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The following is a recommendation for the use of Water Repellents on Cast Stone Material.


Careful quality assurance and control of the cast stone material are important to ensure a professionally finished project. Water repellents are a great way of protecting your stone from efflorescence, mildew, staining, and dirt, from five to ten years, with proper application of most commercial products.


With the use of modern technology, waterproofing cast stone and masonry materials have improved greatly, thus reducing the water intrusion through cast stone and the mortar joints. Studies have even shown that with some repellents, you can reduce freeze-thaw damage, and prevent loss of insulation value.


Silicones, acrylics, siloxanes, and silanes are the most tested and popular water repellents. Silicones provide only a surface film, are generally inexpensive, and last a short time. These products are used mainly to keep cast stone clean during construction, making it easy to clean up the project at the end.


Most acrylics available have poor vapor transmission, inadequate resistance to ultraviolet lights and has low penetration, thus providing only a short-term solution. Additionally, some acrylics have been known to yellow over time or produced an unwanted gloss finish.


Siloxanes, silanes, or a blend of both, are the most recommended forms of water repellent when weatherproofing cast stone is desired. They penetrate exterior surfaces then undergo a chemical reaction with the moisture in the stone to form a weather repellent silicone resin within voids of the cast stone.


Timing of application is important. The cast stone cannot be too wet before application. Keep in mind that if the stone is still wet, if you have just completed the final washdown, or if it has rained within the prior two days of application, there may be too much moisture in the stone. The repellent cannot react with all the moisture in the stone, therefore, water is contained within pores, and penetration is limited.


Air temperature should be 50°F or above, and generally not below 40°F for another 24 hours after application. The application of the water repellent must dry before the stone getting wet, therefore do not apply if rain is in the forecast.

Water repellents are not designed to remedy poor design and use of flashing, lack of weep holes, non ventilated wythe, failure of joint materials, or the use of hard mortar joints where sealant joints should be used. It is important to understand that cast stone is naturally permeable, similar to natural limestone, and architectural concrete.


The purpose of the water repellent is to prevent water intrusion through the outer surface of the cast stone material while allowing sufficient vapor transmission to let moisture out of the wall cavity. Proper design and installation of materials throughout the project improve the quality of the overall project and permits proper weatherproofing, weathering, and the ability to clean the surface, is enhanced should it become dirty.


Proper testing and research should be applied to determine the correct repellent for your application. Studies of similar situations and products will help you to determine which product to use. The manufacturer of the water repellent or the party applying the water repellent should guarantee the application. The cast stone manufacturer is not responsible for the choice of repellent applied, and any resulting damage or discoloration to the cast stone.


Water repellent should be applied only after all work has been complete including pointing, touchup, repair, cleaning, and final inspection has occurred.


Note that careful planning at the start will assist in eliminating disappointment for any homeowner or commercial designer at the completion of the project. There is so little one can do to change the appearance of cast stone after installation; therefore, we strongly suggest that careful consideration and planning are executed from the start.




The information within this and all our bulletins has been provided as a guideline and based upon statistical data and prior uses. We always suggest that you consult with your engineer, architect or contractor for the best design and use of cast stone for your project. Our design team is always available to answer any of your questions. We do not accept any liability from damages resulting from your interpretation of the data contained within.