Floating Dock Materials
Wood

Wood has many advantages for floating structures. It's attractive, economical, naturally buoyant, and easy and familiar to work with. Pressure–treated wood or a naturally rot–resistant wood like redwood, cypress, or cedar should be used to ensure a long service life. Health and environmental issues should be considered when using pressure–treated wood.

Most water–based wood preservatives are suitable for human contact and can be applied by staining or painting. Common water–based preservatives include chromated copper arsenate (CCA), ammoniacal copper zinc arsenate (ACZA), or alkaline copper quat (ACQ). A consumer information sheet should be included with any pressure–treated lumber (figure 3).

Figure 3—Read the consumer information sheet before
working with pressure–treated wood.

Pressure treatment forms an envelope of pesticide-impregnated wood that may be less than half an inch thick, protecting the untreated interior portion of the wood. Treating the wood after all cuts and holes have been made will help keep the treated envelope intact and extend the useful life of the structure. Eliminating field sawing and drilling is also critical to minimizing the introduction of harmful chemicals into the environment.

Chemically treated wood may last 40 years or more, five times or more longer than untreated wood. It is important to know which chemical treatments are appropriate, and how they may cause adverse health or environmental effects.

The subject of chemical treatments for wood is not only complex, it is an area of continuing research and product development. The Additional Information section includes several good resources. Always follow the recommendations in the Best Management Practices for the Use of Treated Wood in Aquatic Environments (Western Wood Preservers Institute 1996). Other sources of good information are the Guide for Minimizing the Effects of Preservative–Treated Wood on Sensitive Environments (Lebow and Tippie 2001) and Best Management Practices for the Use of Preservative–Treated Wood in Aquatic Environments in Michigan (Pilon 2002).

There are several good reasons to use properly treated wood in wet areas and few reasons not to. All of the treatments that are effective in wet areas must be applied using pressure treatment at a certified treatment facility. Spots where the wood was cut or drilled in the field can be treated by hand brushing several coats of copper naphthenate. Copper naphthenate is not an U.S. Environmental Protection Agency (EPA)–controlled chemical, so it can be purchased and applied by field personnel. Both oil–based and waterborne preservatives are suitable for preserving wood in wet environments. Know the characteristics and effects of each type of preservative before deciding which one to use. Water–soluble preservatives, such as borates, are not suitable for wet environments. The borates do not permanently "fix" to the wood.

The availability and use of some preservative–treated wood products may change. Under a pending agreement between the EPA and representatives of the wood–preservative industry, chromated copper arsenate will be phased out of residential use. Chromated copper arsenate can still be used in bridge construction, but its availability may be limited.

Workers need to take safety precautions when handling or disposing of treated wood. Treated wood should not be burned. Some States and other jurisdictions may also impose additional restrictions on disposal.

Each of the preservatives containing copper imparts a color that disappears over time, normally within 2 years. But, depending on site conditions and exposure, the color may last for several months to 3 or 4 years. Stains can be added to the waterborne preservatives at the time of treatment or at any time afterward. Pigments, stains, and dyes mask the normal color of the preservative. Because these materials will penetrate the wood during treatment, future needs for restaining will be reduced. Treatment plants will be reluctant to apply special stains to wood unless they are processing a very large order.

Generally, use galvanized, powder–coated, or painted metal fasteners on any wood members.

Untreated, painted wood will require more maintenance and will have a shorter service life. The wood will need a new coat of paint every few seasons. Old, deteriorated paint tends to trap moisture and hasten decay. A finish, if used at all, should soak into the wood rather than coating it.

Good design and construction techniques can reduce wood deterioration. Most fungi that decay untreated wood require four basic conditions for survival:

• Moisture levels higher than the fiber saturation point in the wood


• Free oxygen


• Temperatures from 50 to 90 °F


• Food (the wood)

Wood that is continually submerged will not decay because no oxygen is present. In addition to treatment, another way to slow fungal deterioration is to minimize the time that wood has a high moisture content. Provide ventilation through wood frames and decks. Fit fasteners flush with the wood's surface to prevent water from accumulating. In general, minimize the areas where water can pool.

Decks—Wood decks remain a popular choice because they are attractive, relatively inexpensive, and easy to assemble and repair. Popular choices include pine, redwood, cedar, cypress, and wood–plastic polymers. Many owners do not treat or paint their decks and allow the wood to age naturally. Untreated decks rot quickly. Decks that are treated with a preservative will last five or more times longer than untreated wood decks.

Frames—Wood has increasingly been replaced by aluminum or galvanized metal for framing, but wood is still common and has many advantages. Wood is easy to work with and workers are often familiar with good construction practices. A rotten wooden frame member is often easier to replace then a comparable section of rusted metal. Wood may also be more attractive and less expensive. A dock's frame comes into contact with water or may be constantly submerged in water depending on the design and loading weights. Wood used for dock frames should be pressure–treated with a preservative. Untreated wood that gets wet and is allowed to dry frequently may last just 2 to 3 years. Properly pressure–treated wood is recommended for framing.

Floats—As wood becomes saturated with water it loses strength and buoyancy. Wooden floats should only be used for temporary, primitive bridges.

Metal

The most common metals used in dock fabrication are aluminum, stainless steel, and galvanized steel.

Steel should be painted, galvanized, or coated with an epoxy or ceramic layer to extend its service life. For maximum effectiveness, coatings should be applied in the shop, after fabrication, so the coating extends into grooves and holes.

Aluminum is lightweight and naturally corrosion resistant. When aluminum interacts with oxygen, a surface layer of oxide is produced that protects the aluminum from further degradation. Aluminum members must be thicker than a comparable steel member to provide equivalent strength. Aluminum will corrode when it contacts a dissimilar metal. Aluminum components should not be joined to steel components.

Stainless steel provides the strength of steel and the corrosion resistance of aluminum, but costs several times more than galvanized steel.

Decks—Most floating structures have timber decking instead of metal. Metal decking usually weighs more than wood and may not fit esthetically into less developed settings. However, docks for heavy traffic areas in developed settings or for motorized users may benefit from metal grating or aluminum sheeting on decks (figure 4). The grating provides additional traction, especially during wet and muddy conditions, and will last longer than wood under heavy use.

Figure 4—This dock at Dworshak Reservoir in Idaho
uses metal grating as its deck. Metal decking
provides drainage and traction but can still
be slick when it is muddy and wet.

Frames—Metal framing is becoming increasingly popular. Lightweight aluminum is a good choice for pedestrian traffic. Galvanized frames also work, but the added weight will require more (or larger) floats.

Steel tubing is popular. Steel tubing can function as both the float and the frame. Framing brackets and crossmembers are welded at each end, and at intermediate locations for long structures (figure 5). Brackets are usually welded to the tubes to attach the deck.

Figure 5—Steel tubing should be framed for adequate structural integrity.

Coatings can increase the service life of the steel members. Apply coatings in the shop after fabrication to further increase the steel's service life. Using thicker steel also increases a structure's usable life, but increases the weight and the cost.

Floats—Metal floats are usually tubes or drums, but almost any closed shape can be used. Metal floats have the advantages of strength, durability and, if properly coated, longevity. Metal floats are generally called pontoons. Most pontoons are aluminum. The wall thickness depends on the pontoons' size and use. Many manufacturers or suppliers provide custom pontoons. Most pontoons have multiple individually sealed air chambers, which may or may not be filled with foam. Connection points may be built in, bolted on, or welded. A punctured tube can sink an entire structure. Foam–filled tubes can provide insurance against this catastrophe.

Recycled industrial drums should not be used as flotation pontoons. Their walls are not thick enough for durability and they may contain toxic residues. Consult reputable suppliers to find flotation devices.

Alternative Materials

A wide variety of materials have been used for docks, including plastics, concrete, fiberglass, and wood/plastic composites (figure 6).

Figure 6—A concrete support makes
an excellent foundation.

Decks—Many distributors offer decking materials made of wood/plastic composites, vinyl, and other plastics engineered to look and behave like wood. Advantages include less maintenance, and less damage from insects, rot, and moisture. In addition, these materials do not twist, cup, or warp as wood decking commonly does. Plastic decking may be made from recycled material. Disadvantages include higher initial purchase price and closer required spacing of supports than wood decking. Some products require extra care because they are more prone to cracking than wood. Holes may need to be drilled. Sawdust from plastic materials will not decompose. Good practices call for cutting these materials away from water and packing out the shavings.

Frames—Plastic products are available for frames. The advantages of plastic include water and decay resistance. Because frame materials have no standard specifications, different manufacturers sell different types of materials. Plastic products are usually made from polyethylene formed into many different shapes. The materials usually function as both the float and the frame. Plastic frames often have molded indentations, holes, tunnels, grooves, or brackets that accept common wood decking and reinforcing members. If you are considering using plastic materials, be aware of strength and repair issues. Many different types of manufacturing processes and chemical additives influence a plastic's strength and durability. Check with the supplier to ensure their product will stand up to its intended use. While plastics can often be repaired if they are damaged, some plastics will require skilled labor to match the appearance and strength of the repaired section to the original member.

Floats—Traditional polystyrene foam blocks or billets must be encased in a protective covering. This material provides excellent flotation, but is susceptible to damage from debris, sunlight, chemicals, and burrowing animals.

Plastic Float Drums—Most common docks use plastic float drums. They are typically constructed of polyethylene by rotational or blown molding. Rotationally molded polyethylene is stronger than polyethylene molded by blowing. Plastic float drums are hollow or may be filled with closed–cell EPS or another type of secondary flotation. Secondary flotation helps absorb shocks during impacts and will reduce the loss of buoyancy if the shell is punctured.

Plastic float drums with EPS inside are an excellent means of flotation. The float drum's tough polyethylene shell has properties that the EPS lacks, including resistance to impact and stress fractures, protection from rodents, and ultraviolet inhibitors that resist breakdown from solar radiation. Plastic does not react with many chemicals and the EPS core provides flotation insurance in case the plastic shell is penetrated. For most applications rotationally molded plastic float drums filled with extruded closed–cell EPS foam are recommended because of their availability and proven functionality.

Fiberglass Float Drums—These float drums are not as widely available as plastic float drums. They share many of the same features, such as resistance to chemicals and rodents. They are generally not as strong as rotationally molded polyethylene, but are lighter.

Foam–Filled Tires—Two different versions are available. Many creative dock owners have built their own floats by filling recycled tires with EPS foam and capping the middle with plywood. This method is not recommended because of the problems of sealing the tire and creating an attachment bracket. Commercially available products have been fabricated to overcome problems associated with foam–filled tires (figure 7). These floats are extremely durable. They are recommended for situations where the structure may be temporarily grounded or in areas with lots of debris. The shape of the tire and the added mass beneath the dock's framework can help keep the dock from being bounced around by waves. Used tires are not very attractive, but they can be hidden with a cleverly constructed skirt.

Figure 7—A cross section of a foam–filled tire.

High–Density Polyethylene (HDPE)—HDPE pipe is similar to common PVC pipe, but it is stronger and better able to handle the rigors of service in docks. HDPE pipe is typically used along the length of the structure, with one pipe on each side that is strapped or slipped into mounting points on the frame. Heavy–duty HDPE piping is recommended. HDPE piping can be used as both the flotation and the framework for the structure when a few additional steps are taken. Pipes should be cross braced with additional piping or other material. Connections for the decking material and the end caps need to be plastic welded. High–strength corrugated HDPE piping is also available.

Concrete—Concrete floating structures are commercially available, but they could not be constructed by a trail crew and may have little Forest Service applicability. Concrete floats are typically constructed by surrounding a block of EPS with reinforced concrete. They are expensive and heavy, but they are tough and extremely stable in water.