lets-do-diy.com logo lets-do-diy.com stripes
Account login  |  Register
Bookmark This Page

    Window replacement

    Summary: Learn how to replace a window, measure for a new window and remove an old window.

    The main window styles found in houses and flats are casement, sash, pivot, awning, bay and bow.

    Casement windows

    Casement windows will have one or more opening sections called casements, which are hinged at the side of the frame. Smaller casements may also be incorporated into the window's design but hinged at the top of the frame.  

    Sash windows

    Sash windows consist of two sliding panels. Traditionally, they were operated by lead weights, but modern sash window will have spiral balances.


    Pivot windows

    Pivot windows have a centrally located pivot mechanism, allowing the window to tilt in any position. The pivot mechanism can either be situated centrally on the vertical sections of the frame or in the middle of the horizontal sections of the frame.


    Awning windows

    An awning window opens from the bottom and pushes out. Traditionally it will have a window stay to keep it in the open position. However, modern friction hinges are being used with awning windows allowing them to be opened at any angle. 

    Bay windows

    The bay window will have a number of fixed and opening sashes or casements. A large bay window may be providing support to the wall above, so it is important to seek advice from a professional builder or a surveyor before you start to remove it. 

    Bow windows

    The bow window will have a curved profile but will comprise of a number flat casements. Once again you should seek professional advice if planning to remove a bow window as large bow windows will have reinforced mullions that offer structural support. 

    Window frame materials

    Window frames are generally made from one of three materials: timber, UPVC and steel.



    The traditional window frame material is timber.  Hardwood frames are very durable and only need to be protected with oil. They can be painted or have a natural wood finish applied. The main disadvantage is the cost as hardwood frames are expensive.  Cheaper softwood frames need regular maintenance and will be prone to rot if maintenance is neglected.



    Modern UPVC frames require little maintenance and provide excellent heat and sound insulation. Although white is the most popular colour, UPVC frames are now available in other colours and in wood-effect finishes. 


    Relatively thin steel frames can support large panes of glass, so are ideal if you want to increase the amount of light in a room. In the past rust has been a problem but today steel frames are treated during manufacturing making them low maintenance. The main disadvantages with steel window frames are condensation and heat loss. Installing double-glazing will reduce heat loss and building regulations may even make this a requirement. 

    Replacing a window

    Before replacing a window there are several important factors to consider. Firstly, you should consult a surveyor to ensure the window you are planning to remove is not structurally important. If the property is listed or situated in a conservation area consult your local Building Control Officer to ensure the existing windows can be removed and the style of window you are replacing them with meets local planning regulations. Finally, safety must not be overlooked. Ideally all windows above the ground floor should open to allow for a means of escape, but if the window is non-opening, it must have breakable glass.

    Here we look at replacing an old timber casement widow with one of a similar style but with a UPVC frame.


    Measuring for a new window

    Accurate measurements are essential for getting the new window to fit properly, so spend some time measuring the window opening and double check your measurements. Remember it is the window opening that you need to measure and not the old frame.

    • Take three vertical and three horizontal measurements. If there is any discrepancy between the three measurements use the shortest.
    • Deduct 10mm (1/2 in) from both the vertical and horizontal measurements. This will give you your frame size.
    • Only include the measurement for the windowsill if it is being replaced too.
    • With a timber-framed wall you will need to measure the thickness of the wall as well.
    • You can now order your new window from the supplier.

    Removing the old window

    • Do not remove the old window until the new window has been delivered.
    • To fit the new window you will need fixings suitable for the type of wall you have. To ascertain whether you have a solid masonry, cavity or timber-framed wall knock away some mortar or pull away some beading or cover strips from around the window.
    • Above the window there will be either a concrete or timber lintel. If possible try to gain access to the lintel, for if your lintel is timber and it is affected by wood rot, it will need to be treated before you install the new window.
    • On arrival check the new window's dimensions to make sure it is the right size and examine it carefully for any damage. When satisfied that everything is in order, you can begin to remove the old window.
    • If the casements are removable, unscrew them or lever them away from the frame with the panes of glass in place. If the casements are fixed, carefully remove the panes of glass as explained in replacing a pane of glass.
    • With the window frame empty of glass it can now be removed. Using a panel saw make a cuts through one vertical side of the frame at its top and bottom. The cuts should be made diagonally, angled towards the centre.
    • Use a nail bar to lever out the wedge-shaped section of frame. Try not to cause any damage to the wall, but sometimes a small amount of damage is unavoidable.
    • With this first section out of the way, it should be easier to remove the other sections of the frame. As you do so, remove any fixings which generally will be in the jambs or sides of the window.
    • Some windows frames will have protruding sections at the corners. These are called horns and also need to be removed.
    • When the old frame and any fixings have been removed brush away any loose debris. Also check that the damp proofing has not come away with the old frame or been damaged.
    • The method for removing a steel window frame is very similar except you will need to use a hacksaw or an angle grinder fitted with a metal cutting disc to cut through the frame. If using an angle grinder it is important you wear goggles, ear defenders and gloves. Also be alert to the danger of sparks from the angle grinder's disc causing a fire.

    Fitting a new window

    All new windows will have their own specific installation instructions, which should be followed. The guidelines below describe the basic method. To secure the window in position it will either have fixings that go directly though the frame into the wall or fixing brackets. If you are fitting a new windowsill, this needs to be fixed to the bottom of the frame prior to installing the window.

    UPVC windows will already be glazed, particularly if the window has double-glazing, but it may be possible to detach the opening casement/s from the main body of the frame. Otherwise, simply opening the casements will enable sufficient access to manoeuvre the window into position. Fitting the window can be awkward for one person, so it is advisable to seek assistance.

    • Lift the window and position it in the rough opening. The new window should occupy the same position as the one it is replacing.
    • Push wedges under the frame or sill so it is level and fits tight against the lintel.
    • Check using a spirit level that the frame is horizontal and vertical, adjusting the frame and wedges until the frame is in the right position.
    • Carefully insert wedges down the sides of the frame. Avoid using too much force as this can knock the frame out of position.
    • To check the frame is square, measure diagonally from corner to corner. If the measurements are the same, the frame is square. If there is a discrepancy between the measurements, adjust the wedges until the frame is square.
    • Open the windows so you have access to the sides of the frame. Using a power drill fitted with a long masonry bit, drill pilot holes for the frame fixings through the sides of frame into the wall at regular intervals to a depth of 100mm (4in), unless otherwise stated by the manufacturer. The number of fixings will depend on the size of the window, but generally they should be positioned about 300mm (1ft) apart. This means for a standard sized window three fixings on each side will be needed.
    • Push the plastic plug of the frame fixing through the frame and into the drilled hole. The plug may need to be gently tapped with a hammer to get the top of the plug flush with the frame.
    • Push the screw in to the plug as far as it will go. Tap the head of the screw with a hammer and then using a cordless screwdriver screw the fixing into the wall but do not over tighten as this can distort the frame.
    • Fit the other frame fixings, checking that the window frame remains square at regular intervals.
    • Fixings can also be fitted through the top of the frame into a timber lintel. However, this is not possible with a concrete lintel. 
    • You may be required to fix the bottom of the frame to the wall underneath, although some manufacturers do not recommend this.
    • Cut off the ends of the wedges and fill any gaps around the window with expanding foam. When using expanding foam it is necessary to protect the UPVC frame with masking tape, as the foam can damage the finish.
    • Once the foam has dried, use a craft knife to trim away any excess. Then to conceal the foam, apply a thin line of mortar around the frame. Wooden mouldings should be used to conceal the foam around timber framed windows and on timber clad homes. With UPVC windows the foam is concealed by fitting the cover strips supplied.  
    • The seal between the frame and the wall is completed with bead of silicone.
    • Finally, replace any casements that you may have removed.

    Return to top of page
    Tools Needed
    • Tape measure
    • Screwdriver
    • Claw hammer
    • Panel saw OR hacksaw (for metal-framed windows)
    • Pry bar
    • Spirit level
    • Power drill/driver
    • Masonry bit
    • Craft knife
    Materials Needed
    • New window frame
    • Frame fixings
    • Masking tape
    • Expanding foam
    • Silicone sealant
    Discuss Project

    Join an existing conversation or create a new thread related to Windows and doors in our DIY forum.