In recent surveys, readers have asked for instructional articles with practical knowledge they can use on the job. In response, Interlock Design is pleased to present this first installment of its new how-to series of feature articles that will run throughout the year. We welcome reader feedback and invite you to contact us at firstname.lastname@example.org with any suggestions or topics you’d like us to cover in the new how-to series.
Why a Raised Patio?
As this issue’s cover story explains, residential outdoor living is a booming market for the hardscape industry. An integral component of an outdoor living space is a raised patio.
Traditionally, a raised patio allows movement from house to backyard without a change in elevation. A homeowner steps out the back door and into the outdoor living space as easily as walking from one room to the next inside the house, creating a seamless transition from interior to exterior space.
“If not done properly, a raised patio can do significant damage to the building that it’s constructed against,” says ICPI Director of Engineering Robert Bowers, P. Eng. The main factors that can cause damage are moisture accumulation and the increased lateral load placed on the foundation, and possibly on exterior above-grade walls.
Most exterior above-grade walls of a house are not designed to have moisture continuously against them, explains Mr. Bowers. Whether they’re brick, wood siding, vinyl or another material, these exterior walls are designed to resist water and shed it—to get wet and then dry out. They cannot withstand a continuously moist environment. Placing compacted soil against these types of walls can trap moisture, resulting in mold, decay and deterioration.
Regarding foundation walls, in most cases they are constructed to bear the weight of the supported structure, the lateral pressure from the soil and not much more. By constructing a raised patio, the lateral pressure against a foundation increases. This presents an increased risk of blowout and basement wall collapse, because the increased load to the wall is not counterbalanced. This is called an unbalanced fill condition. When taking on an unbalanced fill project, an engineer should be consulted to ensure the stability of the project. Additional reinforcement of the foundation wall is sometimes necessary. The cost of the engineer’s involvement will increase the cost of the construction, so it’s important for contractors to include this in the price of their proposals.
“A homeowner should be able to appreciate it,” Mr. Bowers says, “if you say, ‘Hey, I’m concerned that we don’t damage your house in any way and I’d like to have a professional engineer tell us the best way to do this.’”
At the outset of planning, be sure to thoroughly document the existing conditions of the site. Take photos of the exterior walls, the foundation and the basement walls inside and out, carefully inspecting for cracks, bulging and any signs of dampness or water damage.
The most effective way to raise a patio adjacent to a building is with a retaining wall (aka stress relief wall) that faces the building, offset from it by 3 to 4 in. This creates an air gap that prevents the patio from touching the building’s exterior cladding and also allows airflow so any moisture that gets in can dry out (See Figure 1). Additionally, the air gap prevents a raised patio from covering up weep holes. Covering weep holes compromises the exterior above-grade wall venting system, leading to deterioration and potential collapse. For this reason, covering weep holes is a building code violation. At the top of the air gap, cantilevered pavers and screens are common solutions to prevent debris from falling into the gap. A drainage system at the bottom of the air gap is also required. Another option is applying aluminum flashing against the house. This surface, however, cannot block weep holes designed to wick moisture from the walls.
The higher the patio is raised, the greater the complications and potential risks to the foundation. Most homes are constructed with 8 to 12 in. of foundation wall above grade, atop which sits another 12 in. of floor joists. That means the threshold of the back door is typically 20 to 24 in. above grade. For every foot of elevation a wall is built up, roughly 50 to 100 pounds of additional load is applied to the foundation walls. Depending on a number of conditions, it could be even more.
Coincidentally, a raised patio height of 20 to 24 in. is a gray area for determining if additional measures are required to reinforce the foundation wall. For any patio raised above 24 in., it is recommended to have an engineer review the design, test soil quality, evaluate foundation walls and make recommendations. Heights of 20 in. or less generally carry less risk in relation to the loads. Ultimately, each contractor must decide on his or her level of comfort and corresponding liability.
“If you think there’s the slightest possibility you might need an engineer, then you need an engineer,” Mr. Bowers says. “I can’t tell you how many calls I get from contractors who say, ‘I’m not sure but I think I might be doing something that requires an engineer.’ They describe the situation and yes, they should’ve had an engineer involved weeks ago.”
Local building codes also come into play at heights around 24 in. or greater and when adjoining the raised patio to a building exit like a back door. Every building code has specific requirements for steps including tread depth, riser height and pitch, as well as for hand railings and guards. Because many aspects of raised patio construction are governed by building codes, raised patio construction often requires first obtaining a building permit.
Raised Patio Construction
Raised patios are constructed using three basic components: walls, flatwork and steps. But before building anything up, the ground must be broken.
When new home construction is completed, often the soil against the foundation wall is excavated backfill of the soil consisting of silty, clay soil unsuitable for the subgrade of a raised patio. A soil probe or test pit will confirm this and is recommended to determine soil type and quality. A common way to reduce the lateral load applied to a foundation wall is to remove poor quality soil and replace it with a higher quality dense-graded, crushed stone aggregate. As a rule of thumb, the height of the patio determines how deep to excavate and how far out from the building foundation. If a raised patio will be 48 in. (1.2 m) high, dig down 48 in. (1.2 m) and out from the building the same distance.
Subbase, Drainage, Base
Dense-graded, compacted aggregate is commonly used for the base of the wall and the raised patio. For some projects, flowable fill may be advantageous because it’s lighter and does not require compaction. However, it can be more expensive to install and may require time to cure.
Once the subgrade and base for the wall are set, install a 4 in. (100 mm) diameter perforated drainage pipe along the length of the wall that slopes to a drain. For the drainage layer above the drainage pipe, use open-graded, compacted aggregate with ¾ in. (19 mm) minus clean stone (See Figure 2).
When using segmental retaining wall (SRW) units to raise a patio, a conservative rule of thumb is that the maximum height of the wall should be approximately twice the depth of the SRW unit. For heights three times the depth of the SRW unit or greater, geogrid should be used to help stabilize the wall. Most building codes require walls over 48 in. (1.2 m) in height to be engineered, and some jurisdictions have set limits even lower.
A conservative initial design incorporating geogrid could specify continuous layers every 12 to 16 in. (300 to 400 mm) vertically with a length equal to the height of the wall, and not less than 4 ft (1.2 m). This design would only be suitable for typical conditions: dense graded aggregate backfill; pedestrian-only loading with no slope or terraced wall above; a stable, undisturbed subgrade to a maximum total height of 8 ft (2.4 m). If these typical conditions do not exist on the site, or the decision is made to optimize the design, an engineer should be consulted to develop the initial design.
Every building code has requirements for steps. For outdoor applications, a common pitch requirement is 6:12: a 6 inch (150 mm) riser and a tread depth of 12 in. (300 mm). Maximum riser heights of up to 8 in. (200 mm) may be permissible, so check local building codes. The steps must have a consistent tread depth and riser height to prevent a tripping hazard. Complete compaction of base material is extremely important. Flowable fill or a well-compacted, cement-treated aggregate can help minimize the potential for settlement.
SRW units (Figure 3) or concrete pavers (Figure 4) can be used to construct steps. Either way, choose a material that has freeze-thaw durability. Snow removal and deicers can destroy concrete materials not manufactured to freeze-thaw resistance. Some SRW systems have cap units that are not meant to support regular pedestrian traffic, so be sure to choose the proper units if using for steps. If pavers are selected for the steps, it is necessary to build the base out of concrete to prevent “roll over” that occurs if paver steps are not properly supported.
For patios with elevations greater than 24 in. (600 mm), most building codes require a guard or handrail, including minimum height requirements, as well as specifications for resistance to lateral loads. For code compliance, the railing, mount and foundation all must resist the applied load. Generally, there are four types of mounts used to connect the post to the stabilizing foundation: surface, core, side and direct.
Surface mounts are common but also typically the weakest. A plate is welded to the bottom of the post and then connected to the top of the retaining wall with lag bolts or self-tapping concrete screws. Core mounts involve drilling down 18 to 24 in. (450 to 600 mm) into the retaining wall and grouting or epoxying the post directly into the wall. Core drilling can be time-consuming and costly and risks splitting the SRW units under certain conditions in freezing environments. While core-drilled guards are potentially more stable than surface mount guards, neither should be relied upon as the only means of securing the guard.
Side mounts attach handrails to the face of a side wall. When side-mounted handrails are combined with a guard system, they contribute to the stability of the entire guard assembly. The most effective way to secure a guard system is with the fourth type, a direct mount, which attaches to a solid fixed object like a building or caisson (See Figure 5).
The main task in job layout is transferring the final design from paper to the site. Verify access and staging areas; identify slopes and drainage conflicts; install erosion control and containment measures; and provide protection for trees, plantings and structures. Confirm the location of all utilities and buried utility lines, making sure everything is clearly marked. Outline the extent of excavation and the patio, install string lines, and designate finished elevations with stakes, string lines and markings on adjacent structures. Make plans for equipment storage and vehicle parking. And always maintain a clean, organized site to make a favorable impression.
When defining the elevations of a project, identify the critical elevations on existing structures like a doorsill. Typically, critical elevation determines the finished elevation, so it is necessary to calculate backward from the finished elevation down to the starting elevation. Repeat this calculation in several locations on-site and double-check them.
Care must be taken when compacting adjacent to a foundation wall; excessive force may cause cracking. Less force can be used by placing soil in thinner lifts. For the first course of SRW units, dense-graded aggregate base should be compacted to a minimum of 98% standard proctor density (SPD). Although industry guidelines call for 95% SPD for the fill behind the retaining wall, ICPI recommends 98% SPD to minimize the settlement of the pavement surface above. It is important to watch the alignment of the SRW units to ensure they are not pushed out of alignment or rotated forward during compaction.
For bases and fill, in addition to the flowable fill alternative previously mentioned, geotextile or geogrid, cement-treated base (CTB) and asphalt-treated base (ATB) are also options. Installers who have limited experience with these materials and methods should receive technical support prior to selection. A geotechnical engineer’s input may also be necessary to determine the strength of the subsoil and the extent of remediation required.
Raised patios also require adhesives for retaining wall caps, treads and other materials. Adhesives that remain slightly flexible after curing are preferred. Though mortar can be a cheaper option, its use is not recommended in areas with freezing and thawing conditions.
Equipment removal and cleanup are standard operating procedure. After running down punch-list items and performing final inspection, secure a certificate of occupancy and final payment. As a courtesy, provide the homeowner with spare pavers, sand and cleaner. Photograph the completed project for the company’s portfolio and be sure to write a thank-you note for a high-dollar job.
The information provided in this article is from the ICPI Advanced Residential Paver Technician Course manual. To sign up for this course or any other offered by ICPI, visit www.icpi.org/installercourses.