Tradition might have you thinking: “Caulk everything!” While caulking is indeed an overall important protective measure for your siding and home, “more” isn’t always better. What’s more important is focusing on “the where.”
In fact, many newer siding systems, such as James Hardie fiber cement siding, don’t require caulking where you’d traditionally expect to see it. In this post we’ll outline some key locations and why and when they matter — and even introduce a few alternatives to caulking.
Behind Your Siding
During freezing and thawing, frost or condensation can gather on top of your sheathing (behind your siding, but on top of studs). Air circulation and a drainage plane is crucial here to keep things dry.
Above Header Flashing
Anywhere there is metal flashing above a horizontal trim board you’ll want to leave 1/4″ clearance. However, you’ll want to apply a “caulking dam” in the corners. For example, with window trim, flashing will be at top, the caulking dam in the corners, and the sides and bottom of the window trim will be caulked to meet the siding.
Where Two Siding Boards Meet
Rather than caulking where two horizontal boards meet, siding manufacturer James Hardie recommends flashing behind the joint. For example, you can cut a prefinished aluminum trim coil into a square and place it behind the siding joint, where the siding meets with “nominal contact.” If water makes its way into the seam, the metal directs the water back out on top of the lap below it.
Bottom of the Wall/Wall Gaps
Sometimes homeowners want to use caulking to fill the gap at the bottom of a wall, where the siding overlaps the foundation. The most important thing when setting siding above grade is to maintain a 6″ clearance below the first row of siding. If your foundation line is less than the recommended six inches, metal flashing can help solve the issue.
Remember, any water that gets behind the siding needs to be able to drain out the bottom. This is why it’s filling the gap with spray foam or caulking isn’t always the best choice. Instead, use aluminum trim coil, which allows for proper flashing — and also proper drainage and air circulation.
What if Every Joint on My Home is Caulked?
There’s a chance everything is fine, but there are many variables, too. Keep your eye on siding that might be retaining water. Indicators of this could include siding that is swelling, cracking, or peeling underneath the laps. Any of these issues could be worth a close look.
Another area to look for in wood siding is nails that appear oversunk. A common misconception is that the builder may have done this intentionally to ensure nails are embedded into the siding. Although this could be the case, more often than not it’s an indicator that the nail was fastened into a stud, and the wood siding is now swelling, beyond the nail head. If you are concerned with the condition of old siding, call a local expert.
Keeping moisture out and air circulating properly are key to protecting your home from damage. Sometimes caulking is the answer, while other times flashing is a better alternative. Before you run for your caulking gun, always refer to your manufacturer’s installation manual, guidelines and specifications and, when in doubt, contact a siding expert.
13 Comments
Greg Sureseal September 12, 2021
Bryce, the content you’ve shared here regarding the do’s and don’ts of caulking are fantastic. I will follow your guide, and I hope everything will go well for me. I don’t want any problems.
Bryce October 18, 2021
Thanks for the comment. Good luck with your project!
Tamera D Fisher August 09, 2023
Can I caulk James Hardy regular lap siding where the panels overlap?
Kate Acuff August 11, 2023
Thanks you for the question. On regular lap siding, you do not want to caulk the over lap. That’s an exit point if water were to get behind the siding.
Timothy August 20, 2024
Same question, but for slightly older aluminum side where the overlapping (on the side, not bottom) top piece has a bit of a gap?
Roger October 20, 2023
Hi. Should I caulk where the brick meets the aluminum capping? Thanks
Brett Gourley June 13, 2024
Hi, have Hardie cement board siding that was installed 20 years ago and caulked at all the butt joints. Getting ready to paint and the painter is planning to recaulk them. Is this right or wrong? I don’t have metal behind the joints just the wrap. Thanks In advance for your advice.
[email protected] July 03, 2024
Hello
Thank you for reaching out on this question. First and foremost, please refer to the James Hardie installation manual in the resource section of their website with reference for treating butt joints. If there is not flashing behind the lap butt joint, James Hardie would likely recommend caulking the butt joints prior to painting to ensure proper water drainage. Again, please reference the James Hardie website for exact guidelines.
PAM HAMPTON October 25, 2024
There is hardy plank wood siding on our house. There is a “tube”/pipe on the outside of the wood siding that is the vehicle to bring gas into the house for gas logs. There is a gapping “rough’ opening thru which the tube is entering. Can this opening be caulked, or how best to seal off this larger than tube opening. I believe insects are entering the home thru this opening.
Bryce October 28, 2024
Thank you for reaching out. If I understand correctly, this would qualify for a penetration block, and I’d recommend contacting the company that installed your James Hardie siding to see if they can install a trim block for you. Penetration blocks are required if they are a minimum of 1.5″ in diameter. Please refer to the James Hardie installation manual for exact specifications.
Alisa November 02, 2024
Should you caulk just above the metal flashing that runs along the top of the lowest strip of siding on the back of my house?
Kate Acuff March 10, 2025
Great question, Alisa! If you search online for the James Hardie Best Practices, you’ll find their guidelines for caulking. Generally, you would not caulk on top of the flashing, except in the corners as a “caulk dam.”
Joni November 03, 2024
We just bought a very expensive home. It was renovated from stucco to hardie plank. It looks beautiful. Shortly after moving in when the rainy season began, there is water going under our floor and bucking our engineered hardwood floors by most of our windows. There isn’t any visible water signs on the walls or windows only the damaged floors and glue coming up. Could this be caused by poor installation from the hardie plank, they claim they can’t put flashing around the windows because it’s cement on all three stories. Yet water damage only appears to be around the window, not where it is all wall space. Please help
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