How to seal outdoor wood gaps

Updated May 18, 2018

Any outdoor gap between wood siding or trim allows water to enter the exterior walls from rain and humidity. After water enters the exterior wall for extended periods it will enter a home through the floor, ceiling or interior walls. Moisture in walls leads to mould, mildew and contaminates the air inside the home. Sealing all outdoor gaps with caulk protects homes from water damage, expensive repairs and keeps occupants safe from air hazards.

Sand the wood gap on the building or house with medium-grit sandpaper. This action gives the sealant an abrasive area to adhere to the wood.

Place a tube of exterior silicone sealant on a flat surface while supporting it in one hand.

Cut the end off the sealant tube at a 45-degree angle with a utility knife.

Slide the tip of the sealant into the top support ring of a caulking gun and place the tube in the caulking gun body.

Place the caulking gun tip in the outdoor wood gap and squeeze the trigger. The caulk will flow out of the tip. Fill long gaps with caulk by squeezing the trigger and walking while the caulk flows out of the tip.

Fill a deep gap with caulk up to the surface level, let it dry for 24 hours and refill to the surface level.

Sand the caulk with medium-grit sandpaper and paint it to match the colour of the house or building so the repair job is not obvious.


Squeezing the trigger on a caulking gun drives a metal plate into the sealant cylinder to force the caulk out of the tip. Ensure that the gaps are completely covered in caulk in a continuous bead. Any small areas without coverage will enable water to leak through the gap at this point. Place a screw in the tip of the caulk cylinder after use to close off the tip for future use. The screw will not allow air to reach the sealant, dry it out and render it useless. Exterior sealants are generally white in colour and require painting to blend in with a home or building.

Things You'll Need

  • Sandpaper
  • Exterior silicone sealant
  • Utility knife
  • Caulking gun
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About the Author

Mary Lougee has been writing for over 10 years. She holds a Bachelor's Degree with a major in Management and a double minor in accounting and computer science. She loves writing about careers for busy families as well as family oriented planning, meals and activities for all ages.