Post cap lights are the finishing detail that makes a deck or fence look built rather than assembled. A row of matching caps along a railing, all glowing the same warm color at dusk, reads as intentional design. A row of mismatched caps, or caps that do not quite fit the posts, reads as a weekend project that ran out of steam.
I have installed post cap lights on maybe sixty decks and fences, and the single biggest problem is fit. The caps are sold as “4×4” or “6×6” but the actual interior dimension rarely matches the actual post dimension, because lumber is not what it says it is. That sounds like a small thing. It is not. A cap that does not fit either falls off, cracks the post, or sits crooked and looks wrong forever. This guide covers fit first, then material, then light output, because that is the order of importance.
The Sizing Problem Nobody Warns You About
A “4×4” post is not 4 inches by 4 inches. It is 3.5 inches by 3.5 inches. A “6×6” post is 5.5 inches by 5.5 inches. This is true of all dimensional lumber in the US. The nominal size (4×4) is the size when the wood is rough cut at the mill. The actual size (3.5×3.5) is what you buy at the lumberyard after it has been planed smooth.
Solar post cap manufacturers know this, in theory. In practice, the interior dimension of a “4×4” solar cap ranges from 3.4 inches (too tight) to 3.75 inches (too loose). The too tight ones do not fit over the post without sanding the post down. The too loose ones rattle and need shims to stay straight.
Compounding this, pressure treated lumber is wet when you buy it and shrinks as it dries. A 4×4 post measured at 3.5 inches in May might measure 3.35 inches by September. A cap that fit snugly in spring is loose by fall. Cedar and redwood are more dimensionally stable. Composite posts (Trex, Timbertech) run slightly different sizes again, usually 3.5 inches nominal but with a thicker wall.
How to Get the Fit Right
Measure your posts with a tape measure before ordering caps. Do not assume. Measure at the top of the post where the cap will sit, because posts taper slightly and the top is the smallest dimension. Measure two or three posts, because there is variation within a deck.
| Post Type | Nominal Size | Actual Size | Cap Interior Needed |
|---|---|---|---|
| Pressure treated pine 4×4 | 4×4 | 3.5 x 3.5 inches | 3.5 to 3.6 inches |
| Cedar 4×4 | 4×4 | 3.5 x 3.5 inches | 3.5 to 3.6 inches |
| Composite 4×4 | 4×4 | 3.5 x 3.5 inches | 3.5 to 3.6 inches |
| Pressure treated pine 6×6 | 6×6 | 5.5 x 5.5 inches | 5.5 to 5.6 inches |
| Cedar 6×6 | 6×6 | 5.5 x 5.5 inches | 5.5 to 5.6 inches |
| Metal post (steel or aluminum) | varies | 2.5 to 4 inches | Measure and buy adapter sleeve |
Buy caps that specify the interior dimension, not just “fits 4×4.” A listing that says “interior 3.5 x 3.5 inches” is from a manufacturer who actually measured. A listing that says “fits standard 4×4” is a gamble.
For a too loose cap, shim it with thin cedar shingles or composite shim stock. Do not use cardboard or foam, they compress and the cap works loose. For a too tight cap, sand the top of the post with 80 grit sandpaper until the cap slides on. This takes five minutes per post and saves a lot of frustration.
Most post cap lights attach with two or four screws through the sides of the cap into the post. Use stainless steel screws, not the black drywall screws that ship with the caps. The black screws rust within a year and leave streaks down the post. Number 8 by 3/4 inch stainless pan head screws are the right size for almost every cap.
The fit determines everything. A cap that fits right looks built in. A cap that fits wrong looks stuck on, and no amount of light output or fancy material fixes that first impression.
Material Comparison for Post Caps
Post caps sit on top of exposed posts in full sun and rain, year round. The material is under more UV and weather stress than almost any other solar fixture, because there is no overhang and no shading. Here is how the common materials hold up.
Plastic and Resin Caps
The cheapest option, and the most common. ABS or polypropylene plastic, usually black or brown, sometimes with a wood grain texture molded in. The solar panel is on the top face, the LED shines down through a lens on the underside.
The good news is that plastic caps do not rot, do not rust, and do not need finishing. The bad news is that they yellow, chalk, and crack in UV within two to three years. A black plastic cap fades to gray in one summer. A brown plastic cap turns chalky tan. The cheap ones get brittle and crack at the corners where the screws go in.
If you buy plastic, buy UV stabilized polypropylene (not ABS) and accept replacement in three to four years. Expect to pay twelve to twenty dollars per cap. The light output is low (10 to 30 lumens) because the panel is small and the battery is small. These are decorative, not functional lighting.
Metal Caps (Aluminum and Steel)
Cast aluminum is the best value material for post caps. It does not rust, it takes powder coat well, and it survives UV indefinitely. A powder coated aluminum cap looks the same in year ten as year one, as long as the finish is intact. The finish will eventually chalk after five to seven years but the cap itself stays structurally sound.
Steel caps are cheaper than aluminum but rust. Even powder coated steel rusts at the screw holes and any chip in the finish. Avoid steel for post caps unless you live in a dry climate and do not mind repainting. In a humid or coastal climate, steel caps are a two year disposable.
Aluminum caps cost twenty to fifty dollars each, which is two to three times the plastic. They are worth it for the longevity. The light output is similar to plastic (10 to 40 lumens) because the panel and battery are the same size. The metal housing just lasts longer.
Wood Caps
Some post caps are made of cedar or redwood with a small solar light recessed into the top. These look the most natural on a wood deck, because they match the post material. The wood is usually stained or sealed to match the railing.
The problem is that the wood needs maintenance. Every year or two, the cap needs to be re stained or it grays and checks. The solar light recessed in the top is small and the panel fogs. Wood caps are the most aesthetic and the most labor intensive option. Buy them if you enjoy deck maintenance and want the caps to disappear into the railing design.
Glass and Copper
High end post caps use real glass lenses and copper or brass tops. Copper weathers to a verdigris green over a few years, which is the intended look. Brass stays gold. These are the most expensive (forty to one hundred twenty dollars per cap) and the most durable. A copper cap with a glass lens will outlast the deck.
The light output on these is usually still low (20 to 50 lumens) because the panel is decorative and small. You are paying for materials and longevity, not brightness. Buy these for a high end deck where the caps are a visible design element.
Light Patterns: What the Cap Actually Illuminates
Post cap lights fall into three beam patterns, and the pattern matters more than the lumen number for how the deck looks at night.
Downlight Pattern
The LED points straight down, shining a circle of light on the deck surface around the post. This is the most common pattern and the most useful. A row of downlight caps along a railing puts a soft pool of light at the base of each post, which lights the walking path without glare. The light does not reach far (maybe 3 feet from the post) but it is the right kind of light for a deck.
Sidelight Pattern
The LED shines out through the sides of the cap, usually through frosted panels. This makes the cap itself glow but does not light the deck surface. Sidelight caps are decorative. They mark the railing line but do not help you see where you are walking. Buy these if you have other deck lighting (recessed stair lights, post mounted flood lights) and the caps are purely for ambiance.
Uplight Pattern
A few caps shine light up at the underside of a pergola or overhead structure. These are niche. They only make sense if you have a covered deck or pergola and you want to wash the rafters with light. For an open deck, uplight caps just shoot photons into the sky.
The pattern is almost never stated in the listing. You have to look at the photos. If the photo shows a glowing cap with no light on the deck below, it is a sidelight. If the photo shows a circle of light on the deck, it is a downlight. Buy downlight for function. Buy sidelight for looks.
Output and Runtime Realities
Post cap lights are small. The solar panel is the top face of the cap, typically 2 by 2 inches to 3 by 3 inches. That panel charges a small battery (usually 600 to 1500 mAh) which powers a small LED (10 to 50 lumens). You are not lighting a deck with post caps. You are marking the railing and adding a glow.
Realistic output by pattern:
- Downlight caps: 20 to 40 lumens, lights a 3 foot circle on the deck
- Sidelight caps: 10 to 25 lumens, the cap glows but no deck light
- Uplight caps: 30 to 60 lumens, washes a 4 foot square of ceiling
Runtime is the same story as all solar. The caps run at full brightness for 2 to 4 hours, then dim through the rest of the night. By midnight, most caps are at 20 percent output. This is actually fine for post caps, because you want a soft glow late at night, not a bright light. The dimming is a feature, not a bug, if your goal is ambiance.
For a deck where you actually need to see (grilling, dining, stairs), supplement the post caps with dedicated task lighting. A post mounted solar flood light at the grill, recessed step lights on the stairs, and a brighter sconce at the door. The caps tie it together visually but they should not be your only light source.
Color temperature matters on post caps more than on ground lights because the caps are at eye level when you are sitting on the deck. Cool white (5000K) caps glare in your peripheral vision. Warm white (2700 to 3000K) caps read as candlelight and do not glare. Always buy warm white for post caps on a seating deck.
Installation: Getting a Row of Caps to Line Up
The hardest part of post cap installation is not the caps. It is making the posts ready for the caps. If your posts are different heights, the caps will be at different heights, and the row will look wavy.
Before installing caps, sight down the railing from one end. Are all the post tops at the same height? If not, you need to trim them. Use a circular saw or a hand saw to cut the tall posts down to match the shortest one. Measure from the deck surface, not from the ground, because the deck surface may slope.
Cut the posts on a slight angle (about 5 degrees) so water runs off the top. A flat topped post collects water, which seeps into the end grain and rots the post from the top down. This is the single most common cause of post failure on decks, and a post cap does not seal the top unless it is gasketed. Apply a bead of silicone caulk around the top of the post before sliding the cap on. This seals the end grain.
Install the caps in the same direction. Most caps have a solar panel that faces one way. If you install them randomly, half the panels face south and half face north, and the north facing ones underperform. Pick a direction (south is ideal) and aim all the panels that way. On a deck railing, this usually means the panels face the yard.
If your railing runs east to west, the south facing panels get full sun. If your railing runs north to south, the panels face either east or west and get half day sun. East facing panels charge in the morning, west facing in the afternoon. Neither charges fully. For north south railings, consider caps with the panel on a tilt (some caps have an adjustable panel angle) so you can aim it south regardless of cap orientation.
How Many Caps and Where
A common question is whether to put a cap on every post or skip some. The answer depends on the look you want and the budget.
For a formal, symmetrical look, cap every post. This is what most deck designs intend. A cap on every post reads as complete and intentional. Budget roughly twenty to forty dollars per post for aluminum caps, times the number of posts.
For a budget conscious look, cap every other post. This halves the cost and still defines the railing line. The uncapped posts look like they are supposed to be that way if the spacing is even. Do not cap random posts, that looks like you ran out of money.
For a lighting focused look (where the caps are your main deck light), cap every post and choose downlight pattern caps with the highest output you can find. This puts a pool of light at every post base and lights the walking path. You still need stair lighting separately.
Do not mix cap styles on the same railing. Half lantern caps and half flat caps looks like a mistake. Pick one style and use it consistently. If you need to replace a cap later, buy the same style, even if it is not your favorite anymore. Consistency matters more than individual cap design.
What to Buy, by Deck Type
For a pressure treated pine deck on a budget, plastic UV stabilized caps at fifteen to twenty dollars each. Plan to replace them in three to four years. Accept the low output and add stair lighting separately.
For a cedar or redwood deck, cast aluminum caps with a black or bronze powder coat at twenty five to forty dollars each. These match the wood tone and last ten years. Downlight pattern in warm white.
For a composite deck (Trex, Timbertech), buy caps from the same manufacturer as the decking if they make them. The match is perfect. If not, cast aluminum caps in a color that complements the composite. Avoid plastic caps on a composite deck, the material mismatch is obvious.
For a high end hardwood deck (ipe, mahogany), copper or brass caps with glass lenses at sixty to one hundred twenty dollars each. These are the only caps that do not look cheap next to a fifty dollar per square foot deck.
For a fence (as opposed to a deck), the same rules apply but you can get away with lower output since fence caps are purely decorative. Plastic caps are fine for a fence, because you are not relying on them for walking light. Space them every post for a continuous line, or every other post for a dotted line.
The one universal recommendation: buy one extra cap beyond what you need, in the same batch. Caps get discontinued, colors shift between production runs, and you will want a matching spare when one fails. A spare cap in a drawer is worth ten dollars. A mismatched replacement cap on your railing is worth nothing.
