Key takeaways
- Dee Zee states its boxes are deliberately not air tight, because an air-tight seal lets condensation form inside and leaves the contents open to mold or rust.
- A box that is wet after a dry, cold night with no rain or washing has a condensation problem, and a tighter seal makes it worse.
- UWS boxes have adjustable strikers. Loosen, adjust, re-tighten, and lubricate strikers and locks periodically.
- Weather Guard says a box lid may not latch properly until the box is fully bolted down, so a loose mount can open the lid seal.
- Weather Guard's 7730 weather strip kit covers saddle, cross, lo-side and chest boxes, but its 7731 latch and striker kit is the saddle box and chest part. Cross boxes take 70732.
Two answers circulate: make the box tighter, or accept that truck boxes breathe and live with some water. Dee Zee's installation sheet settles it: "Dee Zee tool boxes are NOT air tight. An air-tight seal allows condensation to form inside of the box, causing the contents to be susceptible to mold or rust." The same paragraph says the boxes have "various bulb seals and foam gaskets on the lid to help resist weather elements." The lid keeps rain out; the box still exchanges air. Water that arrives with rain is a fault you can fix. Water that arrives on a dry night is physics, and a tighter seal makes it worse.
Step zero: condensation or a leak?
| What you see | Points to | Why |
|---|---|---|
| Droplets on the underside of the lid and on metal tools, spread evenly | Condensation | The coldest surfaces collect water first, wherever they are |
| Wet after a clear, cold night with no rain and no wash | Condensation | No liquid water reached the box |
| A puddle or wet patch in one corner or along one edge | Leak | Water that enters at one point runs to the low corner |
| Wet only after rain, a car wash or highway spray | Leak | The water arrived as liquid from outside |
| Drip marks or a clean streak running down an inside wall | Leak | That streak points at the entry |
When the picture is mixed, run a dry-night test. Dry the box with the lid open, line the floor with paper towel, close it, and leave it through a night with no rain and no sprinklers. Wet paper or a wet lid underside in the morning means condensation. Dry means the water came in from outside.
Condensation happens when steel falls below the dew point of the air touching it. At 70 F and 70 percent relative humidity the dew point is about 60 F, so a box that cools ten degrees overnight is already making water. The table and the formula are in rust prevention in tool storage. The cure is fewer wet things inside (damp rags, gloves, soaked foam) and desiccant, not extra sealant. The arithmetic in IP ratings on tool boxes shows how a sealed box with one small leak path breathes in humid air on every cool-down and struggles to let it out.
The diagnostic order
If the dry-night test passes and water still appears after rain, work down this list, cheapest and most common first.
| Order | Check | Where the water shows | Fix | Weather Guard part |
|---|---|---|---|---|
| 1 | Lid weatherstrip compression | Along one edge of the lid line, or all round | Clean, then replace the strip if it no longer springs back | 7730 weather strip kit |
| 2 | Striker alignment and latch | Front edge near the latch, lid sits proud on one side | Loosen, adjust, re-tighten, lubricate | 7106 striker kit; latch and striker 7731 (saddle, chest) or 70732 (cross) |
| 3 | Box not bolted down square | Lid gap uneven, latch hard to close | Retighten the mounting clamps or J-hooks | 7733 mounting kit (saddle, cross) |
| 4 | Lid bowed by creep or a dent | Middle of a long side, corners dry | Straighten or replace the lid | Replacement cover, by model |
| 5 | Lock cylinder, grommets, fastener holes | Below the lock, at a wiring hole, along a bolted seam, floor | Reseal or replace the part, treat drilled holes | 7748 lock and key set; 7749 grommet kit |
| 6 | Box-to-cover seam under a tonneau | Back edge of the lid, pooled water on top | Adjust or replace the bridging seal | From the cover maker |
1. Lid weatherstrip compression
A bulb seal (a hollow rubber tube) and a foam gasket both seal only where the lid squeezes them, and both lose that over time through compression set: an elastomer held squeezed does not fully recover. Heat speeds it up, which is why a summer in a truck bed ages a gasket fast.
Check it in three passes:
- Look. Lift the lid and run a finger along the whole seal. Flat sections, cracks, torn corners, seal peeled off its channel, and grit or leaves on the flange are all leaks.
- Press. A bulb seal in good condition springs back when pinched. One that stays dented has taken a set.
- Pull a strip of paper. Close the lid on a strip of paper laid across the seal and pull it out. It should drag. Repeat every 6 in (150 mm) around the lid. Where it slides out freely, the seal is not touching.
Clean first: Dee Zee and UWS both specify mild detergent and rule out abrasive cleaners. If the paper still slides at the same points, the fix is the striker (next check) or a new strip. Weather Guard's 7730 weather strip kit is listed in its parts sheets as "various styles for 1 box," so order it against your model number rather than by appearance. On a Dee Zee, UWS or other brand, order the seal from the box maker. A generic strip of the wrong height either fails to touch or holds the lid open against the latch.
2. Striker alignment
The latch pulls the lid onto the seal by hooking a striker on the box body. If the striker sits slightly high, the latch catches but the seal on that side is never compressed. The lid feels shut and water still runs in.
UWS: "Tool boxes come with adjustable strikers. If the strikers are ever out of alignment, simply loosen, adjust and re-tighten. Periodically lubricate the strikers and locks to ensure optimum performance." Weather Guard, for a knocked striker: "simply use a mallet to re-align," and for the latch: "Lubricate latch pivot points bi-annually with white grease."
To find which striker is off, sight along the lid gap from each end. A gap wider on one side, or a corner you can lift with a finger while latched, marks the striker to move. Adjust in small steps and repeat the paper test: the goal is drag at every point, not a latch that has to be slammed.
3. A lid bowed by creep or damage
A lid seals along its whole edge only if it is flat. If it has bowed upward at mid-span, the corners still clamp and the middle lifts off the seal. The paper test shows it: drag at the corners, nothing in the middle.
Polymer lids bow through creep. Polyethylene deforms slowly under a sustained load at ordinary temperatures, so a lid that carries a coil of hose or a ladder end all summer takes a permanent set, as aluminum vs steel vs polymer truck boxes explains. Metal lids bow when stood on or pried. Check with a straightedge along the closed lid edge; a sealing surface well under a millimeter out of plane can fail a water jet test.
A thicker seal is not the fix, because the latch then fights it at the corners. Straighten or replace the lid (Weather Guard lists covers by model number), then stop loading it.
4. Lock cylinder, grommets and fastener holes
Every hole through the shell is a possible leak:
- The lock cylinder and its bezel. The lock passes through the shell, usually on the front face, so water running down the front meets it. It shows as water directly under the lock. Weather Guard sells the complete lock as a 7748 lock and key set.
- Wiring grommets. Weather Guard boxes carry a rubber 12 V access grommet that you cut to pass a wire. Its manual says not to run an extension cord "through hole without grommet in place." A grommet cut too large, or a hole with no grommet, is an open hole. The grommet is sold as a 7749 kit.
- Bolted seams. Bolt-together boxes such as the Dee Zee DZ8270A are assembled with fasteners rather than welded. Its instructions have the owner cut a gasket to run "from corner to corner" over the end panel slots, then push the button head bolts through that gasket. A short gasket or loose bolt leaks along that seam.
- Mounting holes in the floor. The Dee Zee J-bolt goes through a 13/32 in hole drilled in the box floor. On a crossover the floor sits above the bed rail, where water running off the rail and spray from the bed can reach it.
Treat every drilled hole as both a leak and a corrosion start. Weather Guard's saddle and cross box manual says to "use touch-up paint on any drill holes to prevent oxidation," and its chest manual says to coat drilled holes with zinc-rich primer. UWS asks owners to "treat or seal all fasteners and exposed metal in any holes drilled in vehicle to prevent corrosion."
5. The box-to-cover seam under a tonneau
With a toolbox-edition tonneau cover, the box's rear face is the cover's front sealing surface, and water past that seam pools on the box lid. If the box leaks only along the rear lid edge, and only since the cover went on, suspect the cover's bridging seal. The seam and the order to install everything in are covered in tonneau covers and tool boxes together.
Hose the seam with the cover closed and the box lid latched, then open the cover and look for water standing on the lid.
The hose test, one zone at a time
Hosing the whole box tells you only that it leaks. To find where, wet one zone at a time.
The general method, with how to read each wet pattern, is in the IP ratings guide. The difference here is the order: a truck box has holes low in the shell, so test those before the lid.
Weather Guard parts: which kit fits which box
It is easy to assume one latch kit fits every Weather Guard box. Weather Guard's own parts lists say otherwise, and the difference matters if you own a cross box.
| Part | Description in Weather Guard's sheets | Listed for |
|---|---|---|
| 7730 | Weather strip kit (various styles for one box) | All saddle and cross boxes, all lo-side boxes, all chests |
| 7731 | Latch and striker kit (1 latch, 1 striker, mounting hardware) | All saddle boxes; all -01 series chests |
| 70732 | Latch and striker kit (1 latch, 1 striker, mounting hardware) | All cross boxes |
| 7732 | Latch and striker kit | All -01 series lo-side boxes |
| 7106 | Striker kit (1 striker and mounting hardware) | All saddle and cross boxes; -01 series chests and lo-side boxes (as 7106-1PK) |
| 7748 | Lock and key set (1 lock, 2 keys, retainer clip) | All saddle and cross boxes |
| 7749 | Grommet kit | All saddle and cross boxes |
| 7733 | Mounting kit (4 clamps, 4 bolts, 4 washers) | All saddle and cross boxes |
A Weather Guard cross box owner who orders 7731 gets the saddle box latch. If only the striker is bent, the 7106 kit replaces it alone. Model numbers end in a series code (127-0-01 against 127-0-02) and some parts are listed for one series only, so read the model plate first. A worn lock is also a theft question, covered in jobsite box security.
Should you drill a drain hole?
Forum advice often ends with drilling drain holes. A drain lets water out, but it is also a new opening in the floor. Use it after the leak is fixed, not instead of fixing it. Deburr it and treat the bare edge with touch-up paint or zinc-rich primer on steel, as the makers specify for mounting holes. A drain does nothing for condensation.
Where this answer stops applying
- Your box's manual wins. The adjustment steps come from UWS and Weather Guard; other latches may not adjust at all.
- No box here is rated waterproof. None of the three makers' documents gives an ingress rating, so passing the hose test means weather-tight as tested, not sealed against a pressure wash.
- Under-bed and side boxes take spray from below, so floor holes move up the list.
- Contents that must stay dry belong in a sealed case with desiccant inside the box.