Teak Sash and Sills at the Opening, and When Sapele or Iroko Wins Instead

Why Teak Keeps Landing on the Window Schedule

Teak stays on window schedules because it answers the three hardest demands at an opening at once, resisting decay without chemical treatment, moving very little through a wet season and a dry one, and taking a clear exterior finish that still looks like wood after a few years. Sash and sill live in the worst microclimate on a building. Water runs down the glass and collects at the bottom rail. Sun bakes the exterior face while conditioned air dries the interior face. The assembly is thin, it is joined end grain to long grain, and it has to keep working every time somebody opens it.

Architects who have watched a painted softwood sill fail at the horns in eight years start asking for something better, and the specification tends to land on Teak. The species earns it. Tectona grandis carries natural extractives that shrug off fungal attack and insects, sits around 1,070 lbf on the Janka scale by the figures published in the Wood Database entry for Teak, and holds a machined arris crisply enough that a glazing rabbet or an ovolo sticking survives handling.

None of that makes it automatic. Teak is oily, it is abrasive on knives, and it is not the right call on every opening. What follows is the working detail a millwork house uses to decide, and what changes in the shop when the answer is yes.

What the Opening Actually Does to a Piece of Wood

A window sill is not exterior trim, it is a horizontal drainage component with a fastener field running through it, and it fails for reasons that have nothing to do with how pretty the species looks on a sample chain. Rain hits the glass, sheets down, and arrives at the sill in volume. If the wash is too shallow it sits. If the drip kerf is missing or set too close to the nosing edge, water tracks back along the underside and wets the sheathing behind the trim. Building Science Corporation lays out the general principle in its digest on rain control in buildings, and the sill is where that principle either works or does not.

Then there is end grain. The ends of a sill, the horns that run past the jamb and into the masonry return, wick moisture at many times the rate of a face. A sill that was primed on five faces and cut to length on site is a sill with two raw straws pointed at the weather. The same applies to the bottom rail of a sash and to the mortises that receive the stile tenons.

Movement finishes the job. Every seasonal cycle swells and shrinks the member across its width, and a wide stool fastened tight at both edges with no back relief will cup or split rather than move. Grain orientation and the fastening pattern control that outcome far more than the species name does.

Teak Chemistry Is the Reason It Works and the Reason It Changes the Shop

The same extractives that make Teak durable at an exposed sill are the ones that complicate glue-up and finishing, so a shop that machines it regularly runs a different process than it does for a paint-grade species. Tectoquinone and the associated oils in the heartwood are what discourage fungi and termites. They are also what interferes with adhesive wetting. A Teak sash joint glued straight off the moulder, with no surface preparation, is a joint that can starve.

The fix is well understood. Machine the mating surfaces fresh, wipe them with a solvent to pull the surface oil, and glue inside a short window rather than letting parts sit for a week. Resorcinol and modern one-part polyurethanes both behave, and the wet-use adhesive test methods that back that up sit with ASTM Committee D07 on wood.

Teak also carries silica. That is what dulls a cutter so much faster than the density alone would predict. A knife profile that runs a full week in Sapele may need regrinding partway through a Teak run, which is a scheduling fact rather than a problem, provided the shop grinds its own. J. Gibson McIlvain grinds new moulding knives every week against a profile library holding thousands of ground knives, so a Teak sash sticking or a sill nosing gets a fresh edge instead of a tired one that burnishes the oil into the surface and ruins the finish key.

Sash and Sill Are Two Different Engineering Problems

Sash wants straight grain in narrow sections and joints that will not rack, while a sill wants width, slope and a stable face, and the milling decisions diverge accordingly. Sash stiles and rails run narrow and long. Rift and quartersawn stock is the right call there, because it moves less across its width than plainsawn and because the straight grain keeps a slender stile from bowing between the head and the sill. A stile with a swirl of flat grain through the middle will telegraph that swirl as a bow the first humid month.

Section thickness follows the glazing. Traditional putty-glazed single pane sash is commonly built around a 1 3/8 inch thickness. Drop an insulated glass unit into the same window and the glazing rabbet has to grow, which usually pushes the sash to somewhere between 1 3/4 and 2 1/4 inches depending on the unit thickness and the bead detail. That is the moment a stock window supplier says no and a custom shop says fine. ICD, the architectural millwork division of J. Gibson McIlvain, builds custom doors and windows around the actual opening and the actual glazing package rather than around a catalog module.

Sills carry different loads. The exterior sill needs a wash, usually cut somewhere in the range of eight to fifteen degrees, a drip kerf on the underside set back from the nosing edge, and a horn detail that matches whatever the jamb and casing condition is. Where the head is arched, the casing and the sash head both become radius work, laminated or bricklaid rather than sawn from the solid so the grain follows the curve. Radius trimwork and radius millwork are standing capabilities, not special requests.

Sills, Stools and Aprons Milled to Real Dimensions, Not Catalog Ones

The reason buyers go looking for custom hardwood window sills and aprons is that the dimensions that actually fit a wall almost never match what is on a shelf, and a millwork shop starts from the measured opening instead of the nearest stock size. Modern wall assemblies have made this worse. A wall with exterior continuous insulation, or a masonry opening with a deep return, pushes the interior stool out past anything a lumberyard carries as a stock stool profile. The apron under it has to grow in proportion or the whole detail looks starved.

A stool is milled with a rabbet on the underside that sits down over the sash sill or the jamb, a nosing on the room side, and returns at the ends that carry the nosing profile around so end grain never faces the room. On a deep stool, relief kerfs on the hidden underside plus a fastening pattern that lets the board move are what keeps it flat. Grain orientation helps too. Quartersawn and rift stock moves less across its width than plainsawn, which is why a wide painted stool gets specified that way.

Aprons are where the profile library earns its keep. A replacement apron under a historic window has to match the original sticking exactly, including the returns at each end, or the repair reads as a repair from across the street. If the profile is not already in the library it gets ground to match from a section drawing or a physical sample, and that covers difficult historical restoration profiles as readily as contemporary ones. ICD mills sills, stools, aprons and casing as a matched set off one order so the reveals line up, and the trim profile work runs through the same shop as the sash.

One boundary worth stating plainly. J. Gibson McIlvain supplies and mills. It does not install and it does not take field measurements. The dimensions come from the buyer, the architect or the installing contractor, and the shop mills to them. What arrives is a component set cut to those numbers, with the joinery prepared, so the installer is fitting rather than fabricating on a sawhorse.

When Teak Is Not the Answer at the Opening

Teak is the right species for a clear-finished, high-exposure opening, and for most painted work at a window the better answer is Sapele, Iroko, Afrormosia or Utile. Painted exterior trim has a different set of requirements. Paint has to key to the surface, and an oily species fights that unless the surface is prepared and primed correctly. Sapele holds paint better than Genuine Mahogany and comes in a wider range of sizes and longer lengths, which is why it is the default for painted exterior casing, brickmould and apron stock. Sapele is not milled 1x8 for exterior use, since that width will not stay stable in the weather.

Iroko behaves a lot like Teak in exposure and is easier to source in long clear lengths. Afrormosia machines beautifully and takes a fine edge on a small section, which suits muntin bars and glazing beads. Utile is the practical stand-in for Genuine Mahogany on interior sash and stool work where a warm color under a clear finish is the goal.

The tropical decking species that get named in the same breath, Ipe, Cumaru, Garapa, Jatoba and Red Balau, belong on a deck rather than in a sash. They are extremely hard and carry high oil content, which makes them difficult to glue, and a window is a glued assembly. Ipe, botanically Tabebuia spp, is a superb decking board and a poor sash stile. A millwork house reaches for Sapele, Iroko, Afrormosia, Teak or Utile instead because those five machine, glue and finish predictably in small sections.

Moisture Content Is the Specification Most Window Orders Get Wrong

Interior millwork is milled to an in-service moisture content near 6 to 8 percent and exterior work near 12 to 16 percent, and a window assembly needs both numbers on the same order because it has members living in both climates. The exterior sill, the brickmould and the outer sash face belong to the weather. The stool, the apron and the interior casing belong to a conditioned room. Milling all of it to one number guarantees that half of it is wrong.

The equilibrium moisture content tables in the Forest Products Laboratory Wood Handbook are the reference for what a given climate will drive a member toward, and the practical takeaway is simple. Mill close to where the piece will end up living, not to where it is sitting in the shop.

J. Gibson McIlvain mills interior stool and apron stock to a 6 to 8 percent target and exterior sill and casing stock to a 12 to 16 percent target, then packages the two groups separately so they do not equalize against each other in transit. A section of the mill is dedicated to packaging, and the company runs its own fleet of trucks, which is what keeps a primed sill from arriving with a dented arris or a moisture-swollen end after a week on a common carrier. Delivery runs nationwide, regularly to California.

Priming and Finishing Decide Whether the Sill Is Still Good in Ten Years

A sill that arrives primed on all six faces, with the end grain sealed before it ever leaves the shop, outlasts an identical sill primed on site by a margin that shows up within the first few years. Field priming misses the back face, misses the ends after the cut, and happens in whatever weather the job has that week. Shop priming does not.

J. Gibson McIlvain primes in three levels, in both oil-based and water-based systems, and exterior primed trim carries a fungicide additive. Open-grained and oily species absorb a great deal of finish, so a primer coat evens out what the eventual painted surface will look like. Sanding or buffing after priming knocks back the grain raising while keeping the moulded detail crisp, which matters on a sill nosing or an apron sticking where a heavy-handed field sanding would round the profile into mush.

Teak under a clear finish is its own conversation. The species will weather to silver if left alone, and that is a legitimate design choice on an exposed elevation. Where a client wants the color held, the finish is a maintenance item on a cycle, not a one-time coating. Where Teak is going to be painted, the surface preparation matters more than the paint does, and a shop-applied primer over a freshly machined and solvent-wiped face is the version that holds.

No warranty exists on solid unmodified wood, and no honest supplier offers one. Wood is an organic material responding to its environment. Warranties live with modified products, Accoya, Thermory and Abodo Vulcan, and those three top out around 16 feet because of kiln size, with most sold on metric lengths just under 16 feet. Thermally modified Ash is not available wider than 8 inches, so a wide modified board means Accoya or Abodo Vulcan rather than Thermory Ash.

Documentation Travels With the Material, Not Behind It

A Teak window package on a specified project has to arrive with its paperwork intact, and that is a supply chain question rather than a millwork question. Projects with sourcing requirements in the specification, whether that is an FSC line item or an owner sustainability standard, need the chain of custody to be continuous from import forward. J. Gibson McIlvain is an FSC certified importer and miller, which means the certification sits with the party that actually brings the material in and machines it rather than with a reseller holding a claim it cannot trace.

Where a specification calls in a CITES listed species, documentation and chain of custody travel with the material, and the CITES permit framework governs how that is handled at import. Getting that right at the front of a job is far cheaper than discovering a gap when the millwork is already cut.

Grading conventions matter too, since a window order is usually cut from selected stock rather than run-of-mill. The National Hardwood Lumber Association rules define what a grade means for hardwood lumber, and the Architectural Woodwork Institute standards define what a finished millwork component has to meet once it leaves the shop.

Historic Replacement Work Is Where the Knife Library Decides the Job

Replacing a handful of failed sash and sills in a historic building means matching a profile that no longer exists in any catalog, and either the shop can grind the knife or the job does not happen. The National Park Service guidance in Preservation Brief 9 on the repair of historic wooden windows is unambiguous about repairing in kind, and in a historic district it is often a requirement rather than a preference. Like-for-like means the same profile and frequently the same species.

That is a two-part capability. The profile library holds thousands of ground knives, and new knives are ground custom every week, so a matching run is a lookup where possible and a grind where not. The second part is continuity. A phased restoration that replaces a dozen windows this year and two dozen in three years needs the same profile both times, from a supplier that still exists and still has the record. J. Gibson McIlvain is seventh generation and family run, in business since 1798, and it keeps the knife and the record on file so phase two matches phase one.

Shutters belong to the same conversation. Louver and panel styles both get built to match historically, in style and in species, which is what a historic district review usually asks for when original shutters are missing.

"The window orders that go well are the ones where somebody sent us the real dimensions and told us which side of the wall each piece lives on. We will mill the exterior sill to one moisture target and the interior stool to another, prime what needs priming, and put them on our own truck so they arrive the way they left. What we cannot do is guess at a profile from a photograph and hope it matches the ninety-year-old apron next to it, so send us the section or send us a piece."

Norm Moton, Director of Sales, J. Gibson McIlvain

How J. Gibson McIlvain Would Specify This

Specify the species by exposure, the moisture content by which side of the wall each member lives on, the profile by drawing or physical sample, and the finish level by what you want arriving on site. That is four decisions, and every one of them is answerable before a board gets cut.

Start with exposure. A clear-finished, high-splash or coastal opening is a Teak conversation. A painted exterior opening is a Sapele conversation, in quartersawn or rift where a wide member has to stay flat, and not in a 1x8 exterior width. Small-section sash detail, muntin bars and glazing beads, points at Afrormosia. Interior stools and aprons under a clear finish point at Utile. Iroko covers the Teak brief where long clear lengths are the constraint. J. Gibson McIlvain holds over seven million board feet of exotic and domestic lumber at its Maryland yard, which is what makes lot matching across a twenty-window elevation a real option rather than a hope.

Then split the moisture targets. Interior members at 6 to 8 percent, exterior members at 12 to 16 percent, packaged separately.

Send the profile. A section drawing or a physical offcut is enough. If it is in the library it runs. If it is not, a knife gets ground to match it, historical or contemporary.

Pick the finish level. Three levels of priming are available in oil-based or water-based systems, and exterior work carries a fungicide additive. Decide whether the installer is receiving raw stock, primed stock or primed and sanded stock.

The work splits across two shops that take one order. A full millwork shop sits on site at the lumber yard and handles linear trimwork, finished cladding and primed trim, while the architectural millwork division, ICD, handles the true custom side, sash and custom windows, radius trimwork, box beams, shutters, gates and cabinetry. The ICD working process and the exterior millwork page cover the custom end, and the J. Gibson McIlvain millwork service page covers the linear end. Storage of finished millwork is available where a job needs release in phases rather than all at once.

Bring the drawings, the opening dimensions and the exposure to a conversation at 800-638-9100, and the species, the moisture target, the profile and the finish level get settled in one call rather than three.