If your historic house or building has wood siding, you are likely to find small cracks and splits in the wood. You should repair these cracks and splits when you find them to prevent rain water from seeping into the areas behind the siding boards. Rain water that seeps into these spaces can cause more extensive damage and wood rot.

Evaluate the Damage

Your first step to repair damaged wood siding is to evaluate the extent of the damage. Make a point each spring to closely inspect your exterior wood siding for cracks and splits in the wood. Horizontally laid clapboard siding, drop siding and wood shingles will occasionally split or crack along the wood grain lines. This can also happen to vertically laid board and batten siding. Your approach to repairs will depend on the size of each crack or split:

• For cracks and splits with less than a 1/16-inch gap, follow the instructions below to fill in the gaps.
• For boards or shingles with splits wider than 1/16 inch, you’ll need to replace the damaged piece of wood. See the “Learn More” section below for more information.

Choose an Appropriate Repair Material

You can repair wood siding cracks and splits with less than a 1/16-inch gap by filling in the gaps with an appropriate wood filler. Many different wood fillers are commercially available, but you should never use a filler that cures hard to repair small cracks or splits. Never use architectural epoxies, auto body fillers or any interior-grade fillers on cracked or split siding. Epoxies work well for rotted wood or wood with larger holes, but not for cracked or split wood siding.  As the wood siding contracts and expands from moisture give-and-take, the hardened filler will pop out.

Flexible caulk is the best product to use on cracked or split siding, but not just any caulk will work. Although standard acrylic latex caulk is very flexible, it is too shiny to use as a filler for wood grain splits. Most wood siding has some texture to it, and the most common siding paint is flat, with little to no gloss.

The best caulk to use is a water-based caulk designed for use in wet area tile installations. These caulks come in a wide variety of colors and are applied like any other caulk. Water-based tile caulks are flexible, sticky and available in either a sanded or unsanded variety. You should use the unsanded variety, because a sanded caulk actually contains sand, which gives the caulk too much texture.

Gather Your Tools and Supplies

To repair cracked or split wood siding, you’ll need the following tools and supplies:

• 100 grit sandpaper
• Alkyd oil based primer
• Small artist's brush
• Blue or green painter’s tape
• Water-based tile caulk
• Caulking gun
• Putty knife

Make Repairs

Follow the steps below to fill splits or cracks in your wood siding.

Step 1

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Source: Bob Yapp

Clean any dirt or debris from the split in the siding. If you use water to clean the wood, wait until the wood is dry before you move to the next step. Do not patch wood cracks if the wood is wet.

Step 2

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Source: Bob Yapp

Fold a small piece of 100 grit sandpaper in half and insert it into the split. Move the sandpaper from one end of the split to the other. This sanding will remove any dirt or ultraviolet light damage. Blow or vacuum out the sanded wood debris from the split.

Step 3

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Source: Bob Yapp

Use a small artist's brush to apply a thin coat of alkyd oil-based primer into the crack. Avoid getting primer on the surrounding surfaces.

Step 4

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Source: Bob Yapp

When the primer has dried, apply blue or green painters tape to both sides of the split. The tape will prevent the caulk from spreading outside the split onto the face of the siding.

Step 5

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Source: Bob Yapp

Apply a generous bead of the unsanded tile caulk into the split. Use a putty knife to work the caulk into the split and smooth off any excess.

Step 6

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Source: Bob Yapp

When the caulk has cured (about 24 hours), remove the painter’s tape, prime the caulk with alkyd oil and apply two topcoats of paint to match the painted siding.