A detailed annual survey and analysis of timber sash windows is helpful to determine the extent and precise  causes of any deterioration, and the category of renovation involved. You can do this yourself or employ a professional. If you choose to do this yourself, it is best to record your observations on a sketch elevation of the window.

Look for the following key points:

  • any signs of structural movement which is deforming the opening and damaging the window. (Remember that some signs of movemenet are so old that they have long since been stabilised or repaired, leaving the window in working order; its deformation expresses its age and character.)
  • evidence that the pointing of the frame to the wall opening is cracked, loose, or missing, allowing moisture to penetrate the sash-box timbers.
  • sashes that do not move properly, or at all. This may be due to overpainting of the joinery, stop beads that have been fitted too tightly, pulley wheels that have seized up because of overpainting or lack of lubrication, broken sash cords, swelling due to water absorption, or ineadequate lubrication between the sash and the pulley linings.
  • evidence of water absorption, indicating possible wood decay (wet rot). The signs to look for are:
    • interior paint failure caused by condensation
    • exterior paint failure
    • opening of the frame joints
    • degradation of the wood surfaces (the paint has flaked off) or depressions in the wood surface
    • cracked, loose, or missing putty
    • standing water, especially on the sills

It is important to ensure that water does not enter crucial joints, such as the lower parts of sills or jambs, where deterioration most often occurs. Joints should be kept tightly closed. In addition it is helpful to seal end grains with paint before assembly. You should also watch for any putty failure (which encourages water to sit on flat surfaces of the glazing bars and meeting rails) and for deterioration in the protective paint finish.

If the timber has been affected by rot, the underlying surface will be soft and fibrous. The wood’s moisture content can be measured with a meter; repeated readings of over 20% indicate that wood decay is likely unless steps are taken to dry out the area at risk and to eliminate the cause of the dampness.

It is easy to repair affected areas by cutting out the rotting wood and replacing it with a piece of sound, treated timber. Epoxy resins are sometimes used as a substitute for treated wood in these patch patch repairs.  However, it is important to paint over the repaired area as soon as possible, as resin degrades in ultra-violet light.