Stucco on Wood Frame Homes

By
Home Inspector with Home Inspector/Structural Engineer/Mold Assessor

Exterior walls of a house can have many different finishes such as brick, wood, vinyl, or stone.  Another common finish material is stucco, which has been in use for hundreds of years.  In the U.S. it may also be called "exterior cement plaster".  It provides a durable finish, is economical, and creates an aesthetically attractive surface.  When stucco is applied to an exterior wall, the type of surface to which it is applied governs the method of application.     

          Stucco is basically a thin layer of concrete.  It is porous and will develop very small cracks which, over time, provide a means for water to penetrate via capillary forces and direct flow.  This problem is exacerbated in rainy regions with high winds.  (Welcome to Florida!) Wind creates pressure on exterior surfaces which forces moisture (rain) into and through stucco surfaces.  Therefore, a proper stucco finish must include a means for water to escape from behind its surface.  Note: applying paint to a failed stucco finish is not considered a cure for leaks, which will return. 

A stucco wall finish may appear in good shape, yet there can be unseen problems that, over time, can cause extensive and costly damage to a wood frame structure.  To avoid severe problems, there are several important procedures that must be followed when stucco is installed.        

The exterior walls must be correctly prepared:  (1)  Wall sheathing (plywood or oriented strand board) must be installed with small gaps between each piece to allow for expansion (from humidity) of the sheathing.  The gaps prevent sheathing from buckling when it expands, which otherwise will fracture the stucco finish.  (2) Over the sheathing, typical stucco calls for the placement of two layers of Grade-D building paper to prevent water from reaching the sheathing and structural members.  The paper must also allow moisture vapor to pass through, so the internal wall sheathing can dry should it become wet.  (3) On top of the building paper, a layer of expanded galvanized metal or fiber "lath" must be attached using proper nails and nailing patterns.  Lath provides a mechanical connection to keep stucco attached to a flat wall surface.  Note: both the layers of building paper and lath must each be properly overlapped where sections meet to prevent formation of channels that could allow water into areas behind the paper.  (4) A comprehensive design for placement of contraction joints (control joints) must be created.   As stucco cures, it will contract.  Without predefined joints, it will develop random surface fractures.  Control joints must be provided in any wall area exceeding 144 sq.ft., (12ft x 12ft), or in any surface extending more than 18 feet.   (5) At control joints and around windows, doors, or any other through-penetrations of a wall, specially shaped plastic joints ("M", "W", "slip" joints etc...) must be installed to control water behind the stucco.  Directing and controlling the flow of water behind stucco is paramount to ensure water stays out of a structure and to ensure any such water flows down and out the base of exterior walls.  At the base of a stucco wall, a "weep screed" must be provided to allow water to escape from behind the stucco and prevent it from pooling inside a wall cavity or running into a structure.   

Proper mixing of stucco is also critical.  Different types of stucco from different manufactures carry different blending, mixing and application instructions.  The stucco installation crew must follow those specific directions to the letter.  If the mix ratio or blending process is not correct, the stucco, once cured on a wall surface, may not provide the desired hardness, texture, color, or protection from the weather.  Additionally, the environmental conditions present at the time of installation must also fall within the manufactures guidelines to avoid improper curing. 

With proper wall preparation and mixing of stucco material, it may then be applied in accordance with the manufacturer's instructions.  Common stucco requires three separate coats; first, a base "scratch" coat, then a second "brown" coat, and finally a "finish" coat.  After each coat, a specific length of time must be provided for curing and hardening before the next coat is applied.  After the scratch coat, the installer must wait 48 hours, and after the brown coat, 7 days.   During each period, the stucco must be dampened or "misted" once or twice during the day to ensure adequate moisture is retained in the stucco before the next layer is applied.  The final thickness of stucco on a wood frame house should be at least 7/8".  

If any of the above steps for preparation, mixing, or application of stucco on wood-frame construction are not properly followed, moisture problems may arise.  Problems can include: rotted wall structures, flow of water into a house around windows and wall bases, cracking and buckling of the stucco surface, mold and odors, pealing paint inside or out, and cracking of interior moldings from moisture. 

Finding the source of water leaks in stucco can be difficult, and correcting them can be costly.  Unfortunately, there is no easy way to determine if stucco was or was not installed properly, short of witnessing its installation or conducting destructive testing of wall sections.  To be safe, every new-home buyer should request a home warranty that covers stucco finishes, and EVERY home buyer should have a home inspected for signs of stucco failure before the end of their contract's inspection period.                         

Greg Bertaux is a licensed professional engineer and home inspector.  His company, ISLE   Management Corp., provides property inspection services along the entire Treasure Coast. For more information call (772) 569-2141, or visit  www.IMHomeInspector.com

Copyright 2008 ISLE Management Corporation

 

 

Comments (1)

Anonymous
arthur swords

Greg,

I am building a home in Naples, FL.  The contarctor wants to build the walls with concrete block up to the roof line and then wood frame the gable ends.  The roof pitch will be 12:12 and he says running the block all the way up the gable end and capping the wall with a tie beam will be very expensive.

The walls will then be stuccoed.  I'm concerned with the horizontal joint between the masonry wall and the wood framed gable end wall above.  What kind of joint would you recommend here that will provide an expansion joint and allow any water that may get behind the gable wall to drain?

May 23, 2009 08:50 AM
#1