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Repairing Weather Checks in a Wood Window Sill

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.


We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.



  1. This procedure includes guidance on repairing a wood window sill that shows weathering of the type described in 1.01 B. and C. below.
  2. Weather checks are cracks in the wood that develop when bare wood is exposed to the weather. They begin as small hairline cracks, but exposure to the sun dries out the inner wood causing the crack to widen. Rainwater andfreeze/thaw cycles further exacerbate the problem, makingthe checks wider and deeper.
  3. Weather checks are typically found on the South and West sides of a building where the sun has severely dried out the wood. They can range in size from hairline to 1/4" wide and 3/8" deep.
  4. See "General Project Guidelines" for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
    1. Safety Precautions
    2. Historic Structures Precautions
    3. Submittals
    4. Quality Assurance
    5. Delivery, Storage and Handling
    6. Project/Site Conditions
    7. Sequencing and Scheduling
    8. General Protection (Surface and Surrounding)
      These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
  5. See also "Epoxy Repair For Deterioration And Decay In Wooden Members" for additional guidance on epoxy repair.


  1. A wood window sill in good condition is free from decay and sloped away from the building to shed water. The connection between sill and jamb is tight and well caulked. The sub-sill should have a drip on the bottom that prevents water from entering the building under the window assembly.


  1. Inspect the sills every two years for breaks in the joints or paint film. Spot prime, paint and provide sealant as needed.
  2. Provide ventilation between storm windows and sill by leaving a narrow gap where the two meet.
  3. Remove any impervious coverings (such as sheet metal)that may have been installed over the sill.
  4. Clean clogged gutters.
  5. Cut back large encroaching plants and shrubbery at least 2 feet from the sill (10 feet for trees) to allow adequate air flow between the building and the plants.



  1. Abatron, Inc.
    1. Epoxy wood fillers & consolidants
  2. Roux Laboratories
    1. suppliers of applicator bottles


  1. Epoxy consolidant such as "LiquidWood" (Abatron, Inc.), or approved equal.
    1. Consists of two clear liquids (a resin and a hardener) mixed together in equal volumes.
    2. The mixture is poured or brushed on to the surface where it penetrates and hardens.
    3. Acts as primer for epoxy filler.
  2. Epoxy paste filler such as "WoodEpox" (Abatron, Inc.), or approved equal.
    1. A two-part putty-like filler (a resin paste and a hardener paste) mixed together in equal volumes.
    2. Hardens within 1-2 hours, is lightweight, non- shrinking, heat and weather-resistant.
    3. Applied like a putty; will fill gaps and holes of any thickness and shape.
  3. Oil-base or acrylic latex paint or high performance elastomeric coating.
  4. Rubber Gloves
  5. Disposable vinyl gloves: Available from drug store or pharmaceutical supply distributor in 50 count or larger boxes.
  6. Polyethylene sheeting


  1. Specially ground scraper
  2. Plastic bottles (narrow spouted), like those used for hair dye, to apply the consolidant; having many on hand is recommended. Cleaning of the bottles for reuse is possible.
  3. Applicator bottles: Available from drug store and sold for hair dye application usually in 8 fl. oz. size; Also available in bulk from Roux Laboratories. Roux ColorApplicators lend themselves more easily to cleaning and reuse.
  4. Rags of different sizes to wipe up spills before epoxy has a chance to harden, small rags are recommended for quick one time uses such as wiping off spouts and caps.
  5. Thin wooden sticks, approximately 8" long for scooping out paste and mixing consolidant.
  6. Goggles and a respirator for protection from fumes.
  7. Putty knives for application of filler
  8. Needle nose pliers to pull out hardened epoxy
  9. 1/8"x8"x12" Masonite boards for mixing paste filler
  10. Carbon dioxide fire extinguisher: Curing epoxy creates heat that may cause fire
  11. Stiff, fiber bristle brushes
  12. Paint brushes



  1. Look for breaks in the paint film that may indicate checks below the surface.
  2. DO NOT confuse weather checks with a split sill. Weatherchecks are deep but do not extend through the entire depth of the sill.


  1. Surface Preparation:
    1. Remove loose paint and build-up from the surface of the sill.
    2. Clean out all cracks of any dust and debris using a special scraper and a stiff, fiber bristle brush.
    3. Dry the sill out; cover it loosely with polyethylene sheeting and allow to sit until a low moisture level is achieved in the wood. This may take anywhere from one week to one month.
    4. NOTE: The cracks must be at their widest position when they are filled. Therefore, it is important that the wood be thoroughly dry before proceeding with the repair.


  1. Prime the edges of the checks with epoxy consolidant;inject consolidant into each check using a narrow spouted squeeze bottle; this provides a surface that the filler can adhere easily to. Avoid getting consolidant all over the sill surface.
  2. After the consolidant has cured, apply epoxy paste filler using a putty knife; fill each check completely. NOTE: DO NOT COVER THE ENTIRE TOP SURFACE OF THE SILL WITH EPOXY. THIS MAY LIMIT THE WOOD'S ABILITY TO DRY OUT OR CAUSE IT TO BECOME TOO BRITTLE AND CRACK WITH THERMAL MOVEMENT.
    1. For very deep or narrow checks, spread filler with a very loose consistency into the check using a putty knife. Force it deep into the check with the end of the knife.
    2. Then apply a stiffer epoxy, again forcing the mix into the check with a putty knife.
    3. The epoxy should raise slightly above the surface.
  3. Allow the filler to set, but before it hardens, trim off any excess so that it is flush with the surface using a very sharp hook-type paint scraper.
    Note: If the filler shrinks below the surface of the sill while setting, a second application of epoxy putty will be necessary.
  4. Sand the surface to prepare it for painting.
  5. Apply a paintable water-repellent before priming if desired.
  6. Prime the surface and apply two top coats paint using top quality exterior house paint.