Repairing stucco? Should you replace or resurface?

This is a difficult question and is fully dependant on what you wish to accomplish. If the substrate is wood frame construction and the problems involve leaks, then removal of the damaged stucco and repair of the dry-in is essential, followed by replacement of the stucco. If the problem is cracks due to shrinkage and the substrate is unaffected then a repair strategy involving a dash coat of stucco or filling cracks with an elastomeric sealer prior to repainting would be a good option.

The repair of stucco where it has been applied over a masonry base raises several, more complex, possibilities. While the above methods will work if the cracks are not too pronounced, in some cases you may find that the bond between the masonry and the stucco has failed. Then the existing stucco will have to be removed and we recommend the areas where the stucco has been removed have metal lath installed and then the stucco base restored. If the cracking is wide spread and it is desirable to create a new stucco finish then there are two ways to accomplish this. The first would be to remove the surface coating from the existing stucco by media blasting and/or by high pressure water cleaning followed by treatment with a liquid bonding agent prior to application of the stucco. The use of bonding agents to form a bond to an existing surface is a gamble and the use of blasters and media (sand, crushed walnut shell, or plastic beads) is always messy and potentially toxic.

We recommend a second approach which utilizes expanded metal lath to insure bond. The attachment of metal lath as specified in ASTM 1063 is specifically for new work and specifies a minimum fastener length of 3/4 of an inch when attaching to masonry or concrete. In this case we are attaching stucco to existing stucco, therefore the industry has adopted a standard of attachment using much shorter 3/8 inch stub nails. The stub nails create a bond to the existing stucco without damage to the substrate or the bond between the existing stucco and substrate. The stub nails in this application are spaced approximately six inches on center. This method provides a mechanical bond as opposed to an adhesive bond of the new stucco to the old.

If you have a stucco question our building envelope group is ready to answer your questions or assist you in developing a repair strategy. Contact Gary Pailtorp at gpailthorp@brackenengineering.com.