Floodlight faceplate flush against the junction box

Short answer: put a 4 in [100 mm] round weatherproof box behind the fixture and pull the gasket down to an even compression all the way round. A gap of 1/8 in [3 mm] at any point of the gasket line leaves that stretch of rubber at its free thickness, with zero compression, so the first snow against the wall finds an open seal. Notching the faceplate to clear the proud screw fixes neither real fault. The box behind the plate is an interior box, and the gasket behind the gap is doing nothing. Measure the gap, check the box, and order the round one before the snow sits against the wall.

What the gap tells you

Read the symptoms before you reach for a file or a hacksaw. Each one points at a part.

• Daylight at one corner, closed everywhere else. Something stands proud of the box face on that side, almost always a cable-connector clamp screw or a mounting ear.
• The plate stops short however hard you draw the screws. The proud part carries the load, and the gasket behind it stays uncompressed.
• The fixture's mounting hole lines up with nothing. Floodlights key to round or octagonal boxes with a central hole; a rectangular interior box carries a wall-strap pattern.
• Plated steel box with snap-in knockouts and no flat gasket face. That is an interior box, and nothing on it is made for rain or snow.
• The box rocks when you push it. It hangs on the cable instead of the wall; check what failed on a loose outdoor junction box before anyone works under it.
• Rust or white streaks below the box. Water is already passing, and the visible gap is only the widest path.

The four causes, most likely first

1. An interior box mounted outdoors. Interior boxes are stamped from plated steel, take snap-in knockouts and mount on a wall strap. A floodlight wants a cast, corrosion-resistant box with a flat gasket face and one central hole. Snow against plated steel works into every scratch, and the box rusts outward from the knockouts.
2. A clamp screw standing proud of the box face. Many cable connectors clamp with a screw that sits outside the box. Where its head points at the fixture, it holds the plate off by its own height and unloads the gasket behind it.
3. A faceplate keyed to the wrong pattern. A square, oversized plate over a rectangular box means the fixture was drawn for a round or octagon weatherproof box. The mismatch shows up as crossed holes and a warped seal line.
4. A wall face that is not flat. Brick bumps and proud mortar joints tip the plate, so one side of the joint reopens even with the right box in place.

Measure the gap before you touch anything

Hold a straight edge across the worst point and measure the air gap between plate and box face. Then read the gasket: the flat neoprene gasket in most floodlight kits is about 1/8 in [3 mm] thick. Measure yours, because the arithmetic follows it.

Worked example, for a 1/8 in solid neoprene gasket. Gasket handbooks put solid rubber at 20-30% compression, so take 25% as the target. The squeeze is 0.125 in × 0.25 ≈ 0.031 in [0.8 mm]. The plate therefore has to close to 0.125 − 0.031 = 0.094 in, about 3/32 in [2.4 mm], at every point of the line. Where the proud screw holds the plate a full gasket thickness off, the rubber keeps its free thickness and compresses nothing. A folded sheet of copy paper, about 0.004 in [0.1 mm] per sheet, slides into that joint, and meltwater wicks along the open line. A 3/16 in [5 mm] gasket moves the closure target out to about 9/64 in [3.6 mm]; a sponge gasket needs more squeeze than a solid one at the same thickness.

Two checks tell you which box you have. A magnet that sticks means plated steel, so an interior box; the die-cast aluminum of a weatherproof round box is non-magnetic. Knockouts you can push out with a thumb point the same way, because a wet-location box is cast with threaded hubs. If the box passes both checks and the screw is the only proud point, turn it inward: loosen the connector's locknut about half a turn inside the box, rotate the connector so the screw faces sideways or inward, and retighten.

The fix: a round weatherproof box and an even gasket

1. Kill the circuit. Switch it off and confirm the fixture is dead with a voltage tester before you open anything.
2. Sort the cable first. Interior NM sheathed cable is a dry-location cable, and the run outside the wall is wet. Bring the circuit out as THWN-2 singles in conduit or as UF cable; a 15 A lighting circuit runs on 14 AWG copper. Give the entry strain relief and a drip loop, as laid out in glands, drip loops and drains for an outdoor junction box.
3. Mount the right box. Pick a 4 in [100 mm] round weatherproof box in die-cast aluminum, marked for wet locations. A pancake about 1/2 in [13 mm] deep disappears behind the fixture bell. When you also tuck a driver or a second cable, take a deeper round box or a 3-1/2 in [89 mm] octagon; catalogs run those from 1-1/2 in to 2-1/8 in deep. Check the fixture's own marking too: it must read for wet locations, or the plate and gasket are not the whole story.
4. Fix the box to the structure. Two screws into framing or into masonry anchors, not into the cladding alone. Seal the joint between box and wall the way sealing a junction box to a brick wall against running water shows, and keep sealant off the gasket line.
5. Close the box and leave the weep holes open. NEC 314.15 permits drainage openings of 3 mm [1/8 in] to 6 mm [1/4 in] in boxes listed for damp or wet locations. Those molded holes pass the condensation a sealed box would hold; plugging them turns a drain into a sump.
6. Seat the gasket dry and pull it down evenly. Work the screws in a star pattern until a feeler gauge, or a folded paper sheet, will not pass anywhere on the line. You are closing to the target the arithmetic above gave you, about 0.8 mm of squeeze on a 3 mm gasket.
7. Recheck the screw. If a clamp head still stands proud, rotate the connector as above or fit one whose screw clamps inside the box.

Keep the seal through the freeze-thaw

• Run the paper test again in spring. Rubber takes a set over a winter of compression; if the closure grew past the 2.4 mm target, fit a new gasket. The flat rubber gasket spec guide tells you what thickness and hardness to order.
• Brush snow off the fixture instead of letting it sit against the plate and melt through the weakest point of the line, day after day.
• Leave the weep holes clear. A spider or a wasp nest plugs them in a season; check them whenever you clean the fixture.
• Push the box once a year. Any movement means the fasteners let go, and the cable ends up carrying the weight.

When the box is the wrong spec entirely

Rescue is the wrong move in three cases, and each has its own fix:

• A camera shares the box. A driver, a second cable and camera strain relief change the fill and the entry; the floodlight camera mount strain-relief guide covers that properly.
• There is no box in the wall at all, and the cable comes straight out of the cladding. Mounting an outdoor fixture with no junction box in the wall is its own job with its own parts.
• The old box comes out and leaves holes or a slot in the wall. Cover wall cable entry holes with a weatherproof box rather than a dab of mortar.

The decision in one line. If the box is die-cast, round and marked for wet locations, turn the screw inward and re-close the gasket to the closure target. If it is plated steel with knockouts, swap it for a 4 in [100 mm] round weatherproof box before the first snow sits against it.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions