualities of your uPVC sash window installation.

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Wrapping Up

Remember to double-check measurements, ensure level placement, and secure the window with suitable fixings when installing sash windows. Smooth sliding and a watertight seal are key. If using sealant, match it with your window or brick colour for a tidy finish. Troubleshoot any issues promptly, and don’t forget the importance of a snug fit against the original casement. With attention to detail and careful steps, your sash window installation can enhance your home’s charm and functionality.

Director at Chesterfield Window Centre | Website | + posts

In 2005, I founded Chesterfield Window Centre with a vision to redefine the window and door industry through a commitment to quality, innovation, and customer satisfaction. The journey began with a deep understanding of the market, identifying gaps, and envisioning a business that not only meets but exceeds customer expectations.

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