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

Common

Listed under: Interior, Exterior, finishes, timber and wood-based, paint, timber, windows, moisture, durability, rot, leaks

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.

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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

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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

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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

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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

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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

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