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Step-By-Step Window Installation Guide

Osborn Windows > Windows > Step-By-Step Window Installation Guide

Window Installation: A Step-by-Step Guide

Windows are of course key features of any building, lending it the desired character and style; allowing light and air to enter; and giving occupants a pleasant view of the outside. Careful, professional installation ensures the window looks as good as possible, is fully functional, and is completely sealed against water.

Here is our step-by-step guide to installing a window.


Measure the old windows – inclusive of a 10mm expansion gap either side of the frame; this gives room to square the frame in the aperture. The structure must be secure – and it must be completely safe to remove the old window.

Double-check the size, style and condition of the new window. Check size against survey size and actual opening size.

Remove as much of the old window as possible – glazing, hinges etc., – before taking out the frame. This will reduce the weight making removal easier and safer.

Break the bond around the edge of the frame with a sharp knife, then remove the fixings around the frame. Use a flat pry bar to take the window away from the wall. Ensure the surrounding area is not damaged.

Take off any sealant, debris or mortar around the opening. Once this is done, re-check the measurements.

Prepare the Sills (sometimes spelt ‘cills’)

Sill end caps must be securely fitted to the sill with superglue. The sill can be cut to the required size, or rebated to form a visually pleasing exterior horn around the brickwork.

Sealant must be applied along the sill to prevent water ingress – which might track into the brickwork behind.

Preparing the Window

Prepare the new window by taking off the corner protectors and protective film around the frame. Attach the sill to the frame using PVC screws. If any frame extenders are needed, they should be fixed on at this point. Add lines of silicone to head and sill edges to prevent any water ingress.

Add the New Frame

Place the new frame into the aperture. Use packers around the frame to help ensure the new window is level, square and plumb. The 10mm expansion gap must be maintained. Fixing holes should then be drilled around the frame. Apply silicone to screw holes then add screws to the frame, taking care not to over-tighten. Packers should be adjacent to where the screws go in.

Install Glazing

Remove beading strips, taking care not to mix them up since they must be returned to the same position afterwards. Clean the frame thoroughly. Add bridging packers and install the glazing against them. Add glazing packers to the gap around the glazing. Once in place, apply silicone to secure the glazing. Refit the beads, starting with the shortest ones. Clean the glass and frame, then secure the window to the internal wall with sealant. Remove all tape and clean exterior once again. Use masking tape to add a clean, straight line of sealant around the exterior of the frame. Check the window opens and closes.

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