Timber lintel obstructing fitting of sash windows

Hi Matthew. Let's see if I can explain a few things for you. Traditional stepped checked reveals. Designed to show minimal sight lines. Timber lintel= Bressumer. So this is all correct. This is normally about 2" or 50mm above the centre of the sprung arch or level soldier course. So your joinery would take 2 - 3 measurements. Spring line to spring line. Width spring : face of distance between both reveals Excluding pocket cheeks. Height: Top of cill below to spring line of arch. Lowest point. When fitting the window you can reduce the outside face of the box sash to not interfere with the bressumer. Some times there is a void between the bressumer and the external brickwork to allow you to slide box sash in from the bottom up and not pust straight in, so that why there's a lower void behind the stone cill, they would then cut timber stools below to wedge window in. Folding wedges to each reveal to secure and level box. Then you can nail or screw box at head to bressumer. Not a lot of fixings. Then expanding foam in. Make sure your box joinery has no opening on back box with cheap cardboard. As the foam could get in box or squeeze on box. Thus restricting weight movement. So all your missing is knowledge, a good carpentry joinery book will advise you with some good illustrations. Makes sure you order weights with window or are you going for springs. Don't forget to request a free mouse from the window supplier .

The top of the old sash windows wasn’t as big as the sides where the weights was housed or the bottom where there would have been a thick timber sill. I would speak to the person who makes the new timber sash windows and get the dimensions of their product. If you were to replace with new plastic sash windows you would measure between the external brickwork and stud out behind.

Reduce the height of the working part of the window and make up the difference with an (add on profile)

Hi there, if the wooden lintel is obstructing the height of your windows, you can cut the exact amount carefully off, and make sure the windows fit tight up to it so there is no slack, and the windows will be a nice snug fit