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Replacing complete parts of timber windows

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Cills and bottom rails are the most common widow component that may look like they need complete replacement.  Cill replacement is possible but it’s not always easy to get all of the old cill out from under a frame and will usually cause disruption on the inside.

Opening Sash bottom Rail replace

As hinged or sliding sashes can be removed from the frame it’s job for workshop.  The sashes can be either totally dismantled. Any rail or stile can be replaced replaced with or without taking the glass out.

Window sash bottom rail rotten, easier to replace than repairBottom rail of sash replaced with new timber

You will have to work out how the joints can be reassembled or reworked especially if the adjoining part has suffered. New tenons can be spliced in, mortises, previously with wedged tenons can be cut and a double lap joint used instead. These can be glued and pegged or glued and screwed depending how traditional you want to be. Steel angle reinforcement brackets can be used but I prefer to avoid them.

Sash window rail removed, stile cut for splicing new pieceSash rail replaced and joint rebuilt with spliced piece

Most other parts will only have bottom end damage and so new pieces can be spliced in as long as the joints maintain the required structural strength and loading is passed to good old timber.


Window cill replacing

The frame would need supporting this can be done by temporarily screwing pieces to the frame, supporting and stabilising where required.  You may consider removing the glass first.

New cills can be bought ready moulded, and then planed to the exact sizes if required. Bottom rails can also be bought ready moulded although matching sizes might be difficult to find.  If you have the equipment you can machine your own sections.

An alternative requiring minimal tools is to fabricate your own by gluing various sections together to get the rebates, slopes etc.  With modern adhesives fabricating is an option.

An easier option for a protruding cill is to cut of the exposed and rotten part and replace following similar guidance for splicing. 

Sliding Sash Window Cill Replace

The esiest way to replace a complete cill on a sliding sash window is to remove the frame but if thats not an option:-

Sliding sash window cills are more difficult as the outer linings and pulley stiles are attached to them but as these are highly prized windows it can be a job worth doing.  The Sliding sashes should be removed first and after stabilising the frame then middle of the cill can be cut out. The remaining ends can usually be split along the grain to detach them from the linings and pulley stiles. 

Refitting a complete new cill will have to be done from the inside.  If all the inside mouldings and linings are removed it is possible to fit a complete new cill with glued joints for the stile and linings.

To minimise disruption to the interior decor there is an alternative. The cill can be cut in two with an angled cut .If the bottom of linings are cut as required, each end can be can be fitted and the centre joined. OK not simple but it could well be worth it.



Fixing loose joints

Swollen sticking sashes

Fixing hinges

Replacing window parts

Then: Filling and sanding window repairs


Previous: Making good a timber frame or sash

Please post any questions, comments or suggestions over on the blog


Window repairs intro and index

Investigating the extent of damage

Paint stripping windows

Making good a timber frame or sash

Other Window problems and considerations including sash window double glazing

Replacing window parts

Fixing loose joints

Swollen sticking sashes

Fixing hinges

Filling and sanding window repairs

Glazing and putty

Priming and painting