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What this covers
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Composite deck boards look like a finished product when they arrive. Most of what decides how they look five years later happens during installation, in details that are hard to see once the deck is complete: the fasteners, the gaps between boards and the allowance for movement.
This article explains those details, because they account for many of the visible problems people associate with composite decking, from uneven gaps to boards that bow or butt joints that open in winter.
Composite Boards Move With Temperature
Wood decking moves mainly with moisture. It swells when wet and shrinks as it dries. Composite decking moves mainly with temperature. It expands as it warms and contracts as it cools, and most of that movement happens along the length of the board.
That has two practical consequences. The gaps at the ends of boards matter more than people expect, and the temperature on the day of installation affects how those gaps should be set. A board installed on a hot afternoon is already near its longest. The same board installed on a cold morning will grow.
Manufacturers publish gapping tables for this reason, usually with different end gap requirements depending on board length and installation temperature.
Composite and PVC also differ from each other. Boards made entirely of PVC, with no wood fiber, generally move more with temperature than wood-plastic composites, and their gapping requirements reflect that. Products within the same category vary too, so the gapping table for one brand should never be borrowed for another.
Moisture still plays a small role in some composite boards, particularly uncapped ones, where exposed wood fiber can take on water. Capped boards largely remove that variable, which is one reason they dominate current installations. For practical purposes, though, temperature is the movement to plan around, and the installer is the one who has to account for it on the day.
The direction of movement also explains a common sight: a composite deck that looks perfect in spring and shows slightly wider butt joints in the depths of winter. Within the manufacturer’s tolerances, that seasonal change is normal rather than a defect.
How Hidden Fastener Systems Work
Most composite decking is installed with hidden fasteners rather than screws through the face. Boards have grooved edges, and a clip sits in the groove of two adjacent boards, fastening both to the joist below.
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Fastening method |
How it attaches |
Visible result |
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Hidden edge clip |
Clip in the board grooves, screwed to the joist |
Clean surface, consistent side gap |
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Face screws with color-matched heads |
Screws through the board face |
Visible screw heads, flexible layout |
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Plugged face screws |
Screws countersunk and capped with plugs |
Nearly hidden, slower to install |
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Starter and end clips |
Specialized clips at the first and last board |
Secure perimeter with no face fasteners |
Hidden clips do more than hide screws. They set the side gap between boards automatically, which keeps the spacing consistent across the whole deck. They also allow a small amount of movement, so boards can expand and contract without stressing the fastener.
Side Gaps and Drainage
The gap between the long edges of boards is called the side gap. With clip systems, the clip itself sets it. That gap does two jobs: it lets water drain through the deck and it allows air to reach the frame.
In a wet climate like western Washington, drainage is the more important of the two. A deck with tight or debris-packed side gaps holds water on the surface after every rain, which encourages mildew and slows drying. Consistent clip-set gaps avoid that problem from the start.
End Gaps and Butt Joints
Where two board ends meet over a joist, there is a butt joint. Because composite expands along its length, that joint needs a gap sized for the installation temperature.
Too small a gap and boards can push against each other in summer heat, which can cause bowing or lifting. Too large a gap and the joint opens visibly in cold weather. Getting it right means following the manufacturer’s gapping table on the day of installation, not using a single spacer for every board.
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Installation condition |
End gap consideration |
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Cool day |
Boards will lengthen as temperatures rise, so follow the table’s cool-weather gap |
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Hot day |
Boards are near their longest, so the gap can be smaller per the table |
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Long boards |
Total movement is greater, so gaps are usually larger |
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Dark colors in full sun |
Surface runs hotter and moves more |
Many installers reduce the number of butt joints by using longer boards or by laying out the deck with a breaker board, a perpendicular board that divides the field into sections. A breaker board looks intentional and gives each run of boards a clean termination.
Joist Spacing and Board Span
Composite decking is generally less stiff than wood decking of the same thickness. Manufacturers specify a maximum joist spacing, and it is often closer than wood. Diagonal layouts, which run boards at an angle across the joists, usually require even closer spacing.
That is why the decking choice needs to be made before the frame is built. A frame designed for wood boards at wider spacing may need extra joists to accept composite. Builders who handle the full deck installation process plan joist spacing around the specific board chosen.
Fascia and Perimeter Boards
The rim joists around a deck are usually covered with fascia boards that match the decking. Fascia also expands and contracts, and it needs its own fastening pattern and gaps.
Picture framing, where a border board runs around the perimeter, gives a finished look and hides the cut ends of the field boards. It needs blocking underneath for support. These perimeter details are some of the most visible on a finished deck and some of the easiest to get wrong if the frame was not planned for them.
Stairs and Changes in Direction
Stairs and angled sections concentrate many of these details in a small space. Stair treads are short boards, so their end movement is small, but they carry heavy, repeated foot traffic at the edge. Most manufacturers specify closer stringer spacing for composite treads than for the main deck, and they often require face fastening or specific clips at the tread ends where hidden clips cannot reach.
Changes in direction, such as a deck that wraps around a corner or a herringbone pattern, add more butt joints and more cut ends. Each of those needs support from blocking below and an appropriate gap. Designs with many direction changes take noticeably longer to frame and lay, which is reflected in labor rather than material.
Steps down to a lower level or a landing need the same attention. Every edge where boards end is a place where movement, drainage and fastening all have to be resolved.
Heat, Color and Surface Temperature
Color affects more than appearance. Darker composite boards absorb more sunlight and run hotter on a sunny day than lighter boards. That affects comfort on bare feet and it increases thermal movement, since hotter boards expand more.
In the Puget Sound climate, extreme surface heat is less of a concern than in hotter regions, but a south- or west-facing deck in July can still get warm enough to notice. Lighter and mid-tone boards stay cooler. Some manufacturers produce boards formulated to reduce heat buildup.
The practical takeaway is to consider exposure when choosing color. A shaded deck can carry a dark board without much heat penalty. A deck in full afternoon sun benefits from a lighter tone, both for comfort and for smaller seasonal movement at the joints.
Common Installation Problems and Their Causes
- Uneven side gaps. Usually from face screwing without spacers, or mixing fastening methods.
- Boards bowing in summer. Often from end gaps that were too tight for the installation temperature.
- Open butt joints in winter. Often from gaps set for a cold day being too large, or boards not fastened near the ends.
- Sagging between joists. Joist spacing wider than the manufacturer allows.
- Squeaks. Fasteners not fully seated, or joists not level.
Each traces back to a decision made during installation, which is why installer experience with the specific system matters.
Warranties Depend on Installation
Composite decking warranties are often long, but they are conditional. Most require installation according to the manufacturer’s instructions, including fasteners, gaps and joist spacing. A deck installed outside those instructions may not be covered when a problem appears.
Kent-based builder SmartDecks installs composite decking from established manufacturers, and its composite page describes the material’s resistance to rot, fading and staining. Those properties hold when the installation details are right. The builder’s location and hours are on Google Maps.
The Short Version
Composite decking moves with temperature, mostly along its length. Hidden clips set consistent side gaps for drainage and allow movement. End gaps follow the manufacturer’s table for the day’s temperature. Joist spacing follows the specific board. Get those four things right and the deck looks the same in its fifth summer as it did in its first.