Timber windows – rotted sash

Cause

Water has been trapped in the bottom corners of the sash

Repair

  • First up, identify and deal with the cause of the problem. Repair the sash after that. If the moisture trapped in the sash corners is coming from condensation running down the glass, see the repair for “Severe condensation on the inside of the glass” below.

Cause

Sash joints or glazing rebates were not primed

Repair

  • if the amount of rot is relatively small:

    • scrape out all the rotted timber. Removing the glass will probably be required

    • treat the area around where the rot was removed with a hot air gun to dry the wood and destroy any rot or fungi filaments. Apply two coats of paint-on timber preservative and allow to fully dry

    • apply a two-pot epoxy resin wood sealer following the manufacturer’s instructions

    • fill the excavated area with a polyester fibre-reinforced filler, finishing about 2–3 mm below the finished surface

    • complete the filling with an exterior-grade timber filler and allow to harden according to the manufacturer’s instructions

    • sand the filled area smooth and spot prime as required using a good quality timber primer

    • finish with two coats of enamel paint

    • reinstall the glass. See Cracked or broken glass in timber frames.

  • if the rot is extensive:

    • consider whether to use a timber patch to repair a section of rot or to replace a section of the sash or the whole sash. In either case, the sash must be taken out of the frame

    • replacing the sash altogether may be the best long-term solution (or the only solution), especially if the mortise and tenon joint has failed

    • detailed guidance is provided in the BRANZ Good Repair Guide Timber windows.

Cause

Putty was not sealed to the glass with paint, allowing water to enter behind putty

Repair

  • if the amount of rot is relatively small:

    • scrape out all the rotted timber. Removing the glass will probably be required

    • treat the area around where the rot was removed with a hot air gun to dry the wood and destroy any rot or fungi filaments. Apply two coats of paint-on timber preservative and allow to fully dry

    • apply a two-pot epoxy resin wood sealer following the manufacturer’s instructions

    • fill the excavated area with a polyester fibre-reinforced filler, finishing about 2–3 mm below the finished surface

    • complete the filling with an exterior-grade timber filler and allow to harden according to the manufacturer’s instructions

    • sand the filled area smooth and spot prime as required using a good quality timber primer

    • finish with two coats of enamel paint

    • reinstall the glass. See Cracked or broken glass in timber frames.

  • if the rot is extensive:

    • consider whether to use a timber patch to repair a section of rot or to replace a section of the sash or the whole sash. In either case, the sash must be taken out of the frame

    • replacing the sash altogether may be the best long-term solution (or the only solution), especially if the mortise and tenon joint has failed

    • detailed guidance is provided in the BRANZ Good Repair Guide Timber windows.

Cause

Severe condensation on the inside of the glass

Repair

  • if severe condensation is still a problem, deal with that first:

    • avoid using unflued gas heaters and avoid drying laundry inside – these add a lot of moisture to the air

    • opening windows for at least 10 minutes every day can make a difference to indoor air quality

    • increase the amount of ventilation provided. In particular, add a rangehood over the kitchen stove and an exhaust vent near the shower in the bathroom, both vented to remove steam to outside the house. An alternative to reduce bathroom steam is to fit a proprietary plastic dome over the shower to keep the steam inside

  • if the amount of rot is relatively small:

    • scrape out all the rotted timber. Removing the glass will probably be required

    • treat the area around where the rot was removed with a hot air gun to dry the wood and incinerate any rot or fungi filaments. Apply two coats of paint-on timber preservative and allow to fully dry

    • apply a two-pot epoxy resin wood sealer

    • fill the excavated area with a polyester fibre-reinforced filler, finishing about 2–3 mm below the finished surface

    • complete the filling with an exterior-grade timber filler and allow to harden according to the manufacturer’s instructions

    • sand the filled area smooth and spot prime as required using a timber primer

    • finish with two coats of enamel paint

    • reinstall the glass. See Cracked or broken glass in timber frames.

  • if the rot is extensive:

    • consider whether to use a timber patch to repair a section of rot or to replace a section of the sash or the whole sash. In either case, the sash must be taken out of the frame

    • replacing the sash altogether may be the best long-term solution (or the only solution), especially if the mortise and tenon joint has failed

    • detailed guidance is provided in the BRANZ Good Repair Guide Timber windows.

Cause

The corner joint has separated, admitting water

Repair

  • see repair for “Sash joints or glazing rebates were not primed” above for treating the rot

  • open joints in sashes must be closed and sealed so they do not admit water

  • if the joint opening is small, scrape/sand the paint back until the surface is sound, spot prime and allow to dry. Fill the crack(s) with exterior-grade timber filler, allow to harden. Sand filled areas. Coat with a minimum two coats of enamel paint

  • If the joint opening is more serious, the joint may need to be rebuilt or the sash replaced.