Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.
The Gap at the Bottom: Sealing the Weep Screed Without Rotting Your Home
One of the most common panic calls we receive comes from homeowners inspecting their perimeter: "There is a huge gap between the metal at the bottom of my wall and the concrete! Is my house falling apart? Can mice get in?"
We have good news and bad news.
The Good News: The gap is intentional engineering, not a defect.
The Bad News: If you try to seal it incorrectly with a caulking gun, you will trap moisture inside your walls and rot your framing. This guide explains the physics of the gap and the safe way to close it.
1. Why the Gap Exists (It’s Not a Mistake)
That gap is the result of modern building codes colliding with the reality of concrete.
- Shear Wall Offset: In older homes (Open Stud), the stucco was nailed directly to the studs, sitting flush with the foundation. Modern codes require 1/2" plywood (Shear Panels) over the studs. The weep screed is nailed on top of this plywood, pushing the metal 1/2" to 3/4" away from the concrete foundation.
- Foundation Variance: Metal screeds are perfectly straight. Concrete foundations are not. When the concrete bows inward, the gap behind the straight metal appears wider.
2. The "Do Not Touch" Zone
The Weep Screed is the exit door for your wall system. Gravity pulls moisture down the building paper and out through the small holes in the bottom of the screed.
NEVER apply caulk over the front face or the weep holes of the screed. If you seal the holes, you turn your wall into a swimming pool. The water will back up, saturate the plywood, and rot the mudsill.
3. How to Seal It Safely (Method A: Polyurethane)
If you are concerned about ants or spiders entering through the gap behind the screed, you can seal it, but you must be precise.
The "Undercarriage" Technique:
Instead of caulking the front, apply a bead of high-quality Polyurethane Sealant (like Sikaflex) underneath the metal.
- Bridge the gap between the bottom return leg of the metal and the concrete foundation.
- Do not push the caulk up into the cavity. Just seal the horizontal line where the metal hovers over the concrete.
- This blocks bugs from crawling up, but leaves the internal drainage path open.
4. How to Seal It Safely (Method B: Retrofit Screens)
If the gap is large enough for mice (>1/4 inch), caulk is messy and ineffective. The professional solution is a Weep Screed Screen.
These are stainless steel mesh strips designed to slide up into the gap.
Why they win: The mesh is fine enough to stop mice and large insects, but open enough to allow airflow and water drainage. It is a permanent, mechanical barrier that doesn't rely on chemical adhesion.
5. Avoid Expanding Foam
We strongly advise against using expanding spray foam (like Great Stuff) for this application.
Foam is uncontrollable. It expands upward into the wall cavity, potentially blocking the drainage plane 2-3 inches above the screed. Once it cures, it is nearly impossible to remove without damaging the stucco.
Conclusion: Airflow is Good
Unless you have a confirmed pest infestation, the best advice is often to leave the gap alone. The airflow helps keep the stucco dry and the wood framing healthy. If you must seal it, use the Undercarriage Technique and ensure water can still drip out.
Last week, we shared Integrating Weep Screed with Rainscreens in Stucco Systems. Learn how advanced drainage mats require even larger gaps for ventilation.