Timber windows – rotted sill
Causes
Water sits on the sill if the slope is not sufficient to shed it
Water is getting into the sill/jamb junction
Poor initial substrate preparation
Oil-based (alkyd) paint was applied over water-based (acrylic) paint
The sill was painted a dark colour, increasing thermal expansion/contraction of timber
There was moisture absorption into timber end grain because of inadequate priming
Cause
Water sits on the sill if the slope is not sufficient to shed it
Repair
- if just a section of the sill alone is damaged, cut out and replace the damaged section:
- take out the window and remove the sill from the jambs and mullions
- check that the ends of the jambs and mullions are in sound condition. Replace them if not
- sand the jambs and mullions and prime
- cut out the damaged section of sill and install a new fully-primed section of sill to match the existing. Take a sample of the sill to a joiner to get an exact match if necessary
- reinstall the window and finish with a minimum of two coats of enamel paint
- if the extent of rot is significant, remove the window and sill and replace the entire sill with a pre-primed replacement sill. This may be newly-made to order, or may be second-hand if a matching unit can be found in good condition. If possible, consider a sill with a slightly greater slope to more effectively shed water
- reinstall the window and finish with a minimum of two coats of enamel paint. Follow a maintenance plan to ensure the coating remains in good condition.
- for more details see the BRANZ Good Repair Guide Timber windows.
Cause
Water is getting into the sill/jamb junction
Repair
- repair the rotted sill as above. Ensure that water can no longer get into the sill/jamb junction. Fill any cracks with an exterior-grade timber filler and allow to harden according to the manufacturer’s instructions. Sand filled areas then prime and coat as above
Cause
Poor initial substrate preparation
Repair
- repair as above. Remove all the paint from the sill, prime the timber thoroughly, and recoat with a minimum two coats of enamel paint
Cause
Oil-based (alkyd) paint was applied over water-based (acrylic) paint
Repair
- repair as above. Remove all the paint from the sill, prime the timber thoroughly, and recoat with a minimum two coats of enamel paint
Cause
The sill was painted a dark colour, increasing thermal expansion/contraction of timber
Repair
- repair as above. Consider repainting the house windows/sills a lighter colour that will attract less heat and result in less timber expansion/contraction
Cause
There was moisture absorption into timber end grain because of inadequate priming
Repair
- repair as above, ensuring that the replacement pieces and any cut timbers are thoroughly primed before installation