GRK 16105#9 x 3-1/8" RT Composite Exterior Trim Screws 348 Count

Installing cellular PVC trim and composite fascia can be a lesson in patience if your fasteners aren’t up to the task. I’ve cycled through drywall screws (don’t), generic “deck” screws (better, still messy), and even stainless trim screws that solved corrosion but made mushrooming worse. After several projects with these GRK trim screws, I’ve finally stopped chasing workarounds and started getting predictable, clean results.

What stands out in the design

• Underhead reverse threads: The small band of reverse thread just below the head is the star here. It grabs the top fibers of PVC/composite and draws them back down as the screw seats, which dramatically limits the raised “mushroom” that so often needs trimming or heat-gunning.

• Trim head profile: The head is compact, seats flush, and doesn’t dominate a paint-grade surface. On white PVC, I can leave them exposed without a distracting bump. If you prefer a seamless look, the head recess takes filler well.

• Length and gauge: At #9 x 3-1/8 inches, these are well sized for fastening 3/4 to 1-inch trim and fascia into framing, especially when you want solid bite without resorting to a longer, more visible head. I’ve used them for window and door casings, skirt boards, column wraps, and composite fascia.

• Interior/exterior coating: The gold coating has held up through a few months of exposure and wet/dry cycles in my area without any tea-staining or rust. I won’t claim they’re the answer for high-salinity coastal installations, but for typical exterior trim, the protection has been more than adequate so far.

• Consistency: Box-to-box uniformity matters with finish work. The points are sharp, the threads are clean, and the heads are consistently formed; I never encountered out-of-round recesses or stray burrs that strip bits.

Driving experience and technique

On cellular PVC (think Azek/Versatex), these start easily without a pilot hole under normal conditions. The point cuts in cleanly and the main thread engages fast. I set my drill to a medium torque clutch setting and a moderate speed. With that setup, I get:

• Minimal to no mushrooming on straight-through drives
• Virtually no cam-out
• Predictable seating without snapping the head

On colder days when PVC is brittle or near edges on narrow stock, I still predrill—habit and caution pay dividends in finish work. A 1/8-inch pilot keeps the piece from walking and avoids blowout at the back. For composite fascia (TimberTech/Trex-style), the behavior is similar, though composites are denser; I prefer a quick pilot near edges and ends to reduce chance of micro-cracking.

The reverse-thread band does what it promises. I ran a quick test on scraps: two screws driven without a pilot and two with. On the no-pilot set, one sat perfectly flush with no raised material; the other left a faint ring that knocked down with a thumb swipe. On the piloted set, both seated cleanly with no halo. That’s a big deal—normally I’d count on countersinking or heat to correct mushrooms on composite.

One note: if you overdrive at high speed, you can still create a divot or deform the surface, especially in warm PVC. Let the reverse thread do the work as the head approaches the surface; a half trigger squeeze finishes the seat tidily.

Holding power and finished look

Into SPF framing and plywood backers, the bite is excellent. Two screws at 16 inches on center hold 1x PVC casing tight without telegraphing fastener locations. I like using a shallow back-bevel or adhesive on larger sheets (like wide skirt boards) for belt-and-suspenders; the screws clamp reliably without crushing the foam structure of PVC.

The small head gives a professional finish. On white trim, the gold finish is visible but discreet; a dab of exterior paint disappears it entirely. Fillers adhere well to the recess if you’re chasing a plug-free, monolithic look for high-visibility areas.

Durability and corrosion

I’ve had a gate surround and two window trim packages through a full season with these. No rust streaking, no head degradation. The coating tolerates occasional driver slips without immediately flaking. For coastal, dockside, or chemical-exposure environments, I still prefer true stainless fasteners. Stainless won’t necessarily prevent mushrooming and can be softer under torque, but for severe environments it wins on corrosion. For typical residential exteriors away from salt spray, these GRK screws have been problem-free.

Workflow impact

What these screws really buy you is time and consistency:

• Fewer steps: I’m countersinking less and seldom stopping to clean up raised material.
• Less rework: On large fascia runs, I’m not circling back with a trim plane or heat gun.
• Cleaner sequencing: Because the heads seat predictably, I can set reveals and gaps without fiddling to hide a proud head or a chewed surface.

In practical terms, a porch wrap that would usually have me predrilling every hole and countersinking many now moves faster: quick pilots at edges and cold conditions, straight drives elsewhere. The finish is better with fewer operations.

Where they shine, and where they don’t

They shine:
- Cellular PVC window and door trim
- Composite fascia boards
- Column wraps and skirt boards
- Painted exterior trim where a low-profile head matters

Use caution or consider alternatives:
- Coastal environments: stainless is safer for long-term corrosion resistance
- Structural loads: these are trim screws, not structural fasteners
- Very thin or brittle stock near edges: pilot holes still recommended
- Highly visible, stain-grade installations: even a small head may not be the look you want

Comparisons to common alternatives

• Versus generic deck screws: These win by a mile on finished appearance and mushroom control. Deck screws tend to tear at the surface and leave a bigger head footprint.

• Versus stainless trim screws: Stainless brings corrosion resistance but can be softer and more prone to snapping if over-torqued. Mushrooming isn’t inherently addressed by stainless. These GRKs beat stainless on driving feel and surface quality, but I’ll still pick stainless for high-corrosion sites.

• Versus nails or finish nails with adhesive: Nails minimize visible heads but trade off holding power and long-term stability on PVC/composites, which expand and contract. These screws manage movement better and keep joints tight.

Tips from the field

• Set the clutch and back off as the head nears the surface to avoid overdriving.
• Pre-drill at board ends, near edges, and anytime temps drop—PVC gets brittle in the cold.
• Keep your bit fresh; a clean recess means fewer slips and protected coating on the head.
• For perfectly flush results on pristine fascia, a very light countersink pass still yields the most uniform surface, though I rarely find it necessary with these.
• Space fasteners consistently to manage expansion; let long runs float where appropriate and avoid pinning in the middle.

Value

These sit at a premium price point, but they’ve cut my install time and cleanup significantly. The 348-count box is a practical quantity for a medium exterior trim package, and I’m not tossing a third of the box due to cam-outs or malformed heads. If you care about finish quality on composite or PVC, the extra upfront cost is easy to justify.

The bottom line

These GRK trim screws solve the two biggest headaches in composite and PVC trim work: surface mushrooming and unruly, visible heads. They drive cleanly, hold tight, and leave a finish that needs little to no touch-up. While I still reach for stainless in harsh coastal conditions and I’ll pilot near edges out of habit, these have become my default choice for most exterior trim and composite fascia installations.

Recommendation: I recommend these screws for anyone installing cellular PVC or composite trim who wants a cleaner finish with fewer steps. The underhead reverse thread works as advertised, the small head leaves a professional look, and the coating has handled typical exterior exposure well. If you’re building in a high-salinity environment, consider stainless instead; otherwise, these are a reliable, time-saving fastener that delivers consistent results.