Exterior Caulking:
Which Gaps Need Sealant and Which Don’t
Caulk is the cheapest part of the siding and the first part to fail. It’s also the part people apply in the wrong places.
Cracked, failed or missing exterior sealant showed up in 147 of Russ’s recent reports. The biggest single title was penetration sealant at pipes and vents, which has its own post. This one is about the rest: butt joints in the siding, the joints around windows and doors, and trim.
Some of those gaps need caulk. A few should never get it.
Why sealant fails
Caulk lives in the joint between two materials that move. Wood swells and shrinks with the seasons. Fiber cement moves less, but it moves. Sun bakes the sealant until it hardens and cracks. Russ’s report wording for cracked sealant (42 houses) is simple: replace it to help prevent damage from air, moisture and pests getting in.
South and west walls usually go first. So does any joint that was too thin or too wide for the product, or caulked over dirt or old caulk.
Butt joints in the siding
Where two boards of siding meet end to end, there’s a butt joint. Failed butt joint sealant came up in 22 houses. On fiber cement, the manufacturer’s instructions usually call for a flashing strip behind the joint, a gap, sealant, or a combination, and those details vary by brand and year. The fiber cement siding post covers the common install mistakes.
Open butt joints let rain run straight behind the siding onto the weather barrier. That’s often fine for a while, since the barrier is there for it. It’s the years of it that cause swelling at the board ends and paint failure. The exterior paint post shows what that looks like a few seasons on.
Windows and doors
Sealant around windows came up in 16 houses, and window sealant cracks on new builds in another 12. On new homes, a little cracking in the first year or two is normal as framing dries out. It’s a cheap fix and often a warranty item.
A different finding sits behind the caulk. In a Portland house, the windows had no visible head or sill flashing and were relying on sealant alone to keep water out. Flashing is supposed to do that job, with caulk as the second line. When sealant is the only thing between rain and the wall, any crack in it leads straight in. The kick-out flashing post covers flashing at the roofline.
Inside, Russ notes cracked sealant between window frames and the interior casing too. A Vancouver report called it common.
Where caulk does harm
Not every gap is meant to be closed. The bottom edge of each course of lap siding is supposed to stay open so water behind the siding can drain. Some window installs leave a small gap at the bottom of the trim for the same reason. Brick weep holes are another. Sealing those can trap water inside the wall.
A good rule of thumb: horizontal gaps at the bottom of something are often drainage, and vertical joints and top edges are usually the ones that want sealant. When in doubt, the siding or window manufacturer’s instructions settle it.
Real Inspection Photos
Sealant Findings From Russ's Inspections
Cracked and failed exterior sealant from Russ's recent reports.
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Cracked sealant at a siding joint beside the trim. -
Sealant cracked open along the corner trim. -
Cracked sealant where window trim meets the siding. -
An open joint at the corner of window trim. -
Failed sealant at the side of a window.
What Russ checks
He walks the exterior and looks at sealant at siding joints, trim, windows, doors and penetrations, along with flashing he can see. He doesn’t probe behind siding or remove trim. Failed sealant gets a recommendation for a qualified contractor to reseal. Where flashing appears missing, the recommendation is evaluation by a siding or window contractor.
- Walk the south and west walls first, where caulk fails soonest
- Look at each siding butt joint for open gaps or cracked caulk
- Check the top and sides of window and door trim
- Leave the bottom edges of lap siding and brick weep holes open
- Note new-build cracks for the builder’s warranty list
Cracked and missing exterior sealant came up at 147 of Russ’s inspections. Butt joints, window trim and door trim need it. The bottom edges of lap siding and weep holes need to stay open. Caulk is the second line of defense behind flashing.
The guide to what to look at on the exterior before you make an offer covers siding, trim and grading together. To set up an inspection, you can book online.
Exterior Caulking FAQ
