How to Install Trex Composite Fencing

 Tools

• Drill / Driver (with bits and bit extension)
• Circular Saw (with high-tooth-count blade)
• Miter Saw (with high-tooth-count blade)
• Reciprocating Saw (with metal cutting blade for post stiffener)
• Trim Gun
• Tape Measure
• Pencil/Marker
• Hammer
• Rubber Mallet
• Level
• Speed Square
• String Line
• Stakes
• Chalk Line
• Marking Paint
• Post Hole Digger
• Shovel
• Rake
• Water Bucket
• Cleaning Brush or Rags

Materials

• Panel Kits (includes rails, pickets and hardware)
• Posts
• Post Caps
• Gates (includes hardware)
• Post Stiffener (for gate hinge post)
• Concrete (approximately four 50-pound bags per post)

Step 1: Layout Your Fence on Paper First

Before running string lines or digging post holes, sketch the fence layout on graph paper. A simple drawing helps determine your material list, visualize the project and avoid costly mistakes. You can also use the design tool included with the Trex Fence Cost Estimator. Start by locating your property lines and required setbacks, then measure the perimeter and transfer those dimensions to the drawing.

Step 2: Determine the Distance Between Each Post

For Trex Seclusions®, the ideal post spacing is 8 feet (96 inches) on center from one post to another. However, fence runs rarely divide evenly into 8-foot sections. When dealing with a fence run that's not divisible by 8, there are two common layout options.

The first option is to keep the center fence sections full size and create two smaller sections at the ends of the run. This approach leaves most fence components at their ideal lengths. It also helps conceal the size difference since shorter sections are less noticeable at the ends or corners of a fence run.

The second option is to divide the entire run into equal sections, so each panel is slightly smaller. For example, if a fence run measures 47 feet (564 inches), dividing it into six equal sections results in post spacing of 94 inches on center.

Don’t forget to account for gate openings during the layout process. Trex gates are fixed in width, so post spacing for gates, as provided in the installation guide, should be included on the drawing before post locations are finalized.

Step 3: Find the Lines

Once the layout has been finalized on paper, transfer it to the yard using stakes and string lines. Drive stakes into the ground and stretch mason’s string tightly between them to establish the fence path. The string will act as a visual guide for post placement and help keep the fence straight.

Install the stakes a few feet beyond the corners so they do not interfere with digging the post holes. In most cases, the string line will need to be temporarily removed while digging, then reinstalled in the exact same position when setting the posts. Leaving the stakes in place makes it easier to restore the string line accurately.

Step 4: Mark the Post Holes

Use a tape measure to mark the location of each post hole along the string line. As each location is measured, mark the center of the post hole with ground-marking paint. Position each mark 2-1/2 inches away from the string (half the width of a 4x4 post). Before digging, pull the string aside or remove it. After the holes are dug, restring it between the same stakes. The offset allows the string to run along the edge of each post while keeping the post centered in the hole.

Step 1: Dig the Holes

Renting a post hole digger with a 12-inch auger is easier than hand digging and will save a tremendous amount of time. Rental centers typically offer both one-person and two-person versions. If you choose a one-person model, look for one that includes a long tilting frame with wheels.

Be sure to meet the minimum required depth of 30 inches. There is no harm in digging a couple of inches deeper because the posts are long enough to accommodate the extra depth. However, if you accidentally dig too deep and need to add soil back into the bottom of the hole, thoroughly compact the soil using a tamper or 2x board. Loose soil will settle, causing the post to sink over time.

Step 2: Align the Posts

It’s easier to cut the posts to their final height before installing the fence panels, so set each post into its hole and verify that it’s tall enough to allow trimming later. Each post must be aligned in three different ways. First, the post must be plumb, meaning perfectly vertical. Second, it must align with the string line running along the fence layout. Third, it must be the proper distance from the adjacent posts.

Use a quality level to check plumb on adjacent sides as you align the post. A 4-foot level is more accurate to use than a shorter 2-foot model. Measure carefully from one post to the next to maintain consistent spacing and use a string line to keep the fence straight. Be sure the string line does not touch the posts, or it will throw the subsequent posts out of alignment.

Step 3: Place the Concrete

Using fast-setting concrete that doesn't require premixing before being placed in the hole will save a lot of time and labor. While keeping the post properly aligned and plumb, pour the dry concrete directly into the hole. A hole this size will require four 50-pound bags of concrete per post. Fill the hole to within 3 to 4 inches from the surface of the ground, leaving room for soil on top so grass can grow around the post.

Step 4: Add Water

Once each bag of concrete is placed, add the amount of water recommended by the manufacturer (per bag). If the soil is exceptionally dry, spray the sides and bottom of the hole with water before adding the concrete to help ensure proper curing. Use a stick or piece of rebar to work the water into the concrete and eliminate any air pockets. Throughout this process, continuously check that the post remains aligned and plumb. Keep a bucket of water and a brush or rag nearby and clean any concrete off the posts as you install them. Allow the concrete to cure overnight, then verify that each post is solid before beginning panel installation.

Step 5: Identify the Contour of the Grade

It might seem logical to measure up from the ground and trim every post to the desired height. However, even yards that appear flat have subtle rises and dips. Measuring from the ground at each post can create a choppy fence line, while cutting every post top to a perfectly level line can leave large gaps beneath the fence or bury sections of the bottom rail. If the grade changes are minor, you may be able to eliminate small gaps by adding soil, mulch, or other landscaping materials beneath the fence rather than changing the fence angle.

For more significant changes in grade, identify the natural transition points where the slope changes. Have a helper hold one end of a string a few inches above the ground while you hold the other. Move the string along the fence line, watching how the gap between the string and the ground changes. The points where the gap changes noticeably are the natural transition points. For example, the ground might slope downward for 32 feet, remain relatively level for the next 32 feet, then rise for the final 32 feet.

Use these transition points to divide the fence into long, straight sections that follow the overall contour of the landscape. In this example, each straight section would be 32 feet long, with a transition at every fourth post.

Step 6: Snap a Line Between Transition Points

The posts at each grade transition point should be trimmed so the top of the post is 74-1/2 inches above the ground. This height accommodates full-length pickets in a 6-foot panel kit while allowing the post to extend 2-1/4 inches above the top rail, leaving room for the post cap. After marking the transition posts, snap a chalk line to establish the height of the posts in between. See Chapter 5: Step 1b for how to find the height of posts on a sloped fence.  

Step 7: Trim the Top of the Posts

Use the chalk line as a reference and mark each side of the post using a speed square as a guide. Trim the posts using a circular saw. If you use a smaller saw, you may have to make a cut on each side of the post. 

Step 1b: Find Post Height and Secure Rail Brackets on Sloped Panels

Measure up 74-1/2 inches from the ground on the inside/uphill face of each end post in the sloped section and make a mark. From that mark, measure down 5-3/4 inches and draw a line across the post. This line represents the bottom of the upper rail bracket.

Stretch a string tightly between the layout lines on the two end posts. Using a speed square, transfer the string height to each side of the intermediate posts. These marks represent the bottom of the upper rail brackets. Notice that the brackets uphill will be higher than the corresponding bracket on the downhill side. From each upper bracket mark, measure down 67-7/8 inches to locate the bottom of the lower rail brackets. Finally, trim the post to length (level, not at an angle). 

For sloped sections, cut a second story pole 67-7/8 inches long. Once you’ve established the upper bracket mark with the string line, use this story pole to transfer that location down to the lower bracket.

Step 2: Measure and Mark the Rail Components

If the inside faces of the fence posts are spaced 91 inches apart, the aluminum bottom rail will not need to be shortened. On level fence sections, cut the aluminum rail about 1/2 inch shorter than the distance between the posts to reduce the chances of scratching the posts during installation. It will be easier to see your lines if you use a fine-tipped marker instead of a pencil. Temporarily install the aluminum rail, then hold the rail covers in place and mark them to size. Hold the top rail between the posts at the location of the top brackets and mark it to size.

If the fence section is sloped, the rail components will need to be cut at an angle. To do this, hold each rail component in its installed position and mark the top and bottom edges on each side to establish the correct cutting angle. Check out these additional resources to read about advanced rail cutting techniques. 

Step 3: Cut the Rail Components

Use a carbide-tipped saw blade with a high-tooth count to make the cuts. A 7-1/4-inch blade with 60 teeth, a 10-inch blade with 80 teeth, or a 12-inch blade with 100 teeth will all produce a clean cut on both the composite and aluminum portions of the rails. Until you get used to the process, it’s best to cut each component a bit long and trim it back to size instead of cutting anything too short. Always wear eye and ear protection when operating power tools.