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A Guide to Trench Grates Material

How to choose what your trench grates are made of

Water soaks into a decorative cast iron grate red and brown with developing patina
This raw cast iron trench grate is developing a patina: it will age to a rich red-brown.

Drainage is a ubiquitous part of construction. It’s so common that it often goes unnoticed unless something goes wrong and water backs up around it. Water management is important to all types of building and landscaping from culverts near fields to the slopes of city streets. Trench drains are found in many of these drainage systems. (They’re so common they have a variety of names. Trench is also known as channel drain, strip drain, linear drain, or line drain.) The small channels these furrows create are surface capillaries designed to capture and move water to sewers, bioswales, softscapes, and streams.

Trench is different than weeping tile or French drains in that these are open conduits in the substrate. To prevent tripping people or catching debris, they must be covered with grates. Given the variety of places that these drains may be installed, designers have a lot of choice when picking trench grates material.

There are several factors to consider when choosing trench grate. The contractor might think about aesthetics, cost, wear patterns, total load, weather patterns, and replacement interval. Will the drain be managing rain water alone? Or will it manage salty, chlorinated, or acidic liquids? Will it be part of a containment system for chemical spills?

The size and design of the trench grate also requires some consideration. The design of the grate’s shape is influenced by peak water flow, the type of debris the grate should strain out of the system, compliance with safety regulations, architectural look, and whether or not it will be used in places where people will have bare feet or high heels. Trench grate openings need to be ADA compliant. This means the grate opening size is no greater than ½ inch perpendicular to the direction of travel, so that won’t trap cane tips or wheelchair casters. Some locations might look for “heel-proof” grates, which have smaller openings that high heeled shoes will not slip through. Heel-proof grates can be no larger than 5/16 inch. In situations where there are unlikely to be high heels but there will be a lot of water flow, these grates may not be large enough. This is why heel-proofing is not standard for all trench grate types.

White bar pool grates contrast to deep blue tile around a pool
Plastic trench drain is very common around pools.

Plastic trench drain grates

  • Cost: 💲
  • Durability: 💪
  • Common locations: Pool drainage, decks, indoor drains. Often complements tile.

Poolside, plastic is a very common choice. Plastic is simple to install, inexpensive, and lightweight. Chlorinated water can be hard on many materials—it will degrade plastic over time. However, plastic will tend to bleach and fade rather than corroding, splintering, or peeling. Pool applications often do not need a lot of strength or impact resilience: the most they’ll deal with is a person running over them to cannonball from the pool’s edge. The smooth finish of the plastic grating is a soft material choice for bare feet. Plastic is easily cast, so plastic grates may have complex decorative patterns.

Other applications also use plastic. Often plastic is used outside in residential applications and inside in commercial one. Entire drain systems can be purchased out of plastic, making the act of trench installation simple. This appeals to the residential market. Commercial plastic will often be installed for its non-reactivity in areas that might see spills of acidic liquids. Thicker, stronger plastics are generally used in commercial settings.

Plastic has a few challenges. Thermal resilience is one of them. Plastic can be brittle and more likely to shatter in freezing conditions. Freeze/thaw cycles also may permanently embrittle some plastics.

Plastic does not last as long as other grating materials. Total life-cycle, and how long the grate maintains like new, is dependent on several factors. Climate, mold, maintenance, the liquids flowing through the plastic, maximum load and overall wear may all affect life-cycle.

Galvanized steel channel grates

  • Cost: 💲
  • Durability: 💪💪💪
  • Common locations: Municipal and industrial sites. Often over trench in grass or in concrete.

Galvanized steel is a common material for municipal site fixtures. From a few steps away, galvanized steel may look like a matte ash gray, but close inspection shows a distinctive spangled surface with feather-like deposits of silvery-gray zinc oxide. Zinc creates a layer that both seals the steel physically and provides chemical protection. Over time and in chemical stress, galvanization can wear away and leave the steel open to corrosion. In some places the use life may be decades—but a combination of heavy de-icing chemicals and wear may allow corrosion to set in before then if the zinc oxide is disturbed or chemically stressed.

Most galvanized steel channel grates come in industrial-looking design due to manufacturing considerations: steel does not cast as easily as other materials. Simple slots or bars are common. Steel is often chosen for being a tough, load bearing material. When heavy-duty grates are put over concrete formed drains or steel trench pans, the system can bear the weight of vehicle tires. Steel will also stand up to the wear of many feet. This metal is often used in construction for its material properties. It has some give: rather than breaking, it will dent under heavy impact.

Three black painted cast iron drains against a white background
Gray cast iron is wear-resistant, compression resistant, and vibration dampening.

Ductile / cast iron trench grates

  • Cost: 💲💲
  • Durability: 💪💪💪💪
  • Common locations: Decorative borders, pool and spa, homes and residential complexes, seawalls, parks, office buildings. Often complements brick or stone.

Ductile and cast iron trench grates are inexpensive, hardy, long wearing choices that offer many decorative possibilities due to the material’s castability.

The properties of cast iron make it the most common material used by municipalities for grates and manhole covers. Most cast iron in these situations are known as gray irons. Like steel, gray iron is incredibly strong. Compared with steel, it offers greater vibration damping, which can be useful in hardscape where traffic vibration may affect concrete. Cast iron also does not deform under compression. It’s solid even under a heavy load.

Raw cast iron is that it develops a rich red-brown patina over time that protects it from corrosion. Unlike with galvanized steel, this patina can re-develop even if the original layer is worn, chipped, or scratched. Cast iron grates can also be painted, and black-painted iron is a classic, distinctive choice.

Where there may be strong impact forces, gray iron is not the best choice. Gray iron is brittle. The hard, wear-resistant surface is more likely to shatter than dent—a great feature in some installations, including many grates, where dents may trap water and lead to pitting corrosion. This behavior may be troubling where impact is likely. Ductile iron is not as common in grates, but is used in applications where the grate needs to offer the impact resistance of steel, while still offering the patina and aesthetic of cast iron.

Check out our line of functional and attractive cast iron grates.

Fiberglass trench grates

  • Cost: 💲💲💲
  • Durability: 💪💪💪💪
  • Common locations: Work sites, courtyards, catwalks, storage, some pool and spa applications. Often complements concrete or tile.

Fiberglass-plastic composite grates are corrosion resistant and have greater strength than plastic alone. As such, they’re often used in heavy-duty situations where plastic is desirable but not strong enough. There are lots of options for flow capacity in fiberglass trench, from large square grid for high volume trench to more decorative finishes appropriate for the driveway. Given that fiberglass is built with polymers, different color choices are available.

The reasons people might not choose fiberglass tend to be installation system, cost, and weight. Fiberglass is light: a real bonus in some applications, but mounting systems must stabilize the grate rather than the intrinsic weight of the grate itself and these can become stress points. As a plastic based material, these grates also have a lighter look than metal. Fiberglass generally comes with preformed fiberglass trench pans, rather than being set into frames set into the concrete.

A thin stainless steel trench grate drains water inside a tiled shower
Stainless steel is often used for indoor applications.

Stainless steel channel drain

  • Cost: 💲💲💲
  • Durability: 💪💪 to 💪💪💪
  • Common locations: Showers, tubs, driveways, patios. Complements bathroom tile, stone, and wood.

Stainless steel is generally chosen for corrosion resistance and aesthetic look. Because it is a steel, stainless has tough mechanical properties; because of the cost of material, stainless grates are usually thinner gauge. The silver-glint of polished stainless is one of the reasons it is so prized, and this lighter color often adds to the lighter overall look of the grating. Stainless steel trench grates are also usually constructed of simple machined shapes. Slotted widths are common, or a pattern of holes. Like steel, stainless is harder to cast. These grates therefore often fit well in modern spaces featuring spare geometric designs.

Corrosion resistance is one of stainless steel’s greatest strengths. Passivation creates an almost invisible protective film that prevents flaky red iron oxides from forming. Some grades of stainless are challenged by salts or acids, and so it is important to pick the right kind of stainless for the setting. In a shower or bathroom, 304 stainless might be a fine choice. Outdoor applications that may be exposed to salty water would be better served with 316 stainless.

Aluminum trench grate

  • Cost: 💲💲💲💲
  • Durability: 💪💪💪
  • Common locations: Pools, courtyards, industrial applications, playgrounds, public squares.

Unlike all ferrous metals (steel, stainless steel, and iron), aluminum does not rust. Over a long period of time it may produce a whitish oxide, but in most environments these oxides take many years to form and will not challenge the integrity of the metal. These oxides will not bleed like rust does onto surrounding surfaces.

The silvery shine of the aluminum is like stainless steel: aluminum has a slightly brighter luster. Like steel grating, aluminum is usually machined, not cast. Generally, this means aluminum grates will be made of bars, slots, or holes in the surface, rather than available in more complex cast designs. The similarities in both look and function between aluminum and stainless steel mean that these two metals are often compared against one another.

Aluminum is slightly less strong than steel, and for the same load rating must be thicker. Although aluminum can be a less costly material, the grates may cost more than similar stainless grates for a given load rating. However, due to their added corrosion resistance, and white rather than red oxidization, they are may be the metal of choice in very wet outdoor spaces, especially in places where red oxide staining would be unsightly. Waterparks, spas, and playgrounds sometimes choose aluminum for these reasons.

Close up of perforated bronze grid
Bronze can be found in patterns from basic to sculptural.

Bronze trench grating

  • Cost: 💲💲💲💲💲
  • Durability: 💪💪💪💪💪
  • Common locations: Indoors, marine applications, in contact with other metal, sculptural, and luxury settings

Bronze is a beautiful, sculptural metal that can be crafted into lasting decorative accents. Outdoors, bronze will slowly develop a brown patina, but it does not corrode. Where the patina is disturbed, bronze’s rich golden tone will shine through, as is often witnessed on statues. People’s hands lift the patina at common touch points.

It is common to see boat propellers and accessories made out of bronze, and bronze may be chosen for decking trench drain—either to match the accessories around it, or because of the metal’s properties. Bronze does not corrode in marine environments.

Further, bronze is resistant to all sorts of wear. Although steel may be harder when impacted, bronze is more resistant to galling. Galling occurs when two metals rub against one another. An electrochemical interaction between the two causes metal to pull away and often ball up, much like a sweater pills where it rubs. This property of bronze may be exploited in high-wear situations like train stations or ferry terminals. The combination of aesthetic beauty, corrosion resistance, salt resistance, and ability to handle the scrape of dollies can make bronze the stand out choice. Well-made bronze will generally survive generations, wearing gently to soft curves and glowing polish.

A bronze freedom trail medallion sunk into concrete points the direction of the freedom trail
Bronze plaques show how the material burnishes in the outdoors and under the load of many feet.

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