Determining the suitability of an existing deck for supporting a spa requires careful consideration of load requirements, structural integrity, and local building codes. This guide provides an overview of the factors involved in spa deck design and installation, drawing upon practical experience and engineering principles.
Assessing Deck Load Capacity
The primary concern when installing a spa on a deck is whether the deck can safely bear the weight. This involves understanding the deck's load capacity, which is measured in pounds per square foot (psf). A typical spa can weigh between 100 to 150 psf when filled with water and occupants.
Calculating Spa Weight and Distribution
To determine if your deck can handle the load, calculate the total weight of the filled spa. For example, a spa weighing 5000 lbs will exert a certain amount of pressure per square foot depending on its dimensions. If a deck is 5x7, it will be 142 lbs/sf. Remember that the live load of your tub must include the people in it.
Understanding Live Load, Dead Load, and Combined Load
Decks are designed to withstand two types of loads: live load and dead load.
- Live Load: The weight of temporary or moving objects, such as people and the spa itself.
- Dead Load: The weight of the deck structure itself.
The combined load is the sum of the live and dead loads. The spa's weight, including occupants, must not exceed the deck's live load capacity as calculated by a structural engineer.
Read also: Deck Design for Swim Spas
Structural Considerations
Several structural elements contribute to a deck's load-bearing capacity. The spacing and size of joists, beams, and posts are critical factors.
Joist Spacing and Size
Joists are horizontal structural members that support the decking. The spacing between joists and their size directly impact the deck's ability to handle weight. With an 8’ span and joist spacing on 12” centers, 2x10’s would be more than enough. If you want to stay as close to the ground as you can I think doubled up 2x8’s on 16” centers would be plenty.
Beams and Posts
Beams are structural members that support the joists, transferring the load to the posts. Posts, in turn, transfer the load to the ground.
- Beams: A structural member, usually horizontal and narrow in proportion to its depth, which supports superimposed loads primarily by its internal resistance to bending. The principal structural materials for beams are: steel, light alloy, timber, reinforced concrete, prestressed concrete or composite materials.
- Posts: Vertical supports that bear the weight of the deck.
If you have four "6 x 6" posts in a 5' x 7' configuration, you can easily shore up the joists for sag by attaching doubled "2 x 10" beams to the existing beams with hangers. Double hanger brackets are down at Home Depot for about $1.80 each.
Installation Techniques
Reinforcing an Existing Deck
If an existing deck needs reinforcement, several methods can be employed.
- Adding Additional Support: Attaching doubled "2 x 10" beams to existing beams with hangers can significantly increase load capacity.
- Shoring Up Joists: Reinforcing joists can prevent sagging and improve overall stability.
Building a New Spa Platform
When constructing a new platform specifically for a spa, consider the following:
- Footings: Concrete footings provide a stable base for the posts.
- Posts: Use 6x6 posts sitting on all corners as well as one on each side between the corner posts (a total of eight), all atop concrete support footers.
- Frame: A 2x12 doubled frame, with 2x12's 12" on center, with the "normal" decking boards on top.
- Alternative Thought: Level the ground where we put our spa, then set it on a 6 inch deep bed of gravel with wood framing to keep it in place. Has been there about 4 years and just as level as the day we put it in place. We then built the deck around the spa leaving about 18 inches of the spa coming through the deck- made it a bit easier to climb in and out of and didn't have to account for the weight of the spa for the deck construction.
Integrating the Spa into the Deck
An alternative approach is to integrate the spa into the deck design.
- Partial Inset: Set our spa 1/2 way in to our deck. The seat of the spa is at the same height as the deck.
- Lowered Deck: Our case was different; our deck is 5' off the ground. We had a contractor frame in the hole for our spa(ours is round), and cut the hole in the deck. It is great. We were originally just going to reinforce the deck, and set the spa on top. We had it 1/2 inset to the existing deck. Part of the deck railing was removed, and a second deck, which the spa actually sit on was added. There are two steps down to that lower portion, and it has room to walk around the spa; the maintenance panel is on that side of the spa.
Metal Decking
For some applications, metal decking may be appropriate.
- Design Loads: Design the metal deck for all applicable conditions.
- Framing Plan: The contract documents should provide an explicit framing plan that would show all span conditions for the metal deck to be furnished.
Consulting Professionals
For accurate assessment and design, consulting with professionals is highly recommended.
- Structural Engineers: They can calculate load requirements and design reinforcement plans.
- Building Inspectors: They can ensure compliance with local building codes.
As you have mentioned, in your original post, you have original plans from the original insatllation of your deck. This was a professional installation. At the bottom of the plans you will find the names of the builders, possibly the designer, and the seal for approval from the city/village/county regarding your deck. This has been filed with you city/village/county. I strongly suggest you call these people, to ask them for the proper load requirements etc. regarding your situation. They can work with you and give you EXACT requirements.
Additional Considerations
- Spa Size: Our spa is considered a 5 person and is roughly 7’ by 7’6”. An 8x8 deck would be mighty small for getting in and out of the spa.
- Frost Heave: Have you considered pouring 3 footings on that side and attaching your perimeter beam directly to the footings with brackets? No chance of lateral movement that way and you minimize the step up to the deck. I suppose frost heave could be a problem in you area, we don’t have that in Georgia.
- Manufacturer Recommendations: When we got our spa the manufacture had recommendations for deck construction.
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