The replacing of most window frames is straight forward, while others can be challenging, much depends on the type of window frame that is being replaced.
In older properties the most common window frame was the Sliding Sash and this type of frame was made and fitted in its own unique way. The main reason for this is to do with the sliding sashes. For them to work smoothly, weights and cords were incorporated into the frame design, which means that the sides of the frame are of a hollow box construction, making the actual frame much wider and deeper when compared with it modern day counterpart of similar size glazed area.
When fitted, the majority of this hollow box area is hiden behind the outer brickwork, so that when viewed from the outside only about 25 mm of this part is be visible.The cut through plan view of the drawing here on the right illustrates this.
For this reason, the sliding sash window would be fitted from the inside, behind and up to the outer wall of the cavity wall brickwork, and resting onto a decorative shaped stone sill. The outer brickwork would have either a decorative lintel, or arched brickwork for support. Internally, a timber lintel would span the internal brickwork.
It was also normal practice in the Victorian period that the internal finish surrounding the window frame would be timber lined and finished with architraves.
Given this, it follows that when replacing a Sliding Sash Window with a modern frame that some consideration be given to how the new frame is to be fitted and where it will be located in relation to the brickwork. There are two options available, which are; as the original, which would be internally, behind the outer wall, or in the outer brickwork opening, set back about 25mm from the face. No matter which option taken, the work can get quite extensive, even if the new frame is fitted as the original, this is due to the extra width that was required for the box section design of the sash window.
The modern replacement frame requires less space, and in addition to this, the depth of the new frame is smaller as the openers in a casement frame are all fitted to a single rebate, where the sashes of the Box Sash window are fitted into two channels one in front of the other.
In my opinion, fitting as the original is the easiest option to take, as fitting the frame to the outer wall means removing the stone sill, and making good the opening with matching brickwork. Internally too, extensive making-good is required with both options.The drawing on the left shows a typical arrangement of the brickwork when the old Box Sash window has been removed. In this case the cavity has been closed, and has a vertical DPC to stop the moisture from traveling across the brickwork closer.
Please note that before proceeding with any work of this type you must first satisfy yourself that there is a lintel in place, so that when the old frame is removed the brickwork above is safely supported.
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