Rebar - size advice


New member
Nov 17, 2020
I’m in the process of building a pool (first time ever) and have met with several contractors.
It has come down to 2 of them. Builder 1 uses #3 rebar and builder 2 uses #5 Rebar.

Is #3 rebar “good enough?” We really like builder 1 better but I don’t want to get an inferior pool built either.

Thank you.


Well-known member
Jul 20, 2020
Houston, TX
Post stolen from @ajw22 :

From Construction Best Practices - Further Reading

Rebar and Gunite
Concrete swimming pools are structures and, therefore, by code, require structural engineering. The exact rebar specifications should be designed by a structural engineer for a project.

Without getting into specifics, there is usually a range of specifications for things like rebar size, spacing of the rebar, concrete thickness, concrete strength, etc. At the lower end, the idea is to save money on construction. It is usually worthwhile to go above the minimum to help minimize long term problems.

It is best to inquire about the range of specifications, the cost differences, and the strength differences so that you can make an informed decision.

If you want to know what the specifications of the plans are, ask for a copy of the drawings and discuss them with the PB or the engineer. However if you disagree with the drawing, then you should probably be ready to scientifically / mathematically state your case about why you disagree and which building standard or municipal building code section your objection is covered under.

Try to refrain from requesting that x amount of rebar of your choosing be spaced at y distance. You will only alienate both the PB and Engineer with frivolous requests.

If the soil is an issue, you should get a geotechnical engineer to evaluate and advise. You may need piers if the soil is not considered stable.

For rebar engineering specifications read Reinforcing Steel and Swimming Pool Construction by Pool Engineering.

It's always best to have an engineer design a plan for your specific project. Using "standard" designs only works for nominally standard projects.


TFP Expert
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In The Industry
Aug 10, 2017
Morris Cnty NJ
The spacing will be twice as far with larger rebar. Either way is fine it's the design from the engineer that matters. As long as the drawing is stamped by a PE it should be fine. If your in an area with no inspections or local requirements for engineering and the pool is engineered by the PB himself than it really matters alot more
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TFP Expert
Mar 2, 2011
No. 4 would be the most likely choice. References suggest that #5 and larger can result in “Shadowing”, which is the absence of concrete behind the bars because the bar blocks the shotcrete.

In addition be sure to get at least 4,000 psi rated strength and make sure that the concrete is of adequate thickness.

The structural engineer should certify the plans and that should be adequate.

Because of this, the Building Code places limitations on the size and spacing of rebar in gunite & shotcrete. The maximum size of reinforcement shall be No. 5 bars. There shall be a minimum clearance between parallel reinforcement bars of 2 1/2 inches. Bars larger than # 5 increase the chances of shadowing (voids behind rebar).

Absent special variations, most shells will contain bars no larger than #5s spaced no closer than 2-1/2 inches. This is important, because it ensures that no spaces will be so tight that pneumatically applied concrete cannot completely fill in the spaces behind the rebar. This is why inspectors are so concerned about spots where multiple bars are lapped together. They can easily lead to voids or “shadows” behind the steel that will seriously weaken the structure.

In poured-concrete structures, such as grade beams or friction piles where larger rebar is used, this is not a problem because the concrete is often subjected to vibration while still in a liquid state to cause the material to fill in all voids around the rebar. You can’t do that in a swimming pool or other gunite or shotcrete structure. Instead, you must rely on the material’s self-compacting characteristics to fill the spaces behind the bars.

The upshot of these working-stress-based calculations (and as required by the building codes used throughout the United States) is that most properly designed and constructed swimming pools should be able to support at least twice the load that will actually be placed on them by the weight of the water and the stresses generated by the surrounding soil. Ultimately, the building-code-required safety factors result in good, safe places to be – and offer a very good reason to observe and follow engineering specifications in setting up a steel cage and applying an adequate thickness of concrete to it.

The American Shotcrete Association’s (ASA) Pool and Recreational Shotcrete Committee and ASA Board of Direction have reaffirmed a 4000 psi (27.6 MPa) minimum for in-place compressive strength pool concrete.

In swimming pools, No. 4 reinforcing bar is the minimum size bar that should be used. Depending on the design, we see a lot of pools with No. 5 bars (No. 16M) and No. 6 bars.



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The spacing will be twice as far with larger rebar. Either way is fine it's the design from the engineer that matters. As long as the drawing is stamped by a PE it should be fine. If your in an area with no inspections or local requirements for engineering and the pool is engineered by the PB himself than it really matters alot more
Just one thing to add. Seems obvious but amazing how often this becomes an issue. In addition to the stamped drawing you need to be sure the pool is built to the engineers requirements. This is where inspection comes in to play. If concrete strength, size, spacing, or distance of rebar from either surface of the concrete is inadequate it needs to be repaired before pouring or spraying. Do you trust his inspection, spot check yourself if you know what to look for, or hire an independent inspector? Choice is up to you and right answer depends on your situation.



TFP Expert
Mar 2, 2011
Is a structural engineer giving approval to each design?

Do you have any reason to question the stability of the soil?


New member
Nov 17, 2020
I’m not sure about their use of a structural engineer. I’ll hav to find out.

Regarding the soil, It’s a new neighborhood so I don’t see the cracks in the driveways or sidewalks like my previous neighborhood.


TFP Expert
Mar 2, 2011
To some degree, you have to trust the builder and their reputation. So, you have to decide how much you want to get into their business.

Do they have references?

In my opinion, a good builder should not have a problem discussing the design with a customer. A good builder should be able to explain why they are making the design choices that they make.

After discussing the design, you can request increasing the specifications for more strength, but it will cost more. A good builder should not have a problem upgrading the specifications. Going from #3 bar to #4 bar or to higher psi concrete should not be a problem for a good builder if you want it.

A poor builder will get annoyed because they can’t intelligently defend their choices.

If you want to get into the design, you should know the rebar size and spacing, the psi of the concrete, the thickness of the concrete at different spots (walls/floors etc.) and if there are any special considerations for soil stability and water control.

Will there be any risk of hitting rock or anything that could potentially cost more if it occurs?

A geotechnical engineer’s report of soil analysis and recommendations can be helpful in some cases, but it’s usually not necessary.

Pools built on slopes, expansive soil or backfill require a geotechnical report.

A structural engineer’s approval is usually a good idea and necessary in some cases.

Regarding the soil, It’s a new neighborhood.
The developer should have a geotechnical engineering report that they might be willing to give you.
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