To find the height, measure from the top of the window’s head jamb  to the bottom of the cill  . Measure at the left, middle and right of the window. Use the shortest measurement for the height of your replacement windows

Removal of existing window

1. Cover immediate site with durable dustsheets inside and out.
2. Using screwdriver/nail bar remove all opening sashes.
3. To remove glass in remaining window fixed lights (wear the Heavy Duty gloves and goggles) carefully crack glass with the aid of a screwdriver/punch and light hammer. Always start at the top corner from inside the window, remove cracked shards and deposit into plastic bin. Once all glass has been carefully removed you are ready for the next step.
4. With the aid of a Stanley knife on inside of window, break plaster seal around periphery of window frame.
5. Using crosscut saw, cut through the remaining vertical (mullion) and horizontal (transom) frame members and remove.
6. Paying attention to minimise damage to plaster, brickwork and damp proof membrane cut through the outer frame jambs head and cill of the window frame.
7. With careful application of the nail bar, it should now be possible to lever the framework away from the plaster line with minimum disturbance.
8. Remove any projecting cement accumulations with your masonry chisel and clean away any debris around the opening.

Fitting the New Window

1. Cill (where required): Position the cill profile onto the brickwork, such that the up stand is snug against the plaster line of the jambs (it may be necessary to trim the ‘horns’ to fit around the brickwork.
2. Using the plastic packers, level the cill with approximately 5mm clearance between it and the brickwork.
3. Secure the cill to the brickwork using the 8 x 100 mm fixing bolts positioned approximately 150 mm from each end and at 600 mm centres thereafter. Check for level and do not over tighten the fixing bolts.
4. Run a bead of silicone along the back edge of the cill up stand.
5. Using silicone or superglue, secure the end caps into position. They may need trimming to size if the cill horns have been cut around the brickwork.
6. Remove all glazing beads from the window frame, marking them accordingly to ensure they can be replaced into the original position.
7. Offer the new window into position, with the base snug against the cill up stand, ensuring a tight seal into the bead of silicone. Clean off any excess silicone, which might occur. With spirit level, ensure the new window is positioned vertically and where appropriate hard up against the plaster line.
8. Wedge the window into position using the plastic packers provided.
9. Do not bend the framework by over packing.
10. Open the vents of the window to enable access to the outer frame jambs. Using #8 x 40 posidrive screws, secure the bottom of the window to the cill, 150 mm from each internal corner and 600 mm centres thereafter.
11. Secure the window jambs into the brickwork surround using 8 x 100 mm fixing bolts, again 150 mm from each corner and at +/- 600 mm centres thereafter.
12. In all cases ensure that the screw heads are not standing proud as this may interfere with the glass positioning.Note: it is not advisable to attempt bolt fixing in the head of the window, as this may permanently damage the structural lintel of your window, the best solution for fixing the head is to use expanding foam available upon request.
13. Close and lock all opening sashes, check for square ness within the outer frame.
14. GLAZING Prior to locating the glass sealed units into the approximate aperture, it is necessary to position 25 x 100 x 15 glazing bridges into the frame recess. A dab of silicone will secure the packer in place in preparation for the next step.
15. Position a 28 x 100 x 5 glazing packer onto the bottom glazing bridges. In the case of side hung sashes also glue or silicone secure an additional 28 x 100 x 5 glazing packer onto the glazing bridge opposite the hinge side.
16. Offer the appropriate glass sealed unit into the aperture, resting squarely on the glazing packers, ensuring the glass is central within the opening. Push the glass back into the rebate as far as it will go without using undue pressure.
17. Using hand pressure only, position additional 2 mm glass packers to square up where necessary.
18. Gently unlock and open the sashes to check that no binding occurs within the locking system and no movement occurs between the glass and frame, minor glass packer adjustments may be necessary to achieve perfect squareness.
19. Clip back the pre marked glazing beads into the original position in the following order 1) top 2) Bottom 3) sides.
20. Carefully gun a bead of silicone between the outside masonry and window frame periphery, wiping off any excess.
21. Similarly, gun a thin bead of acrylic between the inside plaster and window from the periphery. Allow this to skin over (1 hour).
22. Remove all protective tape from the faces of the window frame, clean where necessary with warm soapy water.