Why Stucco Cracks
Stucco manufacturers and Plastering Contractors are constantly striving to improve stucco. However, from time to time even stucco manufactured and installed correctly will crack.
Fine sand finish or smooth finish textures accentuate cracks in stucco, while heavier textures tend to hide stucco cracks. Although not all cracks are objectionable, cracks in stucco acceptable to one person might be unacceptable to another person. Cracks can occur in stucco regardless of texture.
Cracks that appear within the first 30 days after installation and are larger than 1/16” (the thickness of a penny) can be filled or repaired. Cracks that are patched and re-appear could indicate a structural or substrate movement problem, necessitating the use of an elastomeric coating. If a crack is visible from more than 10’ away and is a source of leaking, it should be patched. Patching small hairline cracks (smaller than 1/16”) is not recommended. Small cracks will not accept material, and the resulting patch will detract from the natural beauty of the stucco finish and will serve no useful purpose. In this case a skim coat of new stucco finish from corner to corner or from control joint to control joint would be the correct way to repair the stucco cracks and avoid the perceptible patch look.
Plaster cracks form when a stronger force exceeds the restraint capacity of the stucco system. Cracks are “stress-related energy”. The UBC and ASTM do not call for a required hardness (PSI) for Portland Cement Plaster. Minor cracking at the corners of doors and windows and other stress points is reasonable and should be anticipated.
There are two primary reasons for stucco to crack:
May develop as the excess water evaporates from the drying cement mix. Shrinkage cracks can occur very early on and cannot be seen after the finish coat is applied.
Can occur in stucco when stress is transferred to the plaster membrane from various external sources.
Examples of transferred stresses include:
Fill vs. Cut. Does the site have proper compaction? Are there settlement/ or subsidence? Has seismic vibration (earthquake) been a factor?
Concrete creep sag
Common frame trouble spots include offset loads, big spans, point loads, cantilevers, notching green lumber (as lumber is stabilizing with the outside ambient humidity and temperature). For every 4% change in moisture, lumber changes 1% in size. As the lumber dries out, depending on the direction of the grain, lumber will twist and warp in various directions. Water may cause swelling and buckling in plywood or OSB sheeting, especially when there is no gap between the sheets. Moisture that makes its way into the lumber during periods of heavy rain will cause the lumber to swell and put pressure on the stucco membrane.
Plywood or OSB not properly spaced or staggered at vertical horizontal joints. Plywood or OSB not run perpendicular to the studs. Plywood or OSB nailed closer than six inches at the edges may contribute to cracking in stucco by causing the lumber to move outwardly rather than laterally. Wood-based substrates should be gapped a minimum of ¼”.
irregular, inconsistent, or offset framing can cause variations in the stucco thickness, subjecting it to random cracking.
If the framing lumber of sheathing becomes wet, more cracks are likely to appear because the lumber or sheathing may swell putting pressure on the stucco membrane and causing it to crack.
Mechanical vibration and/or other movement.
Wind loads, live loads, and/or dead loads that exceed design specifications. Stucco is not a structural material, it will not hold up the building. If the building moves for any reason, the stucco will develop cracks.
Minor cracking is not likely to contribute to water intrusion; however, large cracks may allow water to reach the back plane or plaster. A properly installed weather barrier will direct any incidental water to the bottom of the wall and allow it to exit at the weeps creed flashing at the bottom of the wall.
A stucco expert can generally determine the cause of some cracks, but at the same time, some cracks can be puzzling. In some situations, it may take a structural engineer to evaluate how or why a building is moving.
Smooth Trowel Finishes
With smooth trowel finishes (Also known as Santa Barbara or Mission Finishes), even minor hairline cracking is more visible. Burn marks and cat faces can also occur in the process of troweling the material down to a smooth finish. This look is either desirable or undesirable, depending on the buyer.
How can we prevent virtually almost no cracks on a stucco system?
A technique that can “almost” assure no cracks will develop in a stucco system involves troweling a fiberglass mesh into a polymer cement basecoat material over the entire stucco base coat followed by application of a 100% acrylic, color-integral, “Synthetic Stucco” finish. This comes with an additional cost, but can virtually eliminate cracks in a stucco system.
What can the General Contractor/Builders do to reduce the likelihood of stucco cracks?
- Use Licensed Plastering Contractors that apply stucco according to local building codes.
- Install Control Joints per Plastering and Lath ASTM 926 and 1063
- Gap sheathing on all sides by 1/8” and nail 6” O.C. to allow for lateral expansion when set
- Load roof tiles five days prior to lathing installations
- Drywall screwing and/or nailing prior to stucco installation
- If stucco is to be painted, allow 28 days for curing cementitious products prior to painting.
- Stagger the mid-span blocking to prevent studs from twisting
- Verify ground compaction or don’t build on fill.
- Proper lumber grades and moisture content
- Verify that the framer correctly installs all structural members