Poured-in-place rubber safety surfacing (PIP) is a popular surface for playground projects. This surface is a low-maintenance unitary system that provides consistent fall protection. Many people do not realize that PIP is porous; therefore, the foundation of the surfacing (the substrate) is just as important as the surface itself. If the substrate is not installed correctly or has improper drainage, the PIP could deteriorate, and without a proper foundation, the PIP's warranty will be affected.
There are three common types of substrates: concrete, asphalt, and compacted aggregate. If budget allows, concrete is preferred as it is the most stable and durable; however, drainage can still be an issue if not properly installed. For instance, a common mistake occurs when a curb is poured around the perimeter of a concrete slab without in-slab drainage or weep holes. When it rains, this area will fill up with water. This scenario and others like it are detrimental to the longevity of the PIP.
Asphalt, new or in good condition, is also an acceptable substrate for PIP. It is slightly less stable than, but can serve as a solid substrate following the same rules as concrete. Whether the substrate is concrete or asphalt, it should be installed with a cross slope of ≤2% in all directions for ADA compliance purposes.
While compacted stone is a valid and less expensive substrate option, it is the most difficult to install correctly and can lead to erosion and settling issues if not built to spec. The following specifications should be followed related to the site grading, compaction, and drainage requirements for installation areas that will have an aggregate substrate:

 
• The final grade or elevation of the compacted stone substrate should be verified to be as indicated on the plans. In other words, check grading elevations compared to curb and sidewalk heights to ensure coordination with stone and PIP thicknesses.
• PIP is highly permeable with an approximate water flow rate of .149 gallons per second. No sheet draining from the top of the PIP should be expected.
• Choosing the stone type depends on what the drainage plan is for the site. There are a few different aggregates that are acceptable for use in building a substrate.
• If the drainage plan calls for water to permeate through the bottom of the stone substrate, the acceptable stone material type is called 3/4"-1" Screened Crushed Stone (commonly referred to as "#57 Stone").
• If the drainage plan calls for water to shed from the top of stone substrate with positive flow out and away from the surfacing area, then 3/4" Minus Crushed Stone (with fines) may be used.
• Stone material must be of an angular/crushed type in all cases; rounded/river rock is not permitted in a play environment.
• Additionally, the cross slope should be maintained at ≤2% in all directions. The surfacing installer can adjust for only +/-0.5" variation within a 10' radius with surfacing materials during the installation process; therefore, the grading must start out smooth and correct because the PIP follows the substrate.
• The compaction rate is recommended at 90 percent or greater such that there is no displacement when walked on. Compaction is facilitated by spraying it with water during the installation process.
• There must be adequate drainage to prevent pooling underneath the surfacing system. In fact, for sites that have a concrete curb and/or sidewalk containment, hard-piped drainage systems with positive flow and discharge outside of the surfacing area are highly encouraged.
A common mistake during a compacted stone substrate installation on a playground is filling the post holes of the play equipment with loose dirt after installation. Instead, post holes, footings, or other excavations within the surfacing area should be filled back to grade with concrete to eliminate the potential of settling, sinking, and ponding underneath the aggregate substrate and ultimately the PIP surfacing system.
Lack of proper drainage and improper compaction of the stone base substrate can cause the PIP surfacing system to develop numerous types of performance and aesthetic issues that are not covered by PIP warranties, so it is of great importance to start with a strong,
well-drained foundation.