Should plastic or foam be used when building a floating dock?

Colourful umbrella on floating dock in middle of lake Photo by Michael Cardoso/Shutterstock

Should plastic or foam be used when building a floating dock? Is one better for the environment?

To resolve this labour issue for dockworkers, let’s start from the beginning. Buoyant foam billets are either expanded polystyrene (EPS)—the white granular material frequently used for beer coolers—or closed-cell extruded polystyrene foam, often known as Styrofoam BB. Plastic floats are generally made of a high-density polyethylene or polyvinyl chloride (PVC) shell filled with air for buoyancy and sometimes expanded polystyrene for additional rigidity.

While there is much debate about the relative merits of foam vs. plastic, closed-cell extruded polystyrene foam is widely accepted as better than EPS which, over time, wicks up water (and possibly oil) and can lose buoyancy. Billets should be snugly encased in wire mesh and/or wood to protect them from gnawing wildlife, which will nibble the underside of your dock and spit out little pieces that end up littering the water. This is not good for the health of animals or the lake.

Advocates of plastic floats like them because they don’t take on water, they maintain their form, and they are available in a greater variety of shapes. They also perform better in ice, are unaffected by animals, and don’t require the additional wrapping that foam does.

DIYers will find foam-free, air-filled plastic floats easiest to work with, as they offer modular construction and are designed with mounting slots so they can be easily bolted to the wood decking. Avoid plastic floats with oddball shapes as they might be difficult to replace later.

What’s most important for a long-lasting dock is the quality of the hardware, materials, and design. Buy all supplies from a good dock builder and find one who knows local conditions well.