Cracking is the formation of cracks in stucco due to shrinkage, expansion, and other movement that stresses the stucco coating.
Cracking is one of the largest complaints from homeowners with stucco homes. Cracks are unsightly and can lead to major water intrusion issues if they aren't addressed in a timely manner. All stucco cracks are caused when the stucco releases stresses placed on it by shrinkage, expansion, or other movement. Usually, stucco releases stress through a series of small cracks spread over the wall surface or at predetermined locations where control joints are placed. However, errors in stucco application can cause the stucco to store the stress until it's forced to crack. These cracks can run deep, exposing the building materials beneath the stucco, such as housewrap, to the elements. Because these materials aren't built to withstand prolonged exposure, water will eventually be able to penetrate them and move toward the interior of the home.
Cause 1:
Insufficient sand
Stucco mix consists of three main ingredients: cement, sand, and water. Cement forms the base of the stucco mix. In some cases, lime is added to the cement base. Sand is added in proportion to the amount of cement and lime. Water is added to achieve a good trowel consistency. The mix proportions must be carefully controlled to create a stucco mix that, when adequately hydrated, allows each coat to relieve stresses through a dispersed pattern of small cracks prior to the application of the next coat.
The amounts of water and cement are usually tightly controlled by the manufacturer’s specifications. With sand, there's more opportunity for error. When there isn't enough sand in a coat, the cement paste shrinks too much. The excessive shrinkage puts excessive stress on the coat, leading to deep cracks on the surface of the wall. These cracks frequently occur at locations where the cross-section area of the stucco is reduced, such as at a window.
Solution
Mix the scratch and brown coats according to manufacturer's instructions. Carefully control the amount of sand in each coat. Be sure to add sufficient sand for shrinkage resistance. On the other hand, don't add too much sand to the mix; if too much sand is added, water will have to be added to compensate, reducing the amount of cement in the mix. Make sure the sand is stucco grade to ensure workability. Always use stucco sand, not mortar sand. Let the mix sit for 20 minutes before remixing it for application. Generally, the brown coat requires more sand than the scratch coat in order to make it weaker. Additional water shouldn't be added, because it will weaken the stucco. If too much water is added, the stucco will lose cohesion and slump on the wall.
ProTip!Store stucco mix on pallets and lay sand on plastic sheets to ensure they won’t be contaminated with soil. Dirty sand reduces the strength of stucco and leads to cracking.
Cause 2:
Inconsistent coat thickness
It’s important to apply the scratch and brown coats consistently over the surface of the wall. When a coat is inconsistent, for instance, ¼” thick in one area and ¾” thick in another, its strength varies. The thin areas won't be strong enough to resist cracking. Achieving a uniform thickness can be particularly difficult with open-frame construction; the stucco is likely to be thinner over a stud and thicker across a stud bay. Stucco is particularly susceptible to cracking around windows and trim boards, where it’s applied thicker to create architectural character.
Solution
Apply the scratch coat at a uniform thickness of 3/8” to ½". Make sure the scratch coat is well embedded in the metal lath. Apply the brown coat at a uniform thickness of ¼” to 3/8”, except at architectural details, where it may be thicker. The brown coat provides architectural character, such as slopes and edges. The brown coat should be thinner than the scratch coat; the scratch coat should be the strongest layer so the stress in the system is between the coats. When screeding the brown coat, control the thickness using reference points on the wall, such as edge trim.
Cause 3:
Insufficient hydration
After a coat of stucco is applied, it should be hydrated. Hydrating a coat of stucco allows it to cure to full strength. When a coat is hydrated, it absorbs the water applied to it and cures. After the coat has absorbed sufficient water, it dries, reduces in volume, and shrinks. The shrinkage builds up stresses in the coat, which it releases in a series of small cracks. The cracks, which aren't deep enough to undermine the integrity of the stucco system, are then covered up by the next coat.
If a coat isn't sufficiently hydrated, it won't have the proper amount of strength it needs. For instance, applying the brown coat to a scratch coat that hasn't sufficiently cured yet will cause the scratch coat to absorb water from the brown coat and continue to cure. The two coats, which are both curing, drying, and shrinking, will essentially act as one coat, doubling the stress they place on the stucco system. The excessive stress will be released as deep cracks. These cracks may run through both coats and even sheer the metal lath. This problem can also occur between the brown and finish coats.
Solution
Adequately hydrate the scratch, brown, and finish coats. All coats should be hydrated in the same manner. To hydrate a coat, flood the wall surface with a gentle stream of water, moving from the bottom of the wall to the top until the water runs off in sheets. Work from one end of the wall face to the other. After reaching the end of the wall, return to the beginning, and apply water again until it runs off in sheets. Then, saturate the wall face a third time in the same manner, until the water runs off in sheets. The second and third passes on the wall will require significantly less water to reach the sheeting stage, because with each pass, the stucco will accept less water. Hydrate each exterior wall at least once, but up to as many as three times, if the wall continues to absorb water. The stucco should retain moisture for the first 48 to 72 hours after it's applied in order to prevent carbonation and cure to full strength. Make sure to apply water under porches, at overhangs, and under popouts; normal runoff won’t reach these areas.
ProTip!Applying water from top to bottom will cause the water to run down in rivulets, potentially eroding the stucco, so make sure to wet stucco from bottom to top.
Cause 4:
Lack of control and expansion joints
Large, uninterrupted areas of stucco will crack when they expand and contract, because expansion and contraction build up stresses in the wall system. The stucco at corners and terminations around windows and doors is particularly susceptible. Even if the stucco is perfectly mixed, applied, and hydrated, cracks will still appear if control and expansion joints aren't installed.
Solution
Install control joints at least every 144 square feet. When possible, align control joints with window or door openings to provide protection against cracks and to create a look that’s more aesthetically pleasing to the homeowner. Install expansion joints between floors and between changes in mass, such as the house-to-garage intersection. Control and expansion joints are designed to relieve stresses on stucco walls to prevent cracking where the stucco expands and contracts.
Cause 5:
Poor sheathing installation
Wood sheathing helps support the wall system. If it’s installed only at corners or above and below windows, it creates an uneven surface for applying the stucco. It’s difficult to apply stucco at a uniform thickness across an uneven surface. An uneven surface acts in the same way as a window opening; it reduces the cross-section area of the stucco as the stucco becomes thinner. If the stresses exceed the capacity of the thinner section to transfer stress, the stucco will crack.
Solution
Install sheathing continuously. If using plywood sheathing, position the boards horizontally. Leave a 1/8” space between the edges of the boards to allow for moisture expansion. For open-frame construction, line wires are installed instead of sheathing to support the wall system. However, sheathed frame construction ensures a more uniform stucco application.
ProTip!Make sure wood sheathing is of the proper moisture content and grade in order to prevent unexpected expansion, which could lead to cracking.
Cause 6:
Incorrect application of metal lath
In a stucco system, a self-furring metal lath reinforces the scratch coat. The furred side holds the metal lath about 1/8" to ¼" off the wall, allowing the scratch coat to fully encase the lath. If the lath is installed backwards or stretched too tightly, the scratch coat won't be fully embedded in and reinforced by the lath. If the scratch coat isn't properly reinforced, it will build up excessive stresses and transfer them to the brown and finish coats, potentially leading to serious cracks.
Solution
Before applying the scratch coat, install self-furring hexagonal metal lath. Make sure the stand-offs are installed against the wall surface, holding the metal lath about 1/8" to ¼” off of the wall. Install the lath continuously across the entire wall, overlapping seams about 2". The lath should also be applied to all architectural features. An alternative to using self-furring lath is to use lathing stand-offs that hold the lath at the appropriate distance from the wall.
