Concrete requirements for a two-post lift

A two-post lift puts the entire weight of a vehicle, plus the leverage of the columns, into a small number of anchors in your floor. The slab is a structural component of the lift, and it is the one the manufacturer did not supply.

Every manufacturer publishes a minimum concrete specification for each model. Read it before you order, because the answer sometimes changes which lift you can buy, and occasionally changes whether you can buy one at all.

What the specification usually covers

• Thickness. Stated as a minimum depth of continuous, reinforced concrete under the base plate.
• Compressive strength. Stated in psi. A garage slab poured to a residential minimum may be below what a lift wants.
• Cure time. New concrete is not at strength on day one. Manufacturers state a minimum age.
• Edge distance. Anchors need a minimum distance from any edge, joint, saw cut or trench. A column set too close to an expansion joint is a column with much less holding power than the numbers suggest.
• Reinforcement. Some specifications assume rebar or mesh.

Figures vary by model and by capacity, and we do not publish a generic number here because a generic number is exactly what gets someone into trouble. The requirement for the model you are looking at comes from that manufacturer's installation manual, and we will send it to you before you commit.

How to find out what you have

Ask, if the building is new enough. A builder, a developer or the original plans may simply tell you.

Core sample. A concrete contractor can take a small core, which gives you thickness directly and can be sent for strength testing. This is the definitive answer and it is not expensive relative to a lift.

Drill test. Drilling a test hole with a masonry bit and measuring the depth to sub-base tells you thickness but tells you nothing about strength. It is better than assuming.

What you cannot do is judge it by looking at the surface, or by the fact that a heavy vehicle parks on it. A slab can carry a distributed load fine and still be inadequate for point loads through anchors.

If your slab is not adequate

You have three real options and one bad one.

• Cut and pour a pad. A concrete contractor cuts out the area under each column and pours a footing to the manufacturer's specification. This is the standard fix and it works.
• Choose a four-post lift. In most configurations a four-post is free-standing, spreads its load over four feet and does not need anchoring. If your slab is the problem, this is often the practical answer rather than a compromise.
• Choose a portable or scissor lift, checking each one's own floor requirement, which is usually lower but is not zero.

The bad option is to fit anyway and hope. Anchors pulling out of thin concrete under load is a specific and well-documented failure mode, and it happens with the vehicle in the air.

Anchors

Use the anchors the manufacturer specifies, at the embedment depth they specify, torqued to the figure they specify. Substituting a hardware-store wedge anchor because it looked similar is the other common way this goes wrong. If an anchor spins rather than tightening, stop: it has not set, and re-torquing it harder does not fix it.

The order of operations

1. Confirm the ceiling height for the model you want.
2. Confirm the concrete specification for that model.
3. Establish what your slab actually is.
4. Then buy the lift.

Doing this in the other order is how a lift ends up in a crate in the corner of the garage for six months.