The Gaps a Painter Must Never Caulk
Why this matters
A crew that caulks everything makes a house look tight on the day it is finished and wet behind the paint a year later. Every exterior wall takes on some water: wind-driven rain past the laps, water around nails and fixtures, vapor from inside. A wall that lasts is built to let that water drain out at the bottom and dry. Some of the gaps a painter sees on a repaint are those exits, and caulking one shuts the water in behind fresh paint, which then blisters and peels from the back, and the siding or trim under it swells and rots. The callback looks like a paint failure and gets blamed on the paint. This card is the list of gaps that stay open, how to recognize each one, and what to do when a previous painter already closed it.
The rule: water in from above, out at the bottom
Siding, trim and veneer are built as overlapping layers, each one shedding water onto the layer below it and out. The joints at the top and sides of a layer are where water gets in; those are what a painter caulks, per 'Caulking and Sealant Application'. The joints at the bottom of a layer are where water that got behind it gets out; those stay open.
So before running a bead along any horizontal joint, ask which way the overlap goes. If the gap is under the bottom edge of something, with the layer above lapping over the layer below, it is a drain. Leave it.
The gaps, in section
LAP SIDING, SECTION BRICK VENEER AT BASE
(outside is right) (outside is right)
WRB wall air brick
|\ board A | space | |
| \ | | |
| \ |\ | |
| | <- butt: paint it | \ |weep| <- OPEN
|\ | | \______|____|__ flashing
| \_| <- under lap: OPEN | | |
| \ water drains here | | |
| \ board B
| |
Lap siding bottoms. Each lap board's thick bottom edge (the butt) sits over the thin top of the board below. Water that gets behind board A runs down the weather-resistive barrier (WRB, the housewrap or felt behind the siding) and out through the small wedge-shaped gap under the butt. Paint the face and the butt edge of every board. Never run caulk along the underside of the lap. A lap caulked shut on a sunny wall can also trap vapor driven inward and outward through the day, and the result is the same: paint lifting off the face of the board below.
Brick weeps. Brick veneer has an air space behind it, and flashing at its base that carries water out through the bottom course. The weeps are the openings in that course just above the flashing: open head joints, small tubes or vent inserts. The IRC section on anchored masonry veneer calls, in substance, for weep holes immediately above the flashing at a maximum spacing of 33 inches on center (R703.8 in the 2021 edition, as adopted by your jurisdiction; check the current edition for the exact subsection). A painter who fills them with caulk or paint, or a landscaper who buries them in mulch, turns the air space into a tank.
Four more that stay open
Window frame weeps. Most vinyl, aluminum and fiberglass windows have weep slots on the outside face of the bottom of the frame, often with small hinged covers. Water that gets past the sash into the frame drains through them. They are easy to paint shut with a careless pass on a frame being painted, and easy to caulk shut by someone sealing the frame to the sill. Mask them or work around them. Old exterior storm windows carry the same thing: small holes or notches in the bottom rail that let water out of the space between storm and window.
The drip gap over horizontal flashing. Where siding sits above a horizontal trim board, a band board or a window head, a Z-shaped or cap flashing runs up behind the siding and out over the trim. The siding above stops short of the flashing, leaving a small gap so water running down the face clears it and does not wick into the cut end of the siding. Siding makers specify that gap, often around a quarter inch on fiber cement; check the installation instructions for the product on the wall. Caulking it seals the cut ends of the siding into standing water on the flashing. Fiber cement swells at those ends and wood rots there.
Siding above a roof line. Where a wall meets a lower roof, the siding is meant to stand clear of the shingles above the step flashing and kick-out flashing, so the siding's bottom edge is not sitting in roof runoff. That clearance is a gap a tidy painter wants to close. It stays open, and the roofer owns the flashing beneath it.
Vinyl siding at its channels. Vinyl siding hangs loose on its nails and moves a lot with temperature, and its J-channels and corner posts carry water down and out. Caulking vinyl to trim or into its channels locks it against movement and blocks the drainage path. Paint it if the maker allows it in the color you are using; do not caulk it.
What is caulked, for contrast
The same walk-around on the same house finds plenty that should be sealed: the vertical joint where siding meets a corner board or casing, the top of a window or door casing where it meets the siding above (unless there is head flashing there with a drip gap), open butt joints in wood trim, nail holes, and penetrations such as hose bibs, vents and wiring where they pass through the wall. Those are top and side joints, and water gets in through them. 'Caulking and Sealant Application' owns which sealant goes where and how to tool it.
What a closed drain looks like on a repaint
- Paint peeling from the bottom of a board or trim piece, along a long horizontal line, while the paint above it is sound.
- Blisters with water in them on a wall that gets direct sun, on the course below a caulked lap.
- Swollen cut ends on fiber cement or soft wood right above a band board or window head.
- A bead of caulk along the underside of the laps, sometimes the full length of a wall, from a previous painter.
- Efflorescence or staining on brick below a weep that is filled or covered.
When you find one, the paint failure is a symptom. Repainting it without opening the drain repaints the same failure.
Worked example: a bid walk on a 1998 two-story
The house. Brick veneer on the first story, wood lap siding on the second, separated by a band board with Z-flashing over it. Vinyl windows. A previous repaint about seven years ago.
What the lead found on the 40-foot front:
- The drip gap above the band-board flashing had been caulked the full 40 feet. The paint on the bottom two courses of lap siding above it was peeling in a horizontal band, and at two butt joints the board ends were soft to a probe.
- Of 16 brick weeps across the front, 4 had been filled with caulk, all at the downspout end, where efflorescence streaked the brick below them.
- On 3 of 5 front windows, the frame weep slots had been painted shut.
- The laps themselves were open. Whoever caulked the drip gap had stopped there.
The plan written on the bid:
- Cut the caulk out of the drip gap along all 40 feet with a sharp knife and a hook, without cutting into the flashing. Let the board ends dry before any primer.
- Probe and moisture-check the two soft butt joints. Both were too soft to hold primer and paint, so the bid carried a carpentry line to replace about 2 feet of board at each, with the new cut ends primed before installation.
- Clear the 4 filled weeps by picking the caulk out with a plastic tool, without enlarging or damaging the joint.
- Open the 3 painted-shut window weeps with a blade, and mask the weeps on every window before the frames are painted.
- Repaint, leaving every gap on this list open, with the crew told on the morning of the job which gaps are which.
The count, checked. The front had four kinds of drain: laps, the drip gap, brick weeps and window weeps. Three of the four had been closed somewhere on the wall; only the laps were still open. Repainting without the first four steps would have put fresh paint over a drip gap that was already holding water.
What would change the answer.
- No flashing over the band board. Then the joint between siding and band board is an entry point, and the right fix is flashing installed by whoever does the carpentry, not a caulk bead, because caulk alone fails in a few seasons at a horizontal joint.
- A pre-1978 house. Cutting out painted-over caulk disturbs a little paint, and scraping the peeling band around it disturbs much more; together that is RRP work unless the total stays under the minor-repair threshold; see 'How to Scrape and Feather Sand Peeling Trim on a Pre-1978 House'.
- The rot was further than the board ends. Damage that reaches sheathing or framing is a carpentry and possibly a moisture-intrusion job for someone other than the painter, and the paint waits for it.
How to verify the drains are open
- Laps: a thin putty knife slides under the butt at points along each wall and comes out with no caulk on it.
- Brick weeps: each one is open to the air space, with daylight or a pick passing into it, and none buried by mulch or soil.
- Window weeps: each slot is open and its cover, if it has one, swings freely.
- Drip gaps: a visible gap between the siding's bottom edge and the flashing along the whole run, with no paint bridge across it.
References
- International Residential Code, R703 exterior covering and R703.8 anchored masonry veneer, as adopted by your jurisdiction
- Siding and window manufacturers' installation instructions for the products on the wall
- See related: Caulking and Sealant Application
- See related: The Exterior Primer Map, Substrate by Substrate
- See related: How to Spray and Back-Roll Exterior Siding