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The replacing of most window frames is straightforward, while others can be challenging, as much depends on the window type being replaced, and how it was originally fitted.
In older properties the window was usually fitted from the inside, behind and up to the outer wall of the cavity wall brickwork, sitting onto a decorative shaped stone sill, so that most of the window frame was hidden. The outer brickwork would have a decorative lintel, or arched brickwork for support. Internally, a timber lintel would span the brickwork over the window frame.
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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It was normal practice in the Victorian period that the internal finish surrounding the window frame would be timber lined and finished with architraves. The Sliding Sash Window is a good example of this type of installation, and is shown in cross section plan view above.
When replacing a Sliding Sash Window with a modern frame, consideration must be given as to where the new frame is to be located in relation to the brickwork. This will either be as the original, which would be internally, behind the outer wall, or externally in the opening of the outer skin of the cavity wall, and 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 required for the box section design of the sash window for carrying the weights.
The modern replacement frame requires less space because it doesn't have to accommodate weights in its design, and this results in a dramatic difference in frame width if the two types were compared. 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 often 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.
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