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            Pre-cast (synthetic) Stone Walls


Several times a week we document defects in the construction of a popular new exterior wall cladding system commonly referred to as pre-cast stone, Cultured Stone™ (a trademark of Owens Corning), manufactured stone, or synthetic stone. The product is attractive and common. However, it's almost never installed correctly. Our experience is that although we document the defects, including warnings of the potentially dire consequences of water intrusion, and recommend an evaluation by a specialist in this type of wall cladding, my clients will disregard the advice given in their report. It is typical for a builder to discredit the home inspector after he leaves the premises. He will tell you not to worry, that the stone wall has two layers of waterproof paper behind it. (It isn't "water proof", it's water resistant, and it may not have been lapped correctly at the seams, and it may not be the correct thickness, and it may not be two layers, and...) He will say "we always do it like this", and "we've never had a problem". Well...this is a relatively new wall cladding system. Give it time! He will. Perhaps you're inclined to accept the builders promise to correct any issues that reveal themselves. And you've been assured by a Realtor that the builder is legally liable for X number of years for structural defects. Ask yourself this question: "what if the builder goes bankrupt, or is bought out by someone else, or what if he dies, or what if the problem doesn't arise until year X+1"?

Builders will sometimes claim that there are no codes that govern the installation of pre-cast stone and that they install it according to the manufacturer's instructions. Not so. This type of wall cladding system is essentially a variation of stucco; it is stucco with a rock stuck to it. There are building codes that address stucco systems, of which this is one.

The main thing to remember about pre-cast stone is that it is porous. It is a masonry product, akin to brick, and it will absorb moisture as will the mortar that binds it to the substrate behind it. Stones with horizontal, flat surfaces are more prone to water intrusion issues, as they have a "shelf" that can drain the water back toward the wall. If water gets behind the stone, it needs a way to drain out. Except when the installation is over masonry or concrete walls, a water resistive barrier, proper flashing, and a weep (or drainage) system is required by code and the manufacturer's installation instructions.


A Few Applicable codes (2006 International Residential Code):

R703.6 Exterior plaster. Installation of these materials shall be in compliance withASTMC926 andASTMC1063 and the provisions of this code.

"R703.6.2 Plaster. On wood-frame construction with an on-grade floor slab system, exterior plaster shall be applied to cover, but not extend below, lath, paper and screed."
Keyword: "screed". i.e, drainage system.

"R703.6.2.1 Weep screeds. A minimum 0.019-inch (0.5 mm) (No. 26 galvanized sheet gage), corrosion-resistant weep screed or plastic weep screed, with a minimum vertical attachment flange of 31/2 inches (89 mm) shall be provided at or below the foundation plate line on exterior stud walls in accordance with ASTM C 926. The weep screed shall be placed a minimum of 4 inches (102 mm) above the earth or 2 inches (51 mm) above paved areas and shall be of a type that will allow trapped water to drain to the exterior of the building. The weather-resistant barrier shall lap the attachment flange. The exterior lath shall cover and terminate on the attachment flange of the weep screed."
Keywords: "shall", "screeds", "2 inches above paved areas", "4 inches above earth"

"R703.8 Flashing. Approved corrosion-resistant flashing shall be applied shingle-fashion in such a manner to prevent entry of water into the wall cavity or penetration of water to the building structural framing components. The flashing shall extend to the surface of the exterior wall finish. Approved corrosion-resistant flashings shall be installed at all of the following locations:

1. Exterior window and door openings. Flashing at exterior window and door openings shall extend to the surface of the exterior wall finish or to the water-resistive barrier for subsequent drainage.

2. At the intersection of chimneys or other masonry construction with frame or stucco walls, with projecting lips on both sides under stucco copings.

3. Under and at the ends of masonry, wood or metal copings and sills.

4. Continuously above all projecting wood trim.

5. Where exterior porches, decks or stairs attach to a wall or floor assembly of wood-frame construction.

6. At wall and roof intersections.

7. At built-in gutters."

Keywords: "shall extend to the surface" i.e., shall be visible, "shall be installed at all of the following locations"





Pre-cast stone wall.                                     Another example.



No weep system between the foundation and the wood framed walls.   Pre-cast stone lacking mortar, exposing the wall sheathing at the wall trim.



Rot observed on wall sheathing from inside the crawl space. This custom home costing > $1,000,000 was still under construction and had never been lived in. There was no weep system to drain the moisture from behind the stone.   Openings for water entry around light fixtures.



Wet sub-floor at the perimeter   Wet sub-floor at the perimeter


Leading Manufacturers Documentation

Owens Corning Flashing Best Practices

Owens Corning Installation Instructions

Centurion Stone Weep Screed Illustration

Centurion Stone Installation Instructions


3rd Party Documentation

Articles by manufacturer associations, engineers and contractors with illustrations.

Installation Guidelines for Adhered Concrete Masonry Veneer

Manufactured Stone Nightmares, by contractor Dennis McCoy

The Perfect Storm Over Stucco, by Joseph Lstiburek, P.Eng., Ph.D



In Summary

Now that you've read additional information from the manufacturers, engineers, and contractors familiar with this wall cladding system, hopefully you are convinced that that my recommendations in your report are sound.

You like the house. You may love it. You want it. But you should want it fixed. So what is the solution to an improper installation? It may involve removing all the stone from the walls and re-installing new ones, properly (the stones cannot be re-used).

Why doesn't the builder just bite the bullet and fix the wall, especially if it's a small area? Usually, it's because they think they are right. They sincerely believe that the stone installation is acceptable. Your house is just like all the other houses they've built. So why is your home inspector making such a big deal out of it? (Keep in mind that a very small percentage of new homes get inspected by private home inspectors and less than one-half of home inspectors write this up as a defect because they are not familiar with the issue.) In my experience, general contractors actually don't know the details about the requirements relevant to this type of wall cladding. Few have read the manufacturer's installation instructions. Instead, they simply hire a well known (and low bidding) stucco company to do the work and trust that they know what they are doing because they are licensed. This is no different, actually, than how they handle the electrical system--they aren't experts in that field either, but trust that the electrician is. A volume builder, especially, is often more interested in selling homes than being an expert in construction standards; they leave those details to the sub-contractors.

Secondly, the general contractor will argue that the home "passed the code inspection". It got a certificate of occupancy (CO), after all.  But you should understand that "the building code" is the minimum expectation required to build a home. Would you be happy with a car that met only the minimum expectation of the government? (If so, I have a Hugo I'd like to sell you.) Today's commonly accepted building code book is about 2 inches thick, over 600 pages in length, filled with technical jargon, and rare is the person who is an expert in every aspect of it. Regarding the code inspection process, most municipal code inspectors are not knowledgeable of the requirements of this type of wall cladding. They have a 'hot list' of things they check for and then it's on to the next house. (Weep holes in brick walls have been required for at least the past 10 years and that's another rule that is often overlooked.)

So, understand that you have a high hurdle to get past initially. And that is convincing the builder (or home seller) that the house construction is suspected of being deficient.

Hurdle number two. The builder doesn't want to spend any money. The cost to the builder would be high. First of all, any work of this sort may delay the closing date (they want to sell the home, remember?). If the house has been on the market for a while, removing the stones may reveal structural damage that the builder or owner does not want to see, because that would involve additional materials, repairs and costs. Finally, and significantly, repairing your house may create a red flag to other homeowners in the area who would see the work in progress; they would want their home repaired too and may hire attorneys. Yikes!

I am never present when my clients discuss the findings of their home inspection with the seller (often the builder). Extremely rare is the client who calls or asks questions of the home inspector after they have received their report. In fact, I am not aware of any client demanding repairs to their exterior walls and having their demands met. I suspect that they just show the builder the home inspection report, listen to the builder's subsequent dismissive comments about the knowledge of the home inspector, and accept the builder's promise to fix anything that may go wrong in the future. Because I am concerned that the significant defects I identify in a report are disregarded, perhaps without full knowledge of the potential consequences of the effects of that action, I have created this document.

My advice is to stand your ground and get the walls examined by a certified specialist (not the mason who installed the system) even if you have to pay for it. If repairs are called for, insist on it. If the seller doesn't want to pay for the repairs and you don't want to pay for the repairs, then consider another home. Otherwise, the home you love may look like the photo below after a few years.


Pretty.                            Not pretty!




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Last modified: 10/04/21