As the weather gets warmer, consumers start to create their outdoor dream space.
By Robert Bowers, P.Eng, ICPI Director of Engineering
ICPI Director of Engineering, Robert Bowers, P. Eng., receives questions from members in need of advice and tips regarding the best practices to use when constructing raised patios.
One of the most recent questions was related to a residential permeable interlocking concrete pavement (PICP) patio. Typically a PICP systems uses a layer of ASTM #2 stone as the subbase, a 4 in. (100mm) layer of ASTM #57 stone above as the base and a 2 in. (50mm) layer of ASTM #8 stone on the top as the bedding layer. Given this is a residential patio application, could the contractor only use a layer of #57 stone for the base or would it need to incorporate #2 stone as well?
ASTM #2 stone provides better structural support for heavier, repetitive loads. It also tends to stay in place better than ASTM #57 stone, once it has been compacted. The drawback to using ASTM #2 stone is that it will typically require heavier compaction equipment and the need to manage another material on site. Given this project is a residential patio and will only experience light pedestrian loading, the ASTM #2 stone subbase is probably not necessary. It was also recommended that a highly permeable geotextile be placed between the ASTM #57 stone and the subgrade to improve separation between the open aggregate and the subgrade soil.
The system of ASTM #2 / #57 / #8 layers of aggregate are recommended because it has been demonstrated to work on numerous projects. However, an engineer familiar with the design of PICP systems may choose to utilize different aggregates.
Another question asked about the recommended installation best practices for a raised patio against brick veneer on a house. The response referred to the ICPI Advanced Residential Technician student manual.
When constructing a raised patio against an existing structure considerations should include: 1) creation of an air gap between the building’s exterior cladding and the raised fill using a retaining wall facing the building and 2) considering the additional load the raised patio will place on the foundation wall of the building.
Considering item 1, it is important to address water and moisture issues. It is important to ensure that rainfall landing on the patio surface is directed away from the house. Typically, a minimum grade of 1.5% is ideal to ensure that water flows across the surface. If the slope is lower than this, the water won’t move rapidly off of the patio. Slopes can be increased up to 4% to help speed the movement of water, but the pavement surface will be pitched. Tables and chairs will be on a noticeable slant, and the area can become a slipping hazard if ice forms on the patio surface. For larger patio areas, surface drains may be constructed to remove water from the surface. For smaller patios, it may be appropriate to have surface water flow across the patio, over the top of the retaining wall, and onto the garden or patio below. The use of stabilized joint sand may be appropriate to ensure that it is not removed by falling or flowing water.
Raised patio construction will likely affect the house. In most cases, there are two types of exterior walls: a) below grade foundation walls and b) above grade exterior walls. During construction, the compaction of fill places large dynamic loads on the adjacent walls. Placement of fill next to a building wall also increases the lateral load applied to the walls. The fill material used to construct the patio has moisture in it that can affect the building if the fill contacts the exterior walls. Below-grade foundation walls are typically constructed using concrete or concrete masonry units (CMU) waterproofed on the exterior with a drainage system incorporated at the base. Foundation walls are designed to withstand moisture against them continuously.
An above-grade exterior wall of a house can be constructed from materials such as wood, vinyl or aluminum siding, cement board, stucco, mortared clay, concrete brick, mortared stone or mortared veneers. These exterior wall materials are designed to support the load of the building above, and to resist the penetration of water. Placing compacted soil next to these types of siding can trap moisture in and against them, which leads to deterioration and eventually failure. Deterioration is accelerated when freeze-thaw conditions also exist. For raised patios with compacted fill material against the exterior wall, it is best to construct a stress relief wall leaving an air gap between the raised patio and the exterior wall. The stress relief wall can be constructed like any other segmental retaining wall on a raised patio project, but the face of the wall faces the exterior wall of the building. The air gap created needs to have drainage so any water that finds its way into the gap can drain out. It is also appropriate to encourage air circulation in the space to minimize condensation and allow the space to thoroughly dry out.
For item 2, when additional fill is placed adjacent to a building, lateral load applied to the foundation wall will increase. This is true even when a stress relief wall is used. For buildings with unbalanced fill conditions, like a basement, the weight of the additional fill can exceed the lateral strength of the foundation wall and can cause the wall to bulge and eventually blow out in to the basement area, potentially causing a collapse of the structure. Extra load is applied from the raised patio fill, but no extra resistance is provided by the foundation wall. Caution should be taken when taking on an unbalanced fill project, and an engineer should be consulted to ensure the stability of the project.
Image Above: Drawing from the ICPI Advanced Residential Installer Course manual.
Have an engineering or technical question? Robert is always ready to respond with the latest technical resources and information.
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