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Structural Load Capacity: Can Your Wooden Deck Support a Swimming Pool
Weight Considerations: Decks, Water, Equipment, Occupants
Most backyard decks are safe to use under normal circumstances where typical occupancy is about 40 to 50 pounds per square foot. However, this is not the case for the installation of pools, where the requirements are much higher as pools need to support 100 psf. This is more than double the current safe load of most decks. Massive stress is created by the water alone. Consider a standard size pool of 12 feet by 24 feet, holding about 20,000 gallons of water, that’s over 160,000 pounds, considering a gallon weighs about 8.34 pounds. That is not considering the additional pumps, heaters, and filters which could weigh an additional 500 to 1,200 pounds. And then there is the weight of the swimmers. Each swimmer could add over 200 pounds to that deck.
Based on a 2023 structural field study conducted by Deck Safety International, nearly 80% of decks that were tested experienced a catastrophic failure at 60 psf (pounds per square foot) illustrating just how rapidly average construction fails. The combination of constant water weight, user-induced vibration, and thermal expansion create a combination of factors that accelerate fatigue more than typical service conditions.
Load Type Average Weight Impact on Deck Water (10,000 gallons) 83,400 lbs Constant, non-dissipating
4 Swimmers 800 lbs Dynamic, cyclic, concentrated
Equipment 1,000 lbs Fixed, vibration-prone
If you want a deck to sustain a 100 psf live load, critical raised deck construction and pool deck structural reinforcements must include: Joist spacing, improved ledger and footings, and pre-planned solid beam placements. More intelligent load distribution (not just heavier) materials will allow for meets the 100 lbs per linear foot (psf) weight.
Also, per the International Residential Code (IRC Section R507.6), permanent pools must be designed and constructed to support at least 100 psf live load, and many local jurisdictions have more restrictive amendments.
Joists: 16" on center spacing will not be adequate. It is an engineering best practice to use 2x10 and 2x12 pressure treated or laminated veneer lumber (LVL) joists so that spacing is 8" to 12" on center to minimize deflection and to allow for adequate distribution of concentrated point loads.
Ledger board: Must be triple bolted, with hot dipped galvanized or stainless steel (not lag screws) through-bolts into solid framing (not just sheathing or siding). Moisture trapped ledger connections are the most common point of failure for pool deck collapses.
Footings: The 6×6 post bases are insufficient. In frost susceptible regions, concrete piers must be a minimum of 12″ in diameter and extend a minimum of 48″ below grade so that heave and settling occur under sustained loads. Depth is a significantly greater factor than diameter with regards to heave and settling.
Deficiencies of this type, combined with cumulative stresses, wood moisture deterioration, and other variables, can result in structural failure in 2 to 5 years. The deck may appear to be in perfectly serviceable condition.
Moisture Management and Material Durability for Swimming Pool Decks
Under Swimming Pools: Capillary Action and Long-Term Rot
Water sits stagnant, and hidden decay problems are not visible in the course of routine inspections. The capillary effect can pull moisture from the stagnant water into critical areas of the structure including, but not limited to, ledger boards, joist hangers, and rim joists. The moisture content of wood must be above 20% for the wood to begin to rot, and in warm and humid areas, this condition can occur dramatically quicker than the average time. The microenvironments of decay around and under swimming pools are estimated to increase decay rates 2 to 3 times compared to other environments.We continually fail to notice the damage these types of structures are making. The damage beyond the surface is the most devastating, and the most destructive is the damage to the strength of the structures. By the time you realize the damage is becoming problematic, you often don't notice the destruction. Among the sources of wood damage, not documented or improperly documented, are the wooden components under swimming pools. While these components may not be documented, the components under wooden decks are sealed and, as documented by the American Wood Protection Association, the components under pools will last, on average, half to three quarters, as long. Because the damage is not visible, the damages from delay can become significantly more expensive, but these damages can be avoided by early detection of damages from metal parts corroding and prevent the damages caused by delamination or warping.
Composite vs Pressure-Treated Wood: a Cost-benefit Analysis of Drainage, Maintenance and Longevity
The Initial selection of the construction material has a direct relationship with the overall lifecycle cost and the safety of the construction.
While Pressure Treated Wood has a more competitive initial cost, it comes with a more expensive overall lifecycle cost as it will require more active management, including annual sealing, biannual inspections of the joints and fasteners, and replacement of boards (which are often sold as a guarantee to not warp, split, or absorb water more than a few inches on the ends) every 5-8 years) due to warping, splintering, etc. ) of Wooden boards, and even when these boards are furnished with new age ACQ and micronized copper azole (MCA) treatments. Moisture, and therefore decay, is often trapped in the ends of the joints, and can create decay at the joints. The wood is so porous that it can create decaying joints at the critical joints of a structure.
In contrast, composite decking is designed with non-absorbing polymers and engineered drainage grooves made to actively withstop the harmful effects of standing water. It is made to never need sealing, and it’s designed to withstand the freeze/thaw cycle for 25 years in a pool environment. Initial costs are 30-40% higher, but with composite decking, the total lifetime maintenance costs (including the costs of maintenance) are reduced by more than 60% and the lifetime liability costs are lowered more significantly than with wood decking.
Safety, compliance, and code concerns with swimming pools and decks
deck design is based on real-world loads (40 psf) that are insufficient for swimming pools (100+ psf)
In the International Residential Code (IRC) Section R507.6, the minimum for a basic standard is 40 psf live load for decks, which are meant to support furniture, small gatherings, and incidental foot traffic. This is not a design for high mass and high-humidity applications that are static, like swimming pools.
The weight of water (around 62.4 pounds per cubic foot) creates extreme challenges. A 24 inch deep wading pool alone puts 125 pounds per square foot on the deck surface. That is over three times the building code limit, and that is not considering the weight of the equipment or the people that will jump in. In terms of deeper water, a 4 foot deep pool, the water pressure is about 250 psf at the bottom. This indicates that a big discrepancy exists between the regulations and the actual consequences. Many builders face problems when they do not foresee these forces in the real world while completing the building.
Engineering plans that are stamped are now required by most building departments. This includes the depth of the footings, attachments of the ledgers, sizes of the joists, and moisture management. It is not sufficient, and it is in fact dangerous to rely on building code compliant and site specific engineering.
Practical Risk Mitigation Strategies: Inspections, Monitoring, and Professional Assessments
To install a swimming pool on a wooden deck, a professional evaluation of the structure is essential. A structural engineer needs to evaluate the load capacity, condition of the joists, ledger-to-house connection, and soil supporting the structure. Examinations of the structure do not reveal all problems. Conditions like decay and micro-fractures around fasteners and moisture buildup beneath the surface can only be determined through diagnostic testing.
Before the installation, moisture sensors should be placed in the deck close to the areas of the most risk (ledger line, post bases, pool edges). A long-term monitoring strategy should be put in place as early warning signs are critical.
Boards that are warped, cupped, and discolored, and efflorescence are signs
The fastener corrosion and rust bleeding from the ledger bolts
The new cracks that form post bases or beam connections
If any of the above flags are present, do not use the pool and consult a professional. Engineering supports that are designed to be safe include sister joists, additional footings and engineered steel supports. The safety of you and the users of the pool depends on accurate measurements and verified data. Never assume that the framing of the existing deck is safe.
FAQ
Can I have a swimming pool on my wooden deck?
In order for a wooden deck to be able to hold a swimming pool, the deck must be structurally supported. For instance, the deck must have new ledgers that are deeper footed, closer joisting, and closer joist spacing.
What is the bare minimum PSF for a deck that supports a swimming pool?
In the International-Residential-Code (IRC) subsection R507.6, a pool deck must support at least 100 pounds per square foot (psf) and is the bare minimum. In most regions, 100 psf is still the least restrictive and tenants and landlords may still be found liable for more support.
What are the most obvious dangers of having a pool on a wooden deck?
The most obvious dangers are structural failure due to an excessive amount of weight on the deck, hidden moisture decay if there is trapped water underneath the deck and pool, and untreated wood hidden underneath the pool will rot if there is no support structure.
Which is better for swimming pool decks, Composite Decking or Pressure treated wood?
Although Composite Decking is more expensive at first, it is far better than Pressure Treated Wood at pool environments because Composite Decking will require less maintenance, lasts longer, and has greater resistance to moisture.
