Home / Exterior Joints You Should Never Caulk Shut

The Gaps on Your House That Need to Stay Open

By pasadena house painting journal

Once you start caulking, it is tempting to keep going and seal every gap you see. That instinct causes real damage. A house is built to shed water on the outside and to let any water that gets behind the cladding drain back out. Some openings on the exterior are there on purpose, and sealing them traps moisture inside walls and window assemblies, where it rots wood, rusts metal and pushes paint off from behind. Knowing which gaps must stay open is as important to good maintenance as sealing the ones that should be closed.

The principle behind drainage gaps

No exterior wall is perfectly watertight. Wind-driven rain, condensation and small leaks all let some water get behind siding, stucco and trim. Modern building practice assumes this and provides a path for that water to escape: a water-resistive barrier behind the cladding, flashing at openings, and gaps at the bottom of assemblies where water can drain. When those drainage paths are sealed, water that gets in has nowhere to go. It sits against the wood or sheathing and causes decay that you may not see until it is extensive.

A simple rule helps: caulk the top and sides of an assembly, and leave the bottom open where it is meant to drain. Water runs down, so the lowest edge is usually where it needs to exit.

Lap siding: the bottom edges

On horizontal lap siding, each board overlaps the one below. The gap under each board's bottom edge, where it rests on the board beneath, should not be caulked. This small space allows any water that gets behind the siding to drain out and lets the back of the boards dry. Sealing it with a bead of caulk along every lap traps moisture and commonly causes the siding to cup, crack and peel. It can also make the siding look as though it has been painted shut, since paint and caulk bridge the gap.

Caulk does belong at the vertical joints where siding meets corner boards, window casings and door trim. Those joints face the weather and would let rain run in sideways.

Window weep holes

Many windows, especially vinyl and aluminum units, have small slots or holes along the bottom of the frame. These weep holes drain water that collects in the sill track. They must stay open. Caulking or painting over them lets water back up inside the frame, where it can overflow onto the interior or seep into the wall below the window.

When cleaning or painting around windows, check that these openings are clear. A thin wire or a soft brush can clear dirt that blocks them, and Santa Ana dust tends to collect in window tracks.

The bottom of window and door trim

On many houses, the joint between the bottom of a window's head flashing and the trim below, or the underside of a window sill, is meant to allow drainage. Head flashing above windows and doors is typically installed so water drains out over the trim. Caulking the bottom edge of a head flashing can trap water behind it. Similarly, the underside of a protruding sill, where the drip edge is, should not be sealed.

The rules differ depending on how the window was installed and what flashing was used, so look carefully before sealing. If in doubt, seal the top and sides of a trim assembly and leave the lower edges open.

Stucco weep screeds

Stucco walls on many Pasadena homes, especially newer and remodeled ones, end at the bottom with a metal strip called a weep screed, installed near the base of the wall above the foundation. It provides a clean edge for the stucco and lets moisture behind the stucco drain out. Do not caulk the weep screed shut or fill its gap with paint or mortar. Also keep soil, mulch and paving from burying it, since that blocks drainage and creates a path for moisture and insects into the wall. Older stucco homes may not have a weep screed, but the principle still applies: the base of a stucco wall needs clearance from soil.

Where a roof meets a wall

Where a roof slopes down against a wall, flashing directs water away. At the bottom of this flashing, a kick-out flashing may turn water into the gutter. These assemblies should be sealed according to how they were designed, but do not caulk the bottom of step flashing or block the outlet where water is meant to leave. If this area is failing, it is a roofing repair, not a caulking job.

Vents and other openings

Foundation vents, attic vents and dryer vents are there for airflow. Never paint or caulk them shut. When painting, mask them or paint the frame only. In fire-prone areas, including the foothill zones near Pasadena, some homeowners replace vents with ember-resistant designs, which is a different project from painting. Keep screens and louvers clear of paint buildup that could reduce airflow.

Garage doors and gates

The bottom of a wood garage door and the base of a gate should be sealed on their cut ends with primer and paint, but the gap below them must stay clear of anything that would hold water against the wood. Weatherstripping at the bottom of a garage door is designed to flex and seal against the floor, not to be caulked.

What to seal

With the exceptions covered, the following joints generally benefit from sealing:

• Vertical joints where siding meets corner boards and trim.
• The tops and sides of window and door casings where they meet siding or stucco.
• Joints between trim pieces, such as where a casing meets a head trim.
• Gaps around penetrations such as pipes, hose bibs, lights and electrical boxes.
• Cracks in stucco and gaps where stucco meets wood trim.
• Nail holes and small splits in trim, filled rather than caulked where possible.

Fixing joints that were sealed by mistake

If a previous paint job caulked the laps of your siding or sealed window weep holes, it is worth correcting. Carefully cut the caulk out of siding laps with a sharp utility knife, taking care not to damage the boards. A thin putty knife run under the lap can break a paint bond. Clear weep holes with a wire or small drill bit used gently by hand. On homes built before 1978, keep chips contained, since old caulk and paint may contain lead. Once drainage is restored, you may see signs of water damage that had been hidden, which should be checked for rot before the next coat.

Good caulking protects a house by keeping water out of joints that face the weather. Leaving the drainage paths open lets out the water that gets in anyway.