Milling Thresholds When Every Opening Sits at a Different Height

Why No Two Openings Land at the Same Height

A hardwood threshold in a commercial building is not a catalog part, it is a dimension lifted off the two floors it joins, and in a building where no two floors are level that dimension changes at every opening. The piece has to carry a rabbet on one side, a different rabbet or a bevel on the other, and an overall thickness that lands both finish surfaces where the drawings say they land. Change either floor and the cross section changes with it.

Look at what stacks up on each side of a doorway. Carpet tile sits close to 1/4 inch. Glue-down luxury vinyl plank runs 2 mm to 5 mm. Sheet vinyl is thinner still. Porcelain over a thinset bed climbs past 1/2 inch, and porcelain over a mortar bed can pass 1 1/4 inch. Solid strip flooring is 3/4 inch plus whatever underlayment went under it. A raised access floor panel in an IT room can sit inches above the slab it covers. A single corridor can touch four of those assemblies in 100 ft.

The slab underneath is not flat either, and it was never required to be. Floor flatness and levelness get measured as F-numbers under ASTM E1155, and a tolerance that passes for a warehouse slab still leaves a doorway low at one jamb and high at the other. Renovation work is worse. Old topping slabs, patched trenches, and two generations of leveling compound mean the difference across one 4 ft opening can vary by a noticeable fraction of an inch from jamb to jamb.

What Code Sets and What It Leaves to the Shop

Code fixes the maximum height and the maximum slope at an accessible opening, then says nothing at all about how the wood gets there. The 2010 ADA Standards allow a vertical change in level up to 1/4 inch. From 1/4 inch to 1/2 inch the change has to be beveled at a slope no steeper than 1 to 2. Anything over 1/2 inch stops being a threshold and becomes a ramp, with its own slope limit and its own handrail and landing rules. The U.S. Access Board publishes the language, and section 404.2.5 applies it to thresholds at doorways.

Egress provisions in IBC Chapter 10 land in the same place for doors on an accessible route. The practical result is a hard ceiling of 1/2 inch of exposed height at the walking surface, with that height beveled rather than left square.

Everything else is millwork. The depth of each rabbet, where the high point of the bevel sits relative to the door stop, how much bearing the piece keeps under the door, whether the low side gets a feathered lip or a square shoulder, the width front to back so the piece spans the wall opening and still tucks under both jamb legs. None of that appears on a code page. It comes from a cross section somebody has to draw and a knife somebody has to grind.

The Threshold Schedule Comes Before the Knife

The only reliable way to buy thresholds for an out-of-level building is to measure the openings and turn them into a schedule, one line per opening, before any stock gets milled. Guessing off the finish schedule produces pieces that fit the drawing and not the building.

A workable line carries the opening number, the clear width, the finish on each side, the measured finish floor elevation on each side taken at both jambs and at the center, the door swing and the stop location, and a note on which way the difference runs. Six measurements per opening sounds fussy until the first piece gets set and reads tight at one end and open at the other. Concrete prep standards such as ASTM F710 govern what the flooring contractor does to the slab, so the elevations worth recording are the ones taken after prep and after any topping has cured.

Who takes those numbers matters. J. Gibson McIlvain supplies and mills, and it does not install or field measure. The general contractor, the flooring contractor or the millwork installer owns the survey, and the shop mills to the numbers on the schedule they submit. That division is cleaner than it sounds, since the party holding the survey is also the party who can walk back and recheck a suspect opening the same afternoon.

"We get a schedule with 40 openings on it and 40 different height differences, and that is normal for a commercial job. Send us the elevations your installer actually measured and we will mill each piece to the opening it belongs to, stamp the opening number on the underside, and ship it in the sequence your crew wants to see it."

Norm Moton, Director of Sales, J. Gibson McIlvain

Grouping Openings So One Knife Covers Many Heights

Forty different height differences do not require forty knives, they require one cross section and a second machining pass that varies by opening. A knife grind is a fixed setup cost in time. Cutting a rabbet to a depth called out on a schedule is not. Group the openings by profile family, run the sticking in one pass, then relieve the underside per line item.

J. Gibson McIlvain holds a moulding profile library of thousands of ground knives and grinds new ones every week, so a threshold cross section that matches an existing building standard usually starts from a knife already on the rack. A profile that is not in the library gets ground to match, whether the match is a historical restoration section or a contemporary detail drawn last month. ICD, the company's architectural millwork division, works the same way on true custom sections planned around a specific space.

The blank matters as much as the knife. Ordering thresholds at a single finished thickness forces somebody to shim the thin openings. Ordering them from a blank milled thick enough to absorb the deepest relief on the schedule lets one stock thickness serve a whole floor, with the variation taken out of the underside where nobody sees it. Grade and workmanship expectations are worth writing down too, and the Architectural Woodwork Institute standards give a spec writer language for that.

Linear trimwork, finished cladding and primed trim run through the millwork shop at the lumber yard. Work that has to be planned around a specific condition rather than run by the foot goes to ICD, the full architectural millwork house off site. Thresholds land across both, which is why they tend to come off one order rather than two.

Resolving the Height Difference at Each Opening

Every opening on the schedule resolves into one of a handful of geometries, and knowing which one applies tells you what the shop mills and what the installing trade still owns. The measured difference drives the choice, not the species and not the door type.

Two of those rows are worth reading twice. A resilient floor that compresses under rolling loads can turn a legal 1/4 inch step into something taller over a few years, so a stepped detail belongs where the adjoining finish is hard rather than soft. The tapered row is the one that separates a milled threshold from a purchased one, since no stock extrusion follows a slab that falls 3/16 inch across a 4 ft opening.

Species That Hold an Edge Under Traffic

A commercial threshold gets kicked, rolled over by carts and wet mopped, so the species question comes down to which woods keep a crisp arris and stay dimensionally calm after machining. The millwork palette is Sapele, Iroko, Afrormosia, Teak and Utile, and all five machine and glue predictably.

Sapele runs roughly 1,410 lbf on the Janka scale and holds a moulded edge cleanly, which is why it is the common default for milled sections in traffic. The Wood Database entry for Sapele covers the interlocked grain that calls for sharp tooling and a slower feed. Utile behaves much the same with a slightly more open texture. Iroko and Teak bring high natural durability, useful at vestibules and shower entries. Afrormosia takes a fine edge and finishes tight.

Domestic stock earns its place on plenty of schedules. Hard Maple at roughly 1,450 lbf is the hardest of the common choices. White Oak at roughly 1,360 lbf is the standard match for oak flooring, and quartersawn or rift White Oak is the version to specify where a threshold has to stay flat. Cherry at roughly 950 lbf is too soft for a corridor opening and is better left to interior trim. Grading language from the National Hardwood Lumber Association keeps a hardwood threshold order unambiguous.

Tropical decking species sit outside this list for a reason. Ipe, botanically Tabebuia spp, along with Cumaru, Garapa, Jatoba and Red Balau, are extremely hard and carry high oil content, which makes them difficult to glue and hard on tooling. A decking yard reaches for them constantly. A millwork house reaches for Sapele, Iroko, Afrormosia, Teak or Utile instead, because a threshold with a machined rabbet and sometimes a glued-up width has to bond and stay bonded. J. Gibson McIlvain holds over seven million board feet of exotic and domestic lumber at its Maryland yard, which is what makes it realistic to pull one lot for 60 openings so the color reads consistent the length of a corridor.

Moisture, Grain Orientation and the Joint That Has to Stay Tight

A threshold that fit perfectly in the shop and opened a gap in February was milled at the wrong moisture content, not designed wrong. J. Gibson McIlvain mills interior millwork to an in-service moisture content near 6 to 8 percent and exterior work near 12 to 16 percent, and a threshold at a conditioned interior opening belongs in the first range.

Grain orientation is the other lever. Quartersawn and rift stock moves less across its width than plainsawn, which is exactly what a wide saddle needs when its rabbet has to keep contact with a tile edge through a heating season. The Wood Handbook from the USDA Forest Products Laboratory publishes shrinkage coefficients if a spec needs the arithmetic rather than the rule of thumb.

Slab moisture deserves a look before anything wood goes near it. A threshold set into a fresh topping slab, or at a slab on grade with no working vapor control, is being asked to live in a different climate than the corridor above it. Building Science Corporation covers what a slab does to the materials sitting on it.

No warranty exists on solid unmodified wood, and no honest supplier offers one. It is an organic material responding to its environment. Warranties live on modified products only. If a project wants one at an exposed exterior-facing opening, Accoya at roughly 1,600 lbf brings acetylation and dramatically reduced shrinkage and swelling, and Thermory and Abodo Vulcan are both carried. Watch the limits. Modified woods top out around 16 ft, most sold on metric lengths just under 16 ft, because of kiln size, and thermally modified Ash is not available wider than 8 inches, so a wide modified saddle means Accoya or Abodo Vulcan.

Curved Openings and Radius Transitions

A curved opening or a floor transition that follows an arc cannot be cut from a straight threshold, it has to be built as radius work from the start. Segmented layup, bent lamination and a profile carried around the curve all get planned before the first blank is selected, because grain direction at each segment decides whether the finished nosing reads continuous or reads like six short pieces butted together.

Radius trimwork and radius millwork are part of what ICD builds, and the arc where a terrazzo lobby meets a wood-floored corridor is the everyday example. The same thinking covers a curved reception transition or a threshold that has to die into a radiused jamb. The ICD working process starts from the space rather than from a standard part, which is the only way a curved transition lands tight at both ends.

Radius pieces also change what the schedule has to say. A straight opening needs two end elevations. An arc needs elevations at intervals along its length, plus a template or a dimensioned plan of the curve itself. ICD builds from that information, and a template taken off the finished floor beats a radius scaled off a drawing every time.

Finishing, Marking and Getting Each Piece to Its Opening

On a 60 opening job, the difference between a smooth set and two weeks of sorting is whether each piece arrived labeled with the opening it was milled for. Pieces that look identical on a pallet stop being interchangeable the moment their undersides carry different relief depths.

Finish behavior is worth settling with a sample before production. Open-grained and oily species absorb a great deal of finish, and a first coat on Sapele, Iroko or Teak can read darker and less even than a small sample panel suggested. A sanded and sealed sample milled from the actual lot tells the architect what the corridor will look like. Where a threshold faces weather and will be painted, Sapele holds paint better than Genuine Mahogany at roughly 800 lbf and comes in a wider range of sizes and longer lengths, though Sapele is not milled 1x8 for exterior use since that width will not stay stable.

J. Gibson McIlvain ships nationwide, regularly to California, and finished millwork can be released against a schedule rather than dumped on site in one load. Thresholds are among the last things installed and among the easiest things to damage while they wait, so staged release against the flooring contractor's sequence protects both the pieces and the floor they land on.

How J. Gibson McIlvain Would Specify This

Specify the cross section family, the species and grade, and the moisture content, then let the schedule carry the per-opening dimensions, rather than forcing one finished part number onto a building whose floors never agreed with each other.

The submittal package that works looks like this. One threshold cross section per profile family, drawn full size with the rabbet depths shown as variables keyed to a schedule. A schedule with one line per opening carrying clear width, finish on both sides, finish floor elevation at both jambs and at the center, door swing and stop location, and a marked high side. Species and grade called out once. Moisture content stated at 6 to 8 percent for conditioned interior openings. Blank thickness set by the deepest relief on the schedule.

From there J. Gibson McIlvain grinds or pulls the knife, mills the sticking in one run, machines the relief per line item, tapers the underside where the schedule shows a difference that varies across the width, marks each piece with its opening number, and ships the lot in the sequence the installer asked for. Curved and true custom conditions route to ICD, the architectural millwork division, on the same order. The company supplies and mills. The installing trade sets, beds and seals the pieces.

Send the threshold schedule, the door schedule and the finish schedule together, along with the cross sections if they already exist. Call 800-638-9100 or start with the J. Gibson McIlvain millwork service to get a profile checked against the library before a flooring crew is standing at an opening holding a piece that does not fit it.