Where Not to Caulk: The Joints You Must Leave Open

More houses have been damaged by caulk in the wrong place than saved by caulk in the right one. The gaps at the bottom of an assembly are not defects — they are the drain.

1. The Principle Behind It

Modern wall assemblies are not designed to be watertight. They are designed to be water-managed. The outer layer — siding, brick, stucco — is a rain screen that sheds most of the water. Behind it sits a drainage plane, and behind that the structure. The whole system assumes that some water will get through the outer layer, and its job is to give that water a fast, downward route back to the outside.

Once you understand that, the rule about where not to caulk becomes almost trivially simple: seal the top and the sides, leave the bottom open. Water arrives from above and travels down. Sealing the top and sides stops it getting in. Sealing the bottom stops it getting out.

The trouble is that an open gap at the bottom of an assembly looks exactly like an unfinished job. It looks like something the builder forgot. Every year, well-intentioned owners and low-cost contractors go around a house closing all of those gaps, and every one of them converts a self-draining detail into a reservoir.

The damage from this is slow and invisible. Water that cannot drain sits against sheathing and framing. Wood at above roughly twenty per cent moisture content supports decay fungi. The rot progresses for years behind an intact-looking wall and is typically discovered only when a window is replaced or siding comes off — at which point the repair is structural rather than cosmetic.

2. Exterior: Leave These Open

Weep Holes in Brick Veneer

The small open joints along the bottom course of a brick wall, usually every few feet just above the foundation or above a window head, drain the air cavity behind the brick. Brick is porous and the cavity gets wet in every serious storm. Those holes are the only way that water leaves. Filling them is one of the most damaging things you can do to a masonry wall, and it happens constantly because they look like missing mortar.

Window and Door Weep Slots

Vinyl and aluminium windows have small slots in the bottom of the outer frame, sometimes with a little flap over them. Water that gets past the outer glazing seal collects in the frame track and exits there. Caulking these — or painting over them, which is nearly as effective at blocking them — fills the track with standing water that eventually finds its way inward.

The Underside of a Window Sill

The joint where the bottom of the window sill or the sill nose meets the siding below should stay open. Anything that penetrates the assembly runs down inside and needs that exit. Seal both vertical sides, seal across the head, leave the bottom alone. This single detail causes more window-related rot than any other.

Beneath Head Flashing

Where a metal drip cap sits above a window or door, the gap under its lower edge is deliberate. The flashing catches water running down the wall and directs it out over the face of the trim. Caulking that lower edge forces the water to travel behind the trim instead, which is precisely what the flashing was installed to prevent.

The Bottom Edge of Lap Siding

Each course of lap siding overlaps the one below, and the small gap at the bottom edge lets the wall behind dry out. Running a bead along the underside of every course seals the drainage plane shut and traps moisture in the back of the boards. On fibre cement in particular, this leads to swelling and delamination at the bottom edge.

Siding Butt Joints

Vertical joints between lengths of lap siding are usually detailed with a piece of flashing behind them. The joint is meant to be a small open gap over that flashing. Caulking it traps water in the end grain of both boards — the single most absorbent part of the plank — and end-grain rot follows.

Where a Deck Ledger Meets the House

This joint should be flashed, not caulked. A deck ledger failure is a genuine safety issue rather than a maintenance one, and sealant is not an acceptable substitute for correctly lapped flashing. If you can see caulk doing that job on your house, it is worth having somebody look at it properly.

Roof Flashing and Shingle Details

Caulk on a roof is a repair of last resort and a short-lived one. Step flashing, valley flashing and chimney counter-flashing all work by overlapping in the direction water travels. Sealant applied over a flashing detail hides the underlying problem for a season or two and makes the eventual repair harder.

3. Interior: Leave These Open

Between the Tub and the Finished Floor

A small gap where the tub apron meets the bathroom floor is a diagnostic feature. If the tub or its drain starts leaking, water appears there and you know within days. Sealed shut, the same leak runs quietly into the subfloor for months. Leaving it open costs you nothing and buys you an early warning.

Baseboard to a Floating Floor

Laminate and engineered wood floors are installed with a perimeter expansion gap because they move seasonally. The baseboard or shoe moulding covers that gap but should not be fastened or caulked to the floor itself. Restrain the floor at its edge and the movement goes somewhere less convenient — usually a buckled seam in the middle of the room.

Around a Water Heater or Furnace Draft Hood

Combustion appliances need their designed clearances and air paths. Sealing gaps around a draft hood or a combustion air opening is a carbon monoxide risk rather than an efficiency improvement. Anything involving a flue or combustion air belongs to a qualified technician, not a caulk gun.

The Perimeter of a Basement Floor Slab

Where an interior drainage system exists, the gap between the slab and the wall is part of it. Water is meant to arrive there and be collected. Sealing it does not stop the water — it just relocates where the water appears, usually somewhere worse.

Around Electrical and Plumbing Penetrations

Hose bibs, dryer vents, electrical boxes and light fixtures all pass through the wall, and they all want the same treatment as a window: sealed across the top and down both sides, open at the bottom. It is a small detail and it is almost universally ignored. A hose bib caulked all the way around collects every drop that runs down the wall above it and holds that water against the sheathing at the one point where the wall has already been drilled through.

Dryer vents deserve a specific mention because the hood itself is designed with a drainage lip. Sealing under that lip defeats it, and the resulting damp shows up as a stain on the interior wall long before anyone connects it to the vent. The same logic applies to any exterior fixture with a shaped back plate — the shape is usually doing a job.

4. What You Should Seal

For balance, the joints that genuinely benefit from a bead: the sides and head of window and door trim where it meets siding; penetrations for hose bibs, dryer vents, electrical boxes and light fixtures, sealed on the top and sides only; the joint between a countertop and a backsplash; the perimeter of a tub or shower where it meets the tile surround; and interior trim joints where the seal is purely cosmetic.

Air sealing inside the building envelope is also worth doing thoroughly — rim joists, top plates, penetrations into the attic. That is a different job from exterior water management and the two get confused surprisingly often. Sealing air leaks on the warm side of the assembly is good practice; sealing drainage paths on the cold side is not.

5. If It's Already Caulked

Finding sealed weep holes or a caulked sill underside on a house you have just bought is common and not a crisis. Open them back up. A utility knife along the joint releases most beads, and a hooked removal tool clears out the rest. On mortar weep holes, a thin blade or a small drill bit reopens them without damaging the brick.

What matters is what you find behind. If the trim is soft when you press it, if paint is blistering below the joint, or if the siding has swollen at its bottom edge, water has been sitting there for a while and opening the drain is only the first step. Get the area open enough to see the sheathing before deciding how far the repair needs to go.

One practical note: reopening a drainage path often produces a visible stain or a dribble the first time it rains, as the trapped water finally leaves. That is the system working, not a new leak.

Do not assume the previous owner was wrong about everything, either. Some beads that look questionable are genuinely load-bearing in a maintenance sense — a well-placed seal on a poorly detailed joint may be the only thing keeping a wall dry, and removing it without replacing the detail underneath makes things worse rather than better. Open one section, see what is behind it, and let that decide how much further you go.

6. The Test to Apply

Before running a bead anywhere on the outside of a building, stand back and ask one question: if water were running down this wall, would this gap let it in, or let it out? A gap on a vertical side or under a horizontal top edge lets water in — seal it. A gap on a bottom edge, or on the underside of anything, lets water out — leave it.

The second question is whether the gap is doing a job that something else should be doing. Caulk is a sealant, not a flashing, not a roof repair, and not a substitute for a properly detailed joint. If the honest answer is that the bead would be holding water back rather than directing it, the joint needs a builder rather than a tube.

Applied consistently, those two questions catch nearly every case on this page without needing to memorise the list.

When you genuinely cannot tell, the safe default is to leave the joint open and watch it through a wet season. An open gap that turns out to have wanted sealing costs you a little air infiltration and can be closed at any time. A sealed gap that turned out to be a drain costs you the sheathing behind it, and you will not find out for years.

Common Mistakes

Sealing every gap you can find

The most damaging habit on this list. A wall that cannot drain rots from the inside while looking perfectly sound from the street.

Caulking the bottom of the window sill

Water gets past the outer seal on every house. The sill underside is where it is supposed to come back out.

Using caulk instead of flashing

Sealant lasts years; flashing lasts decades and works by geometry rather than adhesion. A bead over a flashing failure just delays the diagnosis.

Filling brick weep holes

They look like missing mortar and they are not. They drain the cavity behind the veneer, which gets genuinely wet in any real storm.

Sealing the tub-to-floor gap

That gap is a leak detector. Closing it means the first sign of a problem is a stain on the ceiling below.

Frequently Asked Questions

Where should you never caulk?

Never seal weep holes, the underside of a window sill, the bottom edge of siding courses, the gap beneath head flashing, or the joint where a wall meets a deck ledger. All of these are drainage paths, and closing them traps water inside the assembly.

Should you caulk the bottom of a window?

No. The underside of the sill is the designed exit route for any water that gets past the outer seal. Caulking it turns the wall cavity into a bathtub — water still finds its way in, but it can no longer get out.

Why shouldn't you caulk weep holes?

Weep holes drain the cavity behind brick veneer, and the small slots in a window frame drain the frame itself. They are meant to look like gaps. Sealing them causes moisture to accumulate against the sheathing and the wall structure instead.

Should you caulk siding butt joints?

Generally no. Most fibre cement and wood lap siding is designed to be back-flashed at the butt joint and to breathe. Caulking those joints traps moisture in the end grain, which is exactly where the board is most vulnerable to rot.

Can you caulk between the tub and the floor?

It is usually better not to. A small gap there is your early warning that the tub is leaking. Sealing it hides an active leak while it soaks the subfloor, and by the time it becomes visible the damage is substantial.

Should baseboards be caulked to the floor?

Not to a wood or laminate floor. That floor needs to expand and contract seasonally, which is why it was installed with an expansion gap in the first place. Caulking the baseboard down restrains it and causes buckling or gapping elsewhere in the run.

What happens if you caulk somewhere you shouldn't?

Water still enters — no exterior assembly is perfectly sealed — but it can no longer drain. It accumulates against sheathing, framing and insulation, and the resulting rot is typically discovered years later during a window replacement or siding job.

How do I know if a gap is meant to be there?

Ask which way water would travel. If the gap sits at the bottom of an assembly, or on a face where water would run out rather than in, it is almost certainly drainage. Gaps at the top and sides of an assembly are usually the ones that want sealing.

Related Caulking Guides

How to Caulk Exterior Windows — the joints that should be sealed, and how.
Best Exterior Caulk — what to use where sealing is the right answer.
How to Remove Old Caulk — for reopening a drainage path somebody closed.
Caulk vs Sealant — why the two words describe different materials.

The Bottom Line

Seal the top, seal the sides, leave the bottom open. That one sentence covers weep holes, sill undersides, siding butt joints, head flashing and most of the rest of this page.

If a gap looks like something the builder forgot, assume it is drainage until you have established otherwise. The cost of leaving a gap open by mistake is a small draught. The cost of sealing one by mistake is rotten sheathing.

Where Not to Caulk

In This Guide