Avoid These Common Masonry Repair Mortar Scratch Coat Mistakes
Out of sight. Out of mind. It’s a risky philosophy when repairing and restoring masonry facades—especially when the repairs are deep and budgets are stretched. While a single-mortar approach is always the best path, sometimes a scratch coat is a necessity (if for no other reason than to save money).
It may seem budget-savvy to use an inexpensive scratch coat for most of the depth and then complete the final few inches with a more expensive finishing product. Why put the budget into materials you won’t even see, right? Well, there are several reasons you’ll want to avoid this approach.
First, a scratch coat project requires multiple lifts. Any time you introduce multiple lifts, especially when using different substrate types, you increase the chance of failure.
Furthermore, if the scratch coat is not compatible with the substrate (i.e., contains a bonding agent) you will lose any compatibility offered by your finishing coat. Moisture and salt will be trapped and a failure will be imminent.
Another pitfall to avoid: Contractors frequently choose scratch coat products that use gray Portland cement. Gray Portland cement is full of salt. The salt travels to the surface when exposed to moisture, leading to unattractive efflorescence.
Finally, scratch coat mortars, selected for their low cost, most often have a stronger compression strength than the substrate. The result of this will be the substrate being the sacrificial element as opposed to the repair, which should be sacrificial.
Every circumstance above can cause the stone to deteriorate, resulting in a so-called repair that will fail. That certainly isn’t a cost savings when each substrate layer will later need to be removed to remediate the failure.
Two Safer Repair Mortar Scratch Coat Alternatives for a Lasting Restoration
We always recommend a single lift when possible. The single-lift approach is always the most compatible and will last the longest (maybe even forever). Our formula for proper masonry repair is this: compatibility + proper installation = as close to a forever repair as you can get.
Any Jahn Mortar (engineered for each substrate type) can be used at full depth and in a single lift (at Reagan National Airport, we had single-lift repairs at 10 inches deep!), and they are available in standard and premium colors (custom color matching is available for that rare occasion when a standard or premium color, or combination of those, doesn’t match your masonry).
Ronald Reagan Washington National Airport, Single Lift Repairs
Approx. 10 inches deep.
Finished prior to painting with Potassium Silicate Paint.
So while a single-mortar, single-lift approach is best, what should you do if the budget doesn’t allow for that approach? What if you budgeted for a two-inch depth, but you’ve discovered that you need four or six inches, or more! Budgetary constraints can prevent you from using an expensive engineered mortar at full depth, but there is an alternative: Jahn Backfill Mortar.
Jahn Backfill Mortar was made explicitly for this scenario. Jahn Backfill Mortar is engineered to be completely compatible with any substrate and any Jahn Masonry Restoration Mortar, which must be used for finishing the final 1” or more depth of the repair to color match and texture match the substrate.
The big challenge (and potential pitfall) of using Jahn Backfill is that the scratch coat must be finished properly to guarantee a bond between the Backfill lift and the Jahn Engineered Mortar final lift.
I hope to have helped you see how product selection and professional installation are critical to a successful project. If you have questions on these topics — or anything related to masonry restoration — please get in touch with me at tech@cathedralstone.com.
Have a project with heavy and deep repairs? Check out this post for some helpful tips on supporting heavy and large masonry repairs.
