Although caulk is a versatile and effective way to seal leaks in a wide range of construction materials, as a general rule, you shouldn’t use caulk on wood siding.
In the following guide, we’ll take a closer look at how caulk can damage wood siding, the rare occasions when caulk is okay to use, effective alternatives, and more. Don’t put any caulk on your wood siding until you understand its effects.
Whether you’re new to wood siding maintenance or a DIY veteran, it’s important to understand the names and functions of the basic components.
- Siding – Siding is the exterior side of a structure. Along with the roof, siding protects the building from the elements and helps create the overall style.
- Caulk – A semi-soft sealant used to fill gaps and cracks in a variety of building materials. Caulk originally consisted of cloth fibers and tar, but today is usually made from silicone, polyurethane, or acrylic.
- Flashing – These are thin, flat pieces of metal built into the siding and roofing systems. They act as a barrier and guide to prevent water from entering the structure.
How Caulk Can Damage Wood Siding
Caulk can easily, and rather quickly, cause extensive damage to wooden siding.
More than most building materials, wood changes size and shape in relation to exterior temperature and precipitation levels. The siding manufacturer accounts for these fluctuations, so even though the wood moves, the flashing remains capable of functioning properly.
However, caulk can impair this natural movement. Not only will the flashing fail, which can result in water leaking through to the interior, but the wooden siding can strain and splinter.
When Caulking Is Safe to Use on Wood Siding
Should wood siding be caulked? Sometimes. Caulk doesn’t automatically damage wood siding. When applied in the correct spots and in moderate amounts, caulk plays an important role in regular maintenance and repair.
If the wood siding is properly installed, it won’t need much caulking throughout its lifespan. There are two reasons for this:
- Caulking around doors and windows is completed before the siding is installed
- The boards fit together in a way that they can expand and compress together
So, caulking should never be necessary when installing wood siding. However, it can be used to repair minor cracks, splits, gaps, knots, and other spots that could eventually allow moisture.
Should wood siding be caulked? Follow these general sizing guidelines when deciding whether to repair or replace a damaged wooden board:
- Cracks and splits with a gap less than 1/16 of an inch are repairable with caulk.
- Boards with cracks wider than 1/16 of an inch should be repaired with epoxy or replaced.
Always err on the size of replacement. If you add too much caulk to a severely damaged board, you can further erode the board’s ability to protect the interior of the structure.
The Best Type of Caulk for Wooden Siding
The type of caulk that works best depends on the type of problem you’re repairing.
For knots and larger holes, which are typically found in older and rotted wood, you’ll want to use wood epoxy. It’s a special, wood-safe compound made up of resin and a hardener.
If you need to repair split or cracked siding, use flexible caulk rated for outdoor use. Silicone-hybrid caulk is the best option for several reasons:
- It’s waterproof – Exterior caulk needs to withstand harsh outdoor weather.
- It’s flexible – Even when dry, it remains soft enough to move with the natural wood fluctuations.
- It’s stable – Silicone caulk won’t shrink or crack over time.
Other types of caulk can potentially erode after contact with water or dry so rigidly that they impair the wood’s movement.
Quick Tips for Caulking Around Wood
If you’ve determined that small amounts of caulk will help repair your wood siding, follow these tips and techniques when applying it.
- Clean the Area – Sand down any rough edges and brush away all loose dust and debris before applying caulk.
- Push Instead of Pull – Hold the caulking gun at a 45° angle. Apply the caulk by pushing the tool forward. Pushing helps remove air bubbles and allows the caulk to penetrate deeply into the crack.
- Layer, Don’t Fill – Don’t try to fill the crack or splinter entirely. Instead, apply a thin layer, let it dry, and apply additional layers as necessary.
The best time to apply caulk is on a day with mild, warm temperatures. Ideally, you want the caulk to sit for at least 24 hours before being exposed to rain or other wet weather.
Here are quick answers to common questions about caulking wooden siding.
Does Caulk Hurt Wood Siding?
Caulk doesn’t damage wood siding on contact, but applying too much can prevent the wood from moving due to temperature and pressure changes. If the siding is restricted too much, the flashing can fail, and the wood can splinter.
How Do You Seal Gaps in Wood Siding?
For knots and gaps larger than 1/16″ of an inch, fill the area with wood epoxy. If you want to repair small gaps, use a silicone-hybrid caulk.
Can I Use 100% Silicone-Based Caulk on Wood?
While it won’t hurt the wood, 100% silicone caulk can wash away after exposure to heavy amounts of water. A silicone-hybrid caulk provides all the benefits of silicone caulk but with added moisture resistance. Make sure the caulk is rated for outdoor use.
Can You Caulk Clapboard Siding?
Clapboard siding is designed with gaps between the trim and the siding. Placing caulk in these gaps impairs the ability of water to drain as intended, resulting in excessive moisture damage that will eventually weaken the wood.
Should wood siding be caulked? As a general rule, less is more. Applying significant amounts of caulk to wood siding restricts its natural movement, resulting in damage and decreased water protection.
You shouldn’t need to caulk wood siding much, if at all, until at least 10 years after installation. As the siding ages, you can use small amounts of outdoor-rated caulk to seal gaps, holes, and cracks, ideally those under 1/16th of an inch.
By following the guidelines above, you can keep your wood siding in great shape, so it protects your home from the elements while looking its best!