Part 1 – An Introduction to Stucco.
Exterior Plaster wall systems (commonly known as Stucco) are a popular choice for exterior walls in the Southwest United States, Florida and elsewhere. As a siding material, stucco is;
Economic: stucco is a cost-efficient exterior cladding system
Water resistant: when properly installed and cared for, stucco can protect the building envelope
Breathable: stucco systems allow water vapor out
Durable: stucco is hard enough to last for many decades
Fire Resistant: stucco is non-combustible
Pest Resistant: stucco is not subject to pests that consume
Energy Efficient: stucco provides good insulation performance
Traditional, modern stucco is commonly referred to as three- coat stucco. This is a building system that incorporates the building sheathing (typically plywood or OSB), a moisture barrier (often two layers of Grade D building paper), a metal lath secured to the building with nails (or in some cases staples), and three coats of exterior plaster (a Scratch, Brown, and Finish [or Color] coat). Three- coat stucco is the most common stucco system, and the one that is the focus of this series of articles.
Other stucco systems include Acrylic Stucco, which is based on acrylic resins being substituted for cement based plaster, often applied as a substitute for the Plaster Finish Coat, the misnamed One-Coat system that involves mixing fibers and other non-cement compounds into the plaster in at least two coats, or other siding systems such as EIFS, a system that incorporates a layer of insulation together with fiberglass, non-cement compounds, with a color coat.
Traditional (three-part) stucco, referred to here as “stucco”, is an efficient and durable, if misunderstood system.
The principal misunderstanding is that (plaster) stucco is not a waterproof system; it is a water shedding system.
There are advantages to this. The exterior of unpainted stucco buildings do not have to be perfectly sealed and caulked to prevent all water intrusion, because the moisture barrier is behind the plaster (safe from damaging UV radiation or physical damage). Water shedding involves the performance of the plaster on the exterior of the building after it has absorbed all the water it can (surface saturation). At that point, any amount of water will shed off of the plaster surface. The water then slowly soaks through the plaster, estimates are that it can take up to eight hours of exposure to rain to saturate (soak through).
Water that penetrates through the plaster collects as drops that run down behind the stucco plaster and off of the moisture barrier (often building paper) running down the exiting the building at the stucco weep screeds, located along the bottom of the stucco walls. This way, the water barrier (behind the stucco plaster) is exposed to relatively little water. On the return of fair weather, the stucco plaster dries out slowly in a uniform manner.
One reason traditional stucco is desirable is that it can allow the water vapor that is generated from the interior of buildings (through shower steam and evaporation, cooking, use of clothes dryers, condensation, and other sources) to find its way out. One way to think of this is to look at unpainted stucco as the “Gore-tex”™ of building envelopes, a water resistant material that avoids trapping moisture. Experienced hikers will tellyou that a breathable layer performs much better than a completely waterproof layer because the latter traps moisture, whether the moisture comes from sweat (vapor generated inside buildings), or from rain that drips down the back of your neck (small leaks at eaves, windows, doors, or other cracks).
Of course, the proper functioning of a stucco system pre-supposes that the building incorporates the proper flashing of eaves, gutters, windows and other penetrations into a functional moisture barrier layer (behind the stucco plaster). This is an essential part of the stucco system, since this layer prevents water from entering into interior wall cavities. Attention to detail is required to adequately flash and layer the moisture barrier and tofully caulk all penetrations including the lath nailing (to seal the penetrations in the moisture barrier), a process made a good deal more efficient through the use of self-sealing nails.
The “breathable”, unpainted stucco system just described does have weaknesses. The stucco plaster is a hard, semi-porous material that is susceptible to staining from surface water that can carry contaminants (dirt, soot, other chemicals) that can be absorbed into the plaster. This creates stains, and in the event of long term exposure to surface water, stucco is susceptible to damage from biological growth. This is in part a design consideration to provide building have proper gutters, overhangs and drip edges and it is a protection / maintenance concern to see that gutters and drip edges are kept in good working order.
It is also essential that all landscaping is finished at least 6 inches beneath the stucco weep-screeds. This allows the stucco to perform as designed, and gives the bottom few feet of stucco walls the chance to dry out, and it prevents accidental contact of stucco with damp soils that can “wick” up the stucco wall and eventually into living spaces.
The hardness of (plaster) stucco can lead to cracking, with patching recommended for unpainted stucco for any cracks that can be seen from 10 ft. away, or any cracks that run horizontal in areas that carry surface water runoff (such as from decks or roof runoff). This is in part a cosmetic concern as well as a practical one. Although unpainted stucco does not need to be a perfect seal against rain water, cracking in a surface water condition or extensive cracking thicker than the width of a credit card can lead to excess water making contact with the moisture barrier beneath, which is not recommended.
The same does not hold true for painted stucco. Paint, to the degree that the paint layers are not vapor permeable, changes the structural characteristics of the stucco system by making the paint layers on the exterior the moisture barrier. Paint also cuts off the stucco plaster from direct air contact. Therefore, we believe that no stucco cracking or cracks in joints with windows or other stucco penetrations can be allowed.
The reason this is a structural issue is that any moisture that gets behind the paint layer is inhibited from drying out as quickly or in a uniform manner. This can hold moisture against the moisture barrier for too long (causing water leaks in the house), and in extreme cases (where the stucco stays wet for extended periods), moisture can cause the lime in the stucco plaster to react and decompose the plaster /cement material itself, producing abrittle material that can crumble on contact.
Paint has other downsides; paint is composed of a resin binder with pigments that chemically adhere to the cement plaster. This layer sits on top of stucco texture and washes out stucco’s original texture. Paint also requires additional coatings every 4-10 years depending on the type of paint. Failure to keep up with repainting invalidates paint warranties and risks both damage to walls from water penetration, as well as the significant expense in the event that the paint bubbles or peels off. There are few remedies for that, none of which are inexpensive.
Moisture management is the key to longevity of stucco walls. Unpainted (natural) stucco plaster reveals the state of water absorption by changing color. This is tremendously useful in revealing when stucco is wet for too long or wet in the wrong places. Darkened stucco behind gutters, at window corners, in stucco ceilings, can reveal leaks from gutter nails, inadequate flashing, deck leaks, etc. In fact walking a customer’s project after a rain is one of our standard practices to help spot problems. Unfortunately, once a building is painted, any moisture intrusion problems are hidden until the paint or the stucco system starts to fail, which can be too late to avoid significant dry rot and other moisture related damage.
Alternatives to paint have existed for years. The primary options available are the application of an additional plaster color-coat, or the application of refinishing material such as Dash or Fog Coat. A Dash coat (or slush coat) is a mix of concrete sand and water that is cast onto a cement or stucco wall that cures to a mechanical bond. Fog Coat is a cement based coating, with pigment included, that is designed to refresh stucco walls witha finish that structurally bonds with and becomes part of the plaster stucco finish. All of these options leave stucco with a soft, “natural” look and feel, that leave stucco in its natural state of darkening when wet.
All of these options appear to not inhibit stucco’s ability to breathe. Cement based refinishing is preferred over paint for stucco systems that are mechanically sound, and they provide the lowest lifetime cost for stucco care. This is because cement based touch-up can be done to spot treat areas, as opposed to paint which will need to be regularly re-painted over the entire surface.
There are instances where paint makes sense. For stucco systems that are compromised, in which the moisture barrier behind the stucco plaster has failed or is structurally deficient, paint becomes a lower-cost (if temporary) option compared to the cost of removal and replacement of the stucco wall system. Paint, when repainted regularly, can serve for a long time as the building’s moisture barrier. One option that is becoming increasingly popular is the application of an adhesive layer, covered by an integral color acrylic stucco coat. Ongoing maintenance of either paint or acrylic stucco finishes is needed to keep all windows and other penetrations properly caulked and sealed.
The disadvantages of current cement based finishes such as Fog-coat in practice are that they are difficult to color-match, and currently available material has a limited lifespan, particularly on horizontal surfaces. Stay tuned for the next article in this series in which a new generation material has been developed that has all the advantages of cement based finishes, with durability, color-fastness and an ability to do on-site color matching that allows spot and area treatments of staining to match existing stucco color.
Part 2 – Stucco Care, Esthetic Standards, and Color Matched Restoration.
When considering standards for the repair of stucco, it is necessary to differentiate between structural repair and repair that relates to ongoing maintenance and esthetics.
Structural standards are addressed in the UBC and local building codes, and relate to the integrity of construction including the stability of underlying wall systems that stucco is applied to, proper attachment of the stucco system, soundness of the waterproofing layer including proper flashings and sealing of penetrations, and the proper application of the stucco plaster.
Deficiencies in the structural integrity of a stucco system must be dealt with prior to setting up a plan for repair and ongoing maintenance. There are a wide range of solutions available to address structural problems ranging in cost from a good layer of elastomeric paint, to a new color coat, to the replacement of the entire stucco system. Technical evaluation of the stucco system is recommended to know the nature of the deficiencies and the merits of the chosen remedy.
When establishing preventative maintenance and esthetic standards for stucco, two different sets of standards are recommended; one for unpainted stucco and one for painted stucco. This is because painted stucco is vulnerable to loss of adhesion, which can occur when moisture is trapped behind paint.
Recommended maintenance standards for unpainted stucco include the following;
Repair of cracks visible from 10+ feet away: with a high quality, cement based fill material. Cement fill is preferable because it acts like and bonds with the existing wall. Should hairline (or slightly larger) cracks return it does not necessarily compromise a properly functioning stucco system, filling of such cracks can be considered esthetic. Recurring hairline cracks that emerge over a space of years may be filled as a primarily esthetic remedy. Continued, expanding cracking is evidence of building movement, a potentially serious source of damage.
Repair of all voids: holes in stucco walls should be filled, with lapped paper and secured backing, with a two-step texture repair starting with the restoration of the underlying stucco “plane” and then a substantially matched texture. Repairs should be done with high psi concrete and should not extend beyond the plane of the finished stucco surface.
Stains: should be evaluated for the source of contaminants, and the problem addressed. Stains (and repairs) can then be eliminated with spot or area color-matched refinishing.
Our experience is that most commercially available stucco repair materials are not adequate to provide repairs with a significant lifetime, one measured in decades. The primary reason given to us by an industry veteran to explain this is that popular stucco repair materials contain non-cement fillers and other ingredients to make them more workable, but less durable. Working with high psi cement based materials is more difficult, and requires specific training, but the result is a much more durable repair.
Recommended maintenance standards for painted stucco include the following;
All cracks in the stucco must be filled: this is in the stucco field and where stucco adjoins windows, doors or other penetrations. The material used should be compatible with stucco, waterproofing materials, and paint. The objective should be a *100% seal at all gaps.
Repair of all voids: holes in stucco walls should be filled, with lapped paper and secured lath for backing, following with flush repairs with substantially matched texture.
Stains: should be evaluated for the source of contaminants, and the problem addressed. Stains will typically require painting of whole walls (corner to corner
Paint: should be applied after removing any loose material, surfaces should be cleaned, and an appropriate primer applied. Paint should be applied at the thicknesses recommended by manufacturers, and reapplied within the recommended time period
* Elastomeric caulking is commonly used with elastomeric paint, if this is the case verify that cracks have been opened to the required crack widths to allow for the required caulk thickness.
Failure to reapply paint within the recommended time period, or according to recommended application standards can create problems such as pinholes as the paint dries and pulls apart from itself at a very small scale.
The more stringent maintenance requirements of paint, together with the need to regularly re-apply paint should be seriously considered when deciding how to care for an unpainted home.
Should a Stucco Colormatch restoration certified contractor not yet be available in your area, we recommend the search for a local contractor capable of applying Fog Coat. Fog Coat, while not being the highest quality material available (in our opinion), is still a superior solution to paint (except when paint is being used to defend against an underlying stucco system failure). And once Stucco Colormatch materials are available in your area, we can refinish right over the top of Fog-Coat, as necessary.
Question: How do I tell if my stucco has been painted. Answer: Apply a small amount of water to your stucco’s exterior. If the stucco darkens slowly, with a darker area clearly visible at 30 seconds, your stucco is likely to be unpainted. Painted stucco does not change color when wet.
In summary, stucco is a proven water shedding system that was not designed for paint, a material that depends on adhesion and is vulnerable to trapped moisture. The structural integrity of stucco is a prerequisite. Higher maintenance and esthetic standards are possible, as is the possibility of stucco restoration. Properly cared for, stucco is a great system that can function properly for generations.
Preventative maintenance and stucco care is the subject of the third in our series of articles, in which we will pass along the lessons learned from 20+ years of stucco refinishing.
Part 3 – Preventative Maintenance for Stucco.
Preventative maintenance of stucco begins with good architectural design. Buildings designed with stucco in mind will have roof overhangs adequate to shelter horizontal stucco surfaces, adequate coverage of gutters that have been properly installed, drip edges (at decks and architectural overhangs), adequate clearance at weep screeds, and sprinklers that do not allow water to come into contact with stucco. Our experience is that buildings designed to these standards, where gutters, drip edges and sprinklers have been properly maintained, experience minimal damage or staining and when left unpainted, have period between maintenance and esthetic care measured in decades.
A word on sprinkler and gutter maintenance; sprinklers, if allowed to spray directly onto stucco walls exposes the stucco to long term water exposure. This does several things. The first is that it can activate the lime in the stucco to directly attack the plaster matrix, turning the cement based plaster into a kind of mush, that when dry crumbles to the touch. Of more concern, long term water exposure to the moisture barrier behind the stucco plaster can degrade (or destroy) the moisture barrier which exposes interior walls to direct and long term contact with water.
24 years of stucco maintenance have shown us that sprinklers are “enemy number one”. Beyond that, we have come to see how certain preventative measures can address the lack of stucco protection built into architectural detailing.
When buildings lack proper gutter coverage or drip edges to shield stucco from surface contaminants, there are a few things to look at. The first is to make sure that gutters are properly functioning and are clear of debris. It may be useful to retrofit additional gutters if needed to remove roof water, or install gutter guards (extenders) in cases of gutter overtopping. A bit of creativity may be needed to retrofit drip edges at overhangs (such as at chimneys) or on decks. Drip edges can be the difference between a stain that returns in four years versus a stain that returns in 20 years.
When buildings are built without adequate clearance under stucco weep screeds located at the bottom of stucco walls, the stucco is exposed at this critical “front line of defense”. The risk is that water from sprinklers or from out of the ground can be “wicked” into the framing. Homes can run at a very low humidity due to air conditioning and heating systems, low humidity acts like a vacuum sucking in moisture where it can. When moisture can travel out of the ground, into the stucco, it can be drawn into framing, and into the interior. Removal of landscaping and dirt to 6” below weep screeds is desirable, and has become a code requirement in certain locales, when this option is not assessed as economically feasible, a gravel pocket 10” out from the building and 10” deep, protected by bender board and landscape fabric can serve as a capillary break to limit wicking and provide additional airflow to the base of stucco walls. Care should be taken to make sure that the gravel pockets drain so as not to become a pool of standing water.
In cold climates, where snow can accumulate and bury weep screeds, some additional precautions are recommended. The problem is that warm building interiors can cause snow to be absorbed into the stucco, creating damage and staining. A clear penetrating sealer, re-applied per manufactures recommendations to the lower part of stucco walls can help limit water penetration and staining without excessively compromising plaster stucco’s ability to breathe (verify the vapor permeability of such clear penetrating sealers).
When buildings do not have adequate overhangs to protect horizontal stucco surfaces, the solution we recommend is regular maintenance with a cement based restoration material, with the option of an additional UV / water repellent clear sealer. Short of some kind of metal wall cap or flashing, which would change the aesthetics of the building, there is no good substitute for a predesignated overhang.
Dirty, unpainted, plaster stucco can be cleaned with a soft brush and a diluted mixture of TSP and water. Start with wetting and cleaning the stucco at ground level first, and then slowly work your way up plaster stucco walls. The wet stucco (below) will resist the dirt and chemicals released from the stucco being cleaned (above).
Stucco Colormatch has been providing stucco restoration and maintenance recommendations for clients for over 25 years.
We believe that protection and restoration services work together for long-lasting stucco performance.
Old time stucco craftsman.
Co-Founder, Stucco Colormatch