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Perimeter and Sediment Control with Silt Fences

Silt fences are a standard sediment control measure used to keep sediment from entering waterways. They trap sediment on the upstream side of the fence but allow water to flow through. Silt fences are typically made of porous or filter fabric with high UV stability held up by metal or wooden stakes driven into the ground. They're placed in areas where sediment is likely to accumulate, such as construction sites or near culverts.

Silt fences can also be used for perimeter control. They help keep sediment from entering an area and can help protect against erosion. When used for perimeter control, silt fences and stakes are typically anchored to the ground around the edge of a property or jobsite.

How silt fences work

Silt fences create a physical barrier that sediment cannot pass through. When sediment-laden water flows against the fence, the sediment particles are trapped on the upstream side. The water can then flow through, leaving the sediment behind.

A single 100-foot run of silt fence can hold around 50 tons of sediment. That's enough to fill three or four dump trucks!

You need adequate fencing for silt fences to be effective. The size of the sediment particles you're trying to capture and the amount of sediment-laden water flowing in the area will determine how much silt fence you need.

Silt fence requirements are very state-driven decisions, and there is a lot of variance in the requirements for silt fence height, stake spacing, filter fabric requirements, and whether the fabric needs to be supported or unsupported.

As a rule of thumb, for every 10,000 sq ft of the site you’re fencing, you need 100 ft of silt fence. Additionally, you need to factor in slope angle and length, soil type, rainfall, and site configuration when determining adequate silt fence protection.

Silt fences are most effective when placed in areas with a high sediment load. They're less effective in areas with low sediment loads, or where the sediment isn't moving.

Silt fences are also more effective when placed perpendicular to the direction of flow.
Installing silt fences across ditches, streams, waterways, or other concentrated flow areas is not recommended.

Types of silt fences

There are three major types of silt fences:

  • Type A silt fence
  • Type B silt fence
  • Type C silt fence, with or without wire reinforcement

Type A and Type B differ in that A has a 36-inch-high filter fabric, while B is 22 inches high. Both Type A and B fences have similar fabrics and the same flow rate. However, Type A silt fences are meant to be used on jobsites where the project's life is six months or longer.

On the other hand, Type B silt fences should be limited to minor projects where permanent stabilization is achieved in less than six months. These include residential home sites and small commercial developments.

Finally, type C silt fences are 36" and may or may not include wire reinforcements.
Many DOTs have adapted to the “high flow” silt fence, which is a monofilament geotextile without wire. It performs well without the wire and has less of an environmental impact because there's no wire hauled to a landfill when the project is done.

These high-flow silt fences can handle almost three times more flow rate than Type A fences. Type C fences should therefore be used where velocities or runoff flows are especially high or where the slopes exceed a vertical height of 10 feet.

How to install silt fences for perimeter and sediment control

Silt fences are easy to install and require minimal tools. The first step is to mark the location of the fence. This can be done with surveyor tape, flagging, or other means.

Next, excavate a trench between 4" and 12" deep and 12" wide, or wider still if you aren't digging deep. The sediment barrier should be placed in the trench and covered with soil. Many people fail to bury the fabric, and backfill and compact the trench in which it is placed, which leads to undermining of the silt fence. It's best to bury the fabric at least eight inches deep.

Next, the fence stakes must be installed on the downhill side. You can use a sledgehammer or a Bobcat’s bucket to pound the stakes into place.

Once the stakes are in place, the fence fabric can be attached. This is typically done with 2-3 metal or plastic ties per post. 
Erosion Control - How to Install Silt Fences

Alternatively, you can use the silt fences and stakes that come pre-attached as standard. Be sure the fence is buried completely and the stakes are firm. Backfill completely and pack the soil down.

Remember to avoid long runs of silt fence as these may concentrate the water in a small area where it can easily overflow the fence. When this happens, less sediment is trapped, making your silt fence ineffective.

Instead, you should use J-hooks to break up long fence runs. This technique ensures there are ends turning up the slope, which not only help break up long fence runs, but also provide multiple storage areas. These create ponding, as they act like mini-retention areas.

When installed properly and a filter fabric with high UV stability is used, silt fences can provide years of sediment control.

Expert Tip: Silt fence requirements are very state driven decisions. Therefore, there may be variances across states in the requirements regarding fence height, stake spacing, type of filter fabric, and whether the fabric should be supported or unsupported.

Advantages of using silt fences
Silt fences have several advantages over other sediment control measures.
They're relatively inexpensive and easy to install. They also prevent clogging of drain systems, which can lead to flooding.

In addition, silt fences prevent silt pollution of waterways, which can negatively impact aquatic life. They can also reduce the velocity of runoff water, which can help reduce erosion.

These benefits make silt fences one of several environmentally friendly perimeter and sediment control products available.

Disadvantages of using silt fences
While silt fences have many advantages, they also have some disadvantages. High winds or heavy rains can damage silt fences, which can cause them to fail. Sediment can also build up behind the fence, causing it to become clogged. Sediments that accumulate behind the fence will need to be removed periodically.

Tips for maintaining a silt fence
It's important to maintain silt fences so they continue to work properly. The first step is to inspect the fence regularly. This should be done at least once a week, or after each rain event.

Look for signs of sediment accumulation behind the fence. If sediment has accumulated, and especially if it has reached a third to half the height of the fence, it will need to be removed. This can be done with a shovel or other means.

Also, inspect the fence for damage, such as holes or tears in the fabric, broken ties, or leaning stakes. Any damage should be repaired as soon as possible.

Why White Cap

If you're looking for quality perimeter and sediment control products, look no further than White Cap. We have a wide selection of silt fences, perfect for any job. Our products are high-quality and easy to install, and our knowledgeable pros stand ready to answer your questions.

Browse through White Cap's selection of silt fences for all your perimeter and sediment control needs.

Check out these links for more information on Erosion Control and Geosynthetic Solutions

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