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When properly completed, concrete overlays are a cost effective way to renovate a space for your clients and to add a lot of value to the property. Instead of having to hammer out the concrete to dispose of it and re-do the base, concrete that is in good condition can be overlaid with pavers or other natural stone material.
This is especially useful in the case of a front porch or a swimming pool concrete patio. Both of these options are optimal for a concrete overlay application, along with any other concrete that is in suitable condition.
Paver Overlay: Paver Installation Over Concrete
Overlay pavers are typically thinner than other pavers for pedestrian and especially vehicular traffic. These are also typically less costly than their alternatives, adding to the cost effectiveness of a concrete overlay using these pavers. When deciding whether or not to overlay concrete using these pavers, there are several steps that we need to consider in order to provide our clients with the best possible solution and end product that will last for them.
Concrete Base for Pavers
When starting an overlay project, we need to make a judgement call on whether or not the concrete slab is structurally sound. This means ensuring that there is a proper slope away from the foundation to allow water that enters the system to move away from the house. Any imperfections, cracks, separations of +/-⅜” over 10 feet in the concrete slab may mean that the slab is likely to move and the base of the concrete slab was not properly prepared. Overlaying concrete that is questionable will come back to bite you. However, there are products on the market that allow you to address any inconsistencies in the concrete slab like crack repairs. Any other epoxy, stains, or grease along the outside portion where the pavers or caps will be glued to should be grinded or cleaned to ensure proper adhesion.
We like to ask our potential clients questions like how long the concrete has been there and whether they have they had any issues with it before we start looking it over. This helps us to make our judgement call on whether or not that concrete slab is up to our standard. This information should not be used exclusively. We need to use an 8 foot level to be able to see whether or not there are any major inconsistencies in the concrete. Additionally, we need to inspect the area for cracks in the surface of the concrete and whether or not these are worrisome. One thing we know is that concrete is going to crack. That is just the reality when it comes to pouring a large and rigid surface over ground that experiences movement during freeze-thaw cycles.
Preparing Our Overlay
We like to use a 2” natural stone cap around the exterior of our overlays because we like the finish that it provides especially in high traffic areas like steps where the surface can become worn down. The good thing about natural stone is that it is the same all the way through. It also provides us with a ¼” difference in height between the pavers being laid in the field of this overlay allowing us to line the area with a non-woven geotextile and ¼” of concrete sand. Having the same height cap as the pavers would cause us to have to use just a very light dusting of fine masonry sand in order to create separation between the pavers and the concrete slab.
We use a polyurethane adhesive to glue the caps using perpendicular lines to allow any water to run right under the caps and out. If you ran parallel lines, water would be trapped under the caps causing issues during freeze-thaw cycles and the retention of moisture under the entire overlay system. When adhering these caps, we make sure each piece is in line with one another, level, but with a slight pitch towards the drainage area for water to run directly off the caps and out onto our walkway. In the case of a swimming pool surround, that pitch would be away from the swimming pool itself.
The pavers should never be laid directly on the concrete, as this will trap water that enters the system between the paver and the concrete which will cause issues during freeze-thaw cycles as well as the retained moisture causing efflorescence issues which is a mineral deposit in the concrete that shows in the surface of the stone as a white residue. This can be cleaned, but if moisture persists under the stones it will continuously show.
Especially in concrete slabs that have control joints cut in or if a crack presents itself, the non-woven geotextile will act as a bridge containing the sand in the overlay system while still allowing for the drainage of water. It should be overlapped 12” at the seams. In the case of this concrete porch overlay, the fabric wraps up the sides of the caps to prevent the washout of concrete sand.
ICPI calls for holes to be drilled through the slab along these expansion joints to allow for the free flow of water through the system and into the base material and out to the drainage area. In the case of this front porch where it is covered and a polymeric sand suited for overlay applications allowing just 1% of water through it, not to mention that holes should not be drilled through a concrete front porch, we are not following this step.
Weeping holes in the masonry around the house should be addressed as well. These should never be blocked by the paving stones and an expansion cord or other material should be laid between the pavers and the stones to allow for the airflow behind the masonry. Failure to do so will cause moisture problems for the homeowner in the future.
When screeding the concrete sand on top of the geotextile, we want to match the slope of the concrete slab from our starting point to our ending point. At the caps, we want to have the pavers be approximately ⅛” higher than the caps to ensure the water runs off the caps properly. We also want to factor in the final compaction of the pavers after installing the polymeric sand. This will be minimal considering the ¼” screed layer, but still needs to be taken into account. Using the horizontal lines on our level and knowing the height of our pavers, we are able to easily screed a small front porch by eyeballing it.
Alternatively, you could install a permeable concrete overlay using pavers with sufficient joint widths to allow the water to flow through the joints and onto the concrete slab and out of the system. To accomplish this, you would want to ensure that the concrete slab has a 1/4″ per foot slope away from the foundation. Drilling holes through the control joints would help for that drainage of water, as long as you know that the slab was installed properly with a permeable base. Unfortunately if we did not install the slab or know the contractor who completed the project, we may not be able to know this information without inspecting the base. The non-woven geotextile would be installed with a 1/4″ angular clean stone with at least 0.5″ to 1″ of a setting bed to allow for it to be installed properly underneath the pavers. This would also mean that depending on the height of your pavers, you would need a thicker coping stone around the perimeter of your overlay.
Laying a Concrete Overlay
We can then lay our pavers to complete this porch overlay. Laying the pavers is similar to any other project on a traditional base. Though depending on the accuracy of your screeding without screed rails, you may need to adjust the pavers as you lay with a rubber mallet and level to ensure a proper slope from your foundation.
With your pavers laid, you can then install your polymeric sand or other jointing compound. When installing polymeric sand for your concrete porch overlay, you will want to ensure that you are using a polymeric sand that is specified for overlays. The sand gets swept around the pavers and a rubber mallet is used rather than a plate compactor with a pad or a roller compactor to consolidate the sand to the bottom of the joint. We then top up the sand and do one final pass to consolidate the sand and ensure the sand is ⅛” below the top of the paver or bottom of the chamfer. We can then activate the sand according to the manufacturer’s specifications.
Pavers on Concrete without Sand
You will find in concrete overlays where the stone is laid directly on top of the concrete with no separation, polymeric sand will fail to cure as it requires drainage below it as well. This will also lead to the build up of efflorescence on the surface of the pavers as moisture is retained directly below the pavers causing the white staining to become present. This can be cleaned using an efflorescence cleaner, but as long as there is moisture present in the system it will consistently come back.
This is why we never recommend to lay pavers directly on top of concrete without the non-woven geotextile and a sand to add that level of separation for filtration. However, you can mortar set natural stone if you prefer for your overlay system rather than overlaying using a geotextile and sand. In this case the joints would need to be grouted rather than using a polymeric sand.
Installing pavers over concrete is a great way to dress up an area in order to renovate it and give it a makeover. It is more cost effective than the alternative of having to rip that concrete out and start from scratch. However, you need to be careful when choosing to proceed with this method to ensure the longevity of the project.
Following the installation on the top of your concrete porch, you can dress up the sides of your porch can be veneered with more natural stone and the concrete porch overlay is complete.