Repairing Small Holes and Cracks in Wood Floors

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 specification provides guidance on filling small holes and surface cracks in wood floor boards and filling cracks between wood floor boards.

  2. Cracks the thickness of a dime between floor boards is not uncommon. In fact, plank boards may expand and contract 2-1/2 times that distance.

  3. Cracks can develop between boards from what is known as compression shrinkage or compression set. As the wood absorbs moisture, the floor boards swell. For those boards that swell beyond their allowable range, the boards compress against one another and, sometimes, become damaged. As the moisture level drops, the boards shrink and a gap develops between the boards.

  4. Read "General Project Guidelines" along with this specification. 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). The 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)


  1. Parquet: Small pieces of wood arranged in decorative patterns to form a wooden floor (rather than a floor formed from planks)

  2. Plain/flat sawn: the most common and least expensive method of sawing planks, wherein the log is sawn straight across to form planks

  3. Quarter sawn: the log is first sawn into quarters, which are sawn into plans at a ninety-degree angle from the center of the log, resulting in a very straight woodgrain.

  4. Butt joint: joinery method for plank floors, wherein two pieces are attached at a right angle

  5. Tongue and groove: joinery method for plank floors, where every plank has a slot (or groove) on one edge, and a ridge (or tongue) on the other. These edges fit together with the neighboring planks.

  6. Shiplap: joinery method for plank floors, wherein each plank overlaps another plank.

  7. Doweled: joinery method for plank floors, wherein holes are bored into the planks at corresponding points and attached together with either end of a dowel, or small wooden rod.

  8. Splined: joinery method for plank floors, wherein each edge of the plank is equipped with a groove along its length, and a long thin piece of wood that fits into both grooves is used to hold them together.

  9. Countersinking: Hammering a nail slightly below the wood surface using a nail punch.

  10. Blind-nailing: For tongue and groove joints, wherein the nail is hammered into the tongue at an angle to attach the plank to the framing underneath the floor, so that when the groove is fitted over the tongue, the nail head is hidden.

  11. Screwing and plugging: Boring a small hole into wood using a plug-cutting drill bit, then drilling in a screw so that it is slightly below the surface of the wood. This hollow is the filled in, or plugged, with glue.



Abatron, Inc.

Kenosha, WI



  1. Commercial wood putty such as WoodEpox (Abatron, Inc.), or approved equal.

  2. Colors-in-oil or oil stain pigment to color putty to match wood floor such as Abatron pigments), or approved equal.

  3. Wood glue to reattach splinters.

  4. Sandpaper.

  5. Sawdust.

  6. Varnish or shellac to make a paste filler.

  7. White wood glue.

  8. Tissue paper.

  9. Calcined magnesia to make paste filler.

  10. Strips of cloth, grey felt weather-stripping or varnished hemp to make a fibrous filler.

  11. Linseed oil.

  12. Marine caulking compound to make a caulk filler.

  13. Wood for shims (do not use shingles, they lack compressive strength).

  14. Strips of wood for nailing to underside of floorboards or for filling cracks between boards.


  1. Small putty knife.

  2. Brush or sponge to spread pigment.

  3. Hammer.

  4. Nails.

  5. Screwdriver.

  6. Stiff bristle brush.

  7. Vacuum.



Inspect for the signs of decay or insect infestation such as mold, fungus, bore holes, and sawdust piles.


  1. For Small Holes and Surface Cracks:

    1. Fill with commercial wood putty:

      1. Stain the putty to match the floor by using either colors-in-oils or the settled pigment from the bottom of the stain can.

      2. When staining putty to match wood, it is better to go darker than the wood rather than lighter.

      3. When filling a hole or crack, add filler in layers and allow drying time between layers.

    2. If a floorboard is splintered, glue the splinter down and fill the crack.

    3. If damage is such that it cannot be successfully filled and the board is relatively easy to remove, turn the board over rather than replacing it. The new surface should be sanded to match surrounding boards and may need to be shimmed to make it level with the existing surface.


  1. For cracks between floorboards:

    1. In general, it is best to leave cracks between boards alone. Gaps often diminish as the boards expand in more humid seasons. However, there are several methods for filling cracks between floorboards:

    2. If small, cracks between floorboards can be covered with a new floor finish.

    3. If the underside of the floorboards is exposed, nail strips of wood to the underside of the crack.

    4. Fill with a paste filler (made by user):

      1. Mix sawdust with varnish, shellac or white glue, or

      2. Mix tissue paper, glue size, and calcined magnesia into putty.

      3. Press mixture into crack using putty knife and finish so it is level with the floor surface.

    5. Fill with a fibrous filler (made by user):

      1. Soak cloth strips in linseed oil or glue, or use strands of hemp rope (grey felt weather-stripping may also be used, but is not as stainable as hemp rope).

      2. Pack cloth strips or strands of rope (in layers, if necessary) into the crack using a screwdriver or putty knife.

      3. If desired, stain the filler material to match the floor.

    6. Fill with caulking compound:

      1. If the floor is to be painted, use marine caulking compound to fill the crack. It will expand and contract with the wood and the paint will hide the color difference.

      2. Carefully mask the area before caulking.

      3. Fill crack using a caulking gun or similar injection device.

      4. It is best to fill the crack in the spring or in the fall.

      5. Finish the surface level with the floor.

    7. Fill with a thin strip of wood:

      1. NOTE: This option is not ideal because it reintroduces the potential for compression problems resulting from wood expansion and contraction. However for large cracks that are hazardous, it may be used

      1. Thoroughly clean the opening of dirt and debris using a stiff bristle brush and vacuum.

      2. Slip a thin strip of wood into the crack. Match the depth of the surrounding floor and stain to match.

      3. Nail or glue the strip to only one side of the crack to allow for expansion and contraction.

  1. In extreme cases, the floorboards will have to be taken up and reinstalled. See "Replacing Damaged Floorboards" for guidance.