Pre-Drilling and Countersinking Ipe and Cumaru Decking: Why It Happens on Site and How to Do It Right

Why Decking Gets Drilled on Site, Not at the Mill

Pre-drilling and countersinking is mandatory in Ipe and Cumaru, and it is installation work, not milling work: no millwork house drills fastener holes in decking before it ships. The reason is not capability. It is that a fastener hole has to land on a joist, and where each board meets the joists is only settled while the deck is being laid.

A groove for a hidden clip runs the full length of a board edge, so a mill can cut it without knowing anything about the frame. A pilot hole is different. The installer lays the deck out against the framing as built, cuts boards to length, shifts butt joints onto bearing, and trims ends as field conditions demand. Any one of those routine moves relocates every hole downstream of it. A pallet of factory-drilled boards would take the crew's ability to make a single onsite cut away from them, which is why an experienced installer refuses the idea on sight.

So the sourcing question has a short answer. Buy the boards bare and milled consistent, buy plugs cut from the same lot, and put the drilling in the installation scope where it belongs. The rest of this guide is the technique: where the holes go, what diameter and depth they get, and which fasteners survive wood this dense.

Why Ipe and Cumaru Split Without a Pilot Hole

Ipe at roughly 3,680 lbf on the Janka scale and Cumaru at roughly 3,540 lbf are dense enough that a screw driven without a pilot hole acts as a wedge, not a fastener. The wood does not compress out of the way, so it splits instead.

Density is the headline number. Ipe runs about 1,050 kg/m3 and Cumaru about 1,070 kg/m3, both well above the specific gravity threshold where the USDA Forest Products Laboratory stops treating lead holes as optional and starts treating them as part of the connection design. The Wood Handbook guidance is specific about it: in wood above about 0.6 specific gravity, the lead hole for the threaded portion of a screw should run near 90 percent of the screw's root diameter, with a separate clearance hole through the piece being fastened. Ipe sits far above that line.

Grain structure adds a second failure mode. Cumaru's interlocked grain resists the split that Ipe shows, but it grabs a screw harder and drives torque up, and its waxy extractives load the flutes of a bit fast. Ipe carries silica and dense extractives that dull edges quickly. Crews who skip pilot holes on either species usually discover the problem in the same order: hairline checks radiating from the screw first, then heads snapping off, then a board that has to come back up.

Countersink vs Counterbore: Get the Terms Right Before Anyone Drills

Countersunk and counterbored are two different holes, and the deck's finished look depends on which one the installation notes call for. Crews use the words interchangeably, which causes nobody any trouble until one of them lands in a spec.

A countersink is a shallow conical relief that lets a flat or bugle head pull flush with the deck surface. Nothing covers it. The head stays visible, which is a legitimate finished look on commercial walkways and dock work where inspection access matters more than appearance.

A counterbore is a flat-bottomed cylindrical recess, drilled to a set diameter and depth so a wood plug drops in over the screw head and hides it. That is what most architects mean when they write "drilled and plugged" into a decking section. The drilling then has three parts rather than one: the clearance hole through the board, the counterbore above it, and the plugs themselves, which should be cut from the same species and ideally the same lot so color and grain track the deck.

Write all three into the installation notes: clearance hole diameter, counterbore diameter and depth, and plug species and lot. On site the crew cuts them in one stroke with a self-centering combination bit, so the note is really about naming the bit geometry once and holding every board to it.

Where the Holes Go: Placement That Keeps Dense Boards From Splitting

Hole placement decides whether a face-screwed Ipe deck splits, and the working rules are simple: two fasteners per board per joist, holes about 3/4 inch in from the board edge, and no screw near a board end without a pilot hole.

Edge distance is the first rule. Hold each hole roughly 3/4 inch off the board edge, and set the pair far enough apart across the width that the board cannot rock on a single line of fasteners. Crowd the edge and the screw shank wedges the outer fibers apart; the split may not show until the first dry season pulls the board across its width.

Ends are where dense boards actually fail. End grain within an inch or two of a cut is the weakest wood on the deck, so every end hole gets a full pilot, slightly deeper than the screw, and sits back from the end as far as the bearing allows. Butt joints belong on doubled or sistered joists so each board end lands on its own bearing with its own fasteners; two ends sharing one joist edge guarantees a split or a popped plug within a few seasons.

Consistency is what makes the layout fast. A self-centering jig referenced off the board edge repeats the same hole pair down the entire run, and it can only do that if the boards are milled to a consistent width and edge profile. That consistency is the thing worth specifying from the mill. The holes are not.

Pilot Diameter, Counterbore Depth, and Plug Geometry

Get the two-diameter rule right and the connection works; drill one straight hole all the way through and the board never pulls tight. This is the detail that separates a deck that stays put from one that squeaks and backs out.

The decking board needs a clearance hole matched to the screw's shank, so the threads pass through without biting. The joist below needs a lead hole near 90 percent of the root diameter, so the threads engage without splitting. Drill a single undersized hole through both and the screw threads grab the decking as well as the joist, jack the board up off the frame, and leave a gap that no amount of torque closes. Drill a single oversized hole and nothing holds.

For a #10 screw in 5/4 stock, that means a clearance hole a hair over the shank, a lead hole around 7/64 inch in diameter drilled into the joist, and a counterbore sized to the plug. Standard plug diameters run 3/8 inch, with the counterbore drilled about 1/4 inch deep. Deep enough that the plug has real glue surface. Shallow enough that the screw head still sits on solid wood and the remaining board thickness under the head carries the load.

Plugs go in with an exterior adhesive, grain aligned with the board, tapped home and trimmed flush once the glue sets. Sand across the plug, not around it. Tapered plugs seat tighter than straight ones and tolerate a slightly imperfect counterbore.

Species Data That Drives the Drilling Spec

Hardness and density determine how aggressively a species has to be pre-drilled, and the spread across tropical decking is wide enough that a single drilling spec does not cover all of it. The numbers below are the ones that belong in the submittal.

Species data cross-checks against published references for Ipe, Cumaru, Massaranduba, Jatoba and Garapa. Ipe's ASTM E84 Class A rating, meaning a flame spread index of 25 or less under the ASTM E84 test method, carries weight on commercial and multifamily work where the assembly has to clear a code review.

Fastener Selection and Why Stainless Shears in Ipe

Stainless steel is softer than the hardened carbon steel most deck screws are made from, which is exactly why a pre-drilled hole is not optional when the spec calls for 316. The fastener that resists corrosion is the fastener most likely to snap.

Coastal and marine work calls for 316 stainless. The molybdenum content buys real resistance to chloride pitting, and salt spray finds any lesser alloy eventually. Type 305 stainless handles inland exposure. Neither one has the torsional strength of a hardened screw, so both depend on a correctly sized lead hole to keep driving torque inside what the shank can carry.

Two more things matter at the driver. Use a clutch, not an impact. An impact driver delivers torque spikes that a 316 screw head does not survive in wood this dense, and the head twists off flush with the surface where it cannot be extracted. Wax the threads. A block of paraffin cuts driving torque noticeably and costs nothing.

Head style follows the finish. Trim-head screws countersink cleanly and read as small dark points on the surface. Bugle heads pull flush under a plug. Match the alloy of the screw to the alloy of any framing hardware in contact with it, since dissimilar metals in a wet joint corrode. The American Wood Council maintains the design standards that govern the connection side of this, and ICC publishes the code provisions a deck assembly has to satisfy.

Layout, Joist Spacing, Moisture, and Board Ends

Dense hardwood decking needs joists at 16 inches on center, and every fastener decision downstream is built on that spacing. Stretch it and the boards deflect, the plugs work loose, and the holes stop lining up.

Run the quantities before ordering. A 5/4x6 board covers about 2.1 linear feet per square foot of deck, which sets both the board count and the hole count, since face-screwed work takes two fasteners per board at every joist crossing. On a 400 square foot deck that arithmetic runs past a thousand holes, which is precisely why the drilling rhythm matters: one combination bit, one jig setting, repeated down every run.

Moisture content is the variable that quietly wrecks drilling done too early. Tropical decking should reach the site at 12 to 16 percent and should acclimate on site before installation. Boards drilled well ahead and fastened weeks later at a different moisture content have moved, and across a 16 foot board even modest width change adds up to holes that no longer sit centered on a joist. Drill as you lay, not ahead of the work.

Board ends need their own attention. Every species in the table above splits more readily within an inch or two of an end, so pilot holes near butt joints should be drilled, never driven blind, and butt joints should land on doubled or sistered joists so each board end gets its own bearing and its own fasteners. Edge distance of about 3/4 inch from the board edge keeps the screw out of the zone where the wood has the least to hold.

Face Screws vs Hidden Fasteners

Face screwing with plugs and hidden clip systems solve different problems, and the choice drives the milling order, not the other way around. Decide this before the boards get milled.

Face-screwed decks hold boards flat against cup and offer a mechanical grip that no clip matches. They are serviceable, since a plug drills out and a board comes up. They also put a visible, regular pattern on the surface, plugged or not.

Clip systems need a clip-specific groove, which is production milling a shop cuts the length of the board. Different clip manufacturers use different groove geometry, so the clip gets selected before the mill order, not after the pallet lands. Tongue and groove material is the only profile that works with a concealed fastener through the tongue. Shiplap takes visible fasteners, full stop. And on any grooved or T&G material used vertically or on a soffit, grooves face down so water drains rather than sits.

Whichever path gets chosen, the deck lives or dies on the system around it. Joists at 16 inches on center, airflow under the boards, proper drainage, correct gapping, sealed field cuts and end grain. A species selection alone has never made a deck last. Building Science Corporation has published extensively on the drainage and drying behavior that governs how any wood assembly performs outdoors.

"Customers still call asking to buy it pre-drilled, and I talk every one of them out of it. Layout happens on the deck, not in my shop. The crew cuts boards, walks joints onto bearing, trims ends, and a factory hole pattern dies the first time any of that happens. What we actually sell them is boards milled dead consistent so the jig indexes true on every stick, and plugs off the same lot. Drill on site against the frame you built and the deck goes together clean."

Norm Moton, Director of Sales, J. Gibson McIlvain

How J. Gibson McIlvain Would Specify This

Write the spec so it tells the mill exactly what to make and tells the installer exactly what to drill, and keep every fastener hole in the installation scope.

Start with the material. Ipe or Cumaru decking, kiln dried to 12 to 16 percent, S4S with eased edges, in the dimension the project needs. J. Gibson McIlvain carries a vast Ipe inventory across all standard and custom dimensions and mills to order, so the spec should name the section the design wants rather than working backward from what happens to be on a rack. Call out FSC certification where the project requires it, and note that when a CITES-listed species is involved, documentation and chain of custody have to travel with the material. FSC and CITES both publish the current requirements.

Then put the drilling where it belongs, in the installation notes. Specify a self-centering combination bit with the clearance, pilot and counterbore geometry named, plugs from the same lot as the decking, and the placement rules spelled out: two fasteners per board per joist, edge distance about 3/4 inch, a full pilot at every hole near a board end, and butt joints on doubled bearing.

Fasteners: 316 stainless for coastal exposure, 305 inland, two per board per joist, clutch driver only, edge distance about 3/4 inch, threads waxed. Framing: joists at 16 inches on center, doubled or sistered at butt joints, ventilated underneath. Finishing: seal all field cuts and end grain, gap per the material's moisture content at installation, and expect the surface to weather. No solid wood carries a warranty, since it is an organic material that responds to its environment, and any deck spec that promises otherwise is writing a check the wood cannot cash.

For a quote on Ipe or Cumaru decking with plugs cut from the same lot, call J. Gibson McIlvain at 800-638-9100 or see the Ipe decking line. Material ships nationwide, California included, on contractor and commercial volume.

Where Face-Screwed Decks Actually Fail

Almost every failure on a face-screwed tropical deck traces back to one of four things, and none of them is the species.

Holes drilled as a single diameter through board and joist together. The board never pulls tight, the gap collects water, and the screw backs out within a season. Impact drivers on 316 stainless, which leaves snapped heads buried in the deck where nothing extracts them. Plugs glued with an interior adhesive or cut from a different lot, which pop out or read as a color mismatch once UV does its work. And joists at 24 inches on center under material that needed 16, which lets the boards deflect until the fasteners loosen and the plugs break their glue line.

Every one of those is an installation decision. The WoodWorks technical library and the grading standards published by the National Hardwood Lumber Association are useful reading for anyone writing this section of a spec, and on public projects the US Access Board guidance governs surface gaps and openings that a decking layout has to respect.