Slat spacing, span and attachment are not three separate decisions on a hardwood trellis or privacy screen, they are one decision made three times, and the frame is where the three of them meet.
A screen fails in a small number of ways. Slats cup and the clean shadow line turns ragged. A long slat sags in the middle of its run because nothing carries it there. Water sits in a joint that had no way to drain. Or the whole panel racks against the wall it was hung on, since the fastening pattern was drawn for a picture frame and the panel behaves like a sail.
Every one of those is a shop decision before it is a site decision. Slat section and unsupported span control deflection and cupping. Gap width controls both the privacy you actually get and the wind the panel has to shed. Attachment controls whether the assembly can move seasonally without splitting itself apart. Get the three of them agreeing with each other on the shop drawing and the panel behaves for decades. Draw them independently and the screen becomes a maintenance item within a few seasons.
ICD, the architectural millwork division of J. Gibson McIlvain, builds this category as true custom work planned around a specific opening rather than cut down from a stock panel. That is the whole difference. A stock lattice panel is sized to a pallet. A screen milled to a drawing is sized to the balcony rail it screens, the entry it flanks or the mechanical yard it hides, with the slat spacing derived from the sightline you actually need to block.
Spacing is a sightline geometry problem, and the useful number is the ratio of gap to slat depth, not the gap alone.
Designers ask for a gap dimension. The gap by itself tells you very little. What determines whether a person on the far side sees through is the relationship between the width of the opening and the depth of the slat, read at the angle a viewer actually stands.
Take a nominal 1x4 laid flat with a 1 inch gap. The slat is around three quarters of an inch deep, so anyone standing more than a step off axis looks straight through. Turn the same board on edge so that three and a half inches of depth face the viewer, keep the same 1 inch gap, and the screen closes almost completely by the time the viewer moves fifteen or twenty degrees off perpendicular. Same open area. Completely different privacy. That is why a deep fin screen with generous gaps can outperform a shallow slat screen with tight gaps, and why it carries less wind load doing it.
The second number worth fixing early is open area, meaning gap divided by the slat-plus-gap module. A screen at roughly forty to fifty percent open reads as a screen, moves air, and does not behave like a solid wall in a gust. Below about twenty percent open you have built a wall with decorative reveals, and you should be talking to your structural engineer about the load path rather than about millwork. Above about sixty percent you have a trellis, which is a fine thing to build, but it is a plant armature and not a privacy device.
For a growing trellis the arithmetic runs the other way. Climbers want something to grab. A grid on roughly six to eight inch centers in both directions suits twining vines, and the member facing the wall needs a standoff so tendrils can wrap it. Wisteria and other heavy woody climbers are a structural client in their own right and want members sized well beyond what the wind alone would ask for.
| Infill pattern | Privacy behavior | Wind behavior | What the frame has to carry | Milling note |
|---|---|---|---|---|
| Flat slats, face out, narrow gap | Blocks head on, opens up off axis | Highest pressure for a given look, low open area | Stiff stiles and at least one intermediate rail on taller panels | Simple eased-edge profile, easiest to prime evenly |
| Slats on edge, deep fin | Closes hard off axis, stays open on axis | Sheds well, high open area for the privacy delivered | Rail-to-slat connection carries torsion, not just weight | Housed grooves or tenons, more machine time per slat |
| Fixed louver blades at an angle | Directional, blocks a chosen view cone | Deflects and drains, predictable pressure | Angled housings in both stiles, tight tolerance | Blade profile and pitch cut from a dedicated knife |
| Square grid, half lapped | Light screening, reads as trellis | Very low pressure, high open area | Perimeter frame only, the grid stiffens itself | Half lap joints must drain, no flat water traps |
| Diagonal grid or eggcrate | Moderate, varies with depth of the cell | Low pressure, but catches debris in the cell | Corner joinery works hardest, racking is the risk | Compound angle cuts, more setup, more waste |
Unsupported span is limited by cupping and appearance long before it is limited by strength.
Nobody snaps a hardwood slat with a windstorm on a residential screen. What actually goes wrong is visual. A slat with too much unsupported length picks up a slow curve, and once one slat in a run of forty curves out of plane, the eye finds it instantly. Shadow lines are unforgiving that way.
The practical control is a thickness-to-span relationship plus an intermediate rail. A four quarter slat run flat wants intermediate support before it gets much past four to five feet of clear span. Five quarter and six quarter stock buys real distance. Turning the same slat on edge multiplies stiffness in the direction that matters, which is one more reason the deep fin pattern is efficient. Where a panel has to reach eight feet or better without a visible mid rail, the answer is a heavier slat section, not a longer thin one.
Cupping deserves its own note, since it gets blamed on species more than it deserves. A slat cups when one face gains or loses moisture faster than the other. That is an installation and ventilation condition. A screen pinned flat against a stucco wall with no air behind it will cup regardless of what you milled it from, and a screen with air on both faces stays flat in species people are suspicious of. The Wood Handbook from the USDA Forest Products Laboratory is the reference for the shrinkage numbers behind that behavior, and it is worth having open when you are sizing gaps.
Quartersawn and rift stock moves less across its width than plainsawn. On a screen where the slats are painted, or where the panel is held to a tight reveal against a jamb, specifying quartersawn or rift for the slats is money spent on flatness rather than on figure. It also gives a more uniform paint surface, since the grain pattern that telegraphs through a film is largely a plainsawn phenomenon.
The species that machine and glue well for exterior millwork are Sapele, Iroko, Afrormosia, Teak and Utile, and the tropical decking woods are not the default here.
A millwork shop and a decking yard reach for different boards, and a screen is millwork. Sapele runs around 1,410 lbf on the Janka scale, mills cleanly, takes glue, and comes in a wide range of sizes and long lengths. Iroko is stable outdoors and weathers to a settled gray without drama. Afrormosia holds crisp arrises, which matters when the whole design is edges and shadow. Teak needs no argument. Utile machines much like Sapele and stretches the sizes available when a job wants a lot of one section.
Ipe, Cumaru, Garapa, Jatoba and Red Balau are excellent decking. They are also extremely hard and carry high oil content, which makes them difficult to glue and hard on knives. A trellis panel or a louvered screen is a glued and machined assembly with mortises, housings and mitered corners in it. That is exactly the work those species fight. Where a screen is genuinely an extension of an Ipe deck and the client wants the match, it can be done, but the shop reaches for Sapele, Iroko, Afrormosia, Teak or Utile when the assembly has joints in it. Botanically Ipe is Tabebuia spp, and the reference data on comparative hardness and movement lives at the Wood Database if you want the side by side.
Western Red Cedar remains a legitimate default and always has been. It is light, stable, naturally durable and easy on hardware. Specify the grade deliberately. CVG, clear vertical grain, is the choice where the screen is painted or where the reveal is tight and you want no knots in the shadow line. STK, select tight knot, is the choice where the character is wanted and the budget is real. Both are correct answers to different questions.
Where a CITES-listed species is part of the package, documentation and chain of custody travel with the material, and that paperwork should be part of the submittal rather than an afterthought at delivery. The CITES listings and the FSC chain of custody framework are both worth understanding before the species is locked. J. Gibson McIlvain is FSC certified and supplies Sapele, Iroko, Afrormosia, Teak and Utile with the documentation in order.
The frame is the part that fails, so it gets the real joinery and the slats get room to move inside it.
A privacy screen is a stile and rail assembly with an infill. Load goes into the slats, out through their end connections, into the rails, into the stiles, and finally into whatever the panel is fixed to. Every one of those handoffs is a place where a shortcut shows up later.
Slat ends want a housed groove or a tenon, not a face screw through the frame. A screw through the stile into slat end grain is a hinge, and it will work loose. Housing the slat lets the frame carry it while allowing the slat to grow and shrink in width inside the groove. Corners of the frame want a proper mechanical joint. Mortise and tenon with waterproof adhesive is the standard, and the joint should be drawn so water leaving the panel has a path down and out rather than a shelf to sit on.
Where a screen is built as a hollow structural element, a post wrap or a heavy top cap, the box beam joinery choice applies directly. Butt joints give the most minimal profile, the fastest turnaround and the easiest assembly on site, but they leave visible seams and visible end grain and they are the weakest. Dado joints are strong and still easy to assemble on site, though the seams and end grain still show. Lock miter joints show no visible seams and are the strongest of the three. On an exposed exterior screen post, lock miter is usually worth the extra machine time, since a seam that opens on a south elevation is a seam everyone sees.
J. Gibson McIlvain mills the slat stock, stiles, rails and cap profiles on its own moulders, with straight line and gang rip saws, band resaws and trimming saws feeding the run. The moulding profile library holds thousands of profiles and new knives are ground custom every week, so a screen with a slat profile drawn specifically for one elevation is a routine job rather than a special request. Details on how those runs are set up are at the ICD trim profiles page.
"The screens that come back to us with problems almost never have a slat problem. They have a frame problem or a fastening problem. We spend more drawing time on how a panel hangs and how it drains than on the pattern itself, because the pattern is what the architect sees and the connection is what decides whether it still looks that way in ten years."
Norm Moton, Director of Sales, J. Gibson McIlvain
Attachment has to allow the panel to move, drain and dry, and it has to land on real structure rather than on cladding.
Screens get hung on three things. A wall, a set of posts, or a guard assembly at a balcony edge. Each one changes the detail.
On a wall, the panel needs a standoff. An inch or better of clear air behind the panel lets both faces of every slat see similar conditions, which is the single most effective cupping control available. That gap also keeps the back of the frame out of standing water and lets the wall assembly dry. The reasoning behind ventilated, drained claddings is laid out well in the Building Science Corporation discussion of drainage gaps, and a screen hung on a wall is functionally a rainscreen element whether anyone calls it that or not.
Fasteners land in blocking or in framing, never in the cladding itself. A panel four feet wide and eight feet tall in a gust is a meaningful lateral load on whatever it is attached to, and lag screws into siding will pull. Where the screen sits at a balcony or a guard, the connection becomes a code conversation, and the load path and connection design belong to the project engineer. The American Wood Council codes and standards library and the International Code Council are the right starting points for what the local jurisdiction will actually enforce.
Hardware metallurgy is not optional on hardwood. Sapele, Iroko, Afrormosia, Teak and Utile all carry extractives that react with ordinary steel, leaving black streaks that no amount of cleaning removes. Stainless is the specification. Slot the holes in the frame where a fastener crosses the grain of a wide member, so seasonal movement has somewhere to go instead of splitting the stile.
One thing to be clear about. J. Gibson McIlvain supplies and mills, and does not install or perform field measurement. The shop delivers panels, frames, slat stock and cap profiles built to the approved drawing, with the joinery cut and the priming done, and the installing contractor sets them, flashes them and makes the structural connections. Coordinating the blocking layout is the installer's scope, and the drawing should show where blocking has to be for the fastening pattern to work.
Exterior screen stock is milled near 12 to 16 percent moisture content, and the gaps in the design are sized around what happens after that.
Interior millwork is milled to an in-service moisture content near 6 to 8 percent. Exterior work is a different target, near 12 to 16 percent, since the material is going to equilibrate outdoors no matter what it left the kiln at. Milling exterior slats to interior moisture content guarantees swelling, which closes gaps, pinches slats in their housings and pushes frames apart.
Design gaps accordingly. A slat housed in a groove needs clearance at the bottom of the housing so the slat can grow in width without bottoming out and levering the frame. A run of slats stacked with a nominal gap will see that gap narrow in a humid August and widen in a dry January, and the drawing should state the gap at a stated moisture condition rather than as an absolute. Panels set into a masonry opening need a perimeter joint that is a sealant joint, not a tight fit.
Wide members are where movement shows first. That is why quartersawn and rift material is worth specifying for slats over about four inches wide, and why Sapele is not milled 1x8 for exterior use. Too wide to stay stable. Where a design genuinely calls for a wide exterior board, that is a signal to look at a modified product or to break the width into two members with a reveal between them, which usually looks better anyway.
Modified woods solve the movement problem on screens, and their real constraint is length.
Accoya is acetylated wood with dramatically reduced shrinkage and swelling, running around 1,600 lbf on Janka, and it takes paint better than nearly anything else available for exterior use. On a painted screen with fine slats and tight reveals, it is a strong candidate specifically because the coating is not being asked to stretch over a moving substrate. Abodo Vulcan and Thermory are the thermally modified options, both carried, both stable, both weathering predictably.
Know the ceiling. Modified woods, Thermory, Abodo Vulcan and Accoya, top out around 16 feet, and most are sold on metric lengths just under 16 feet, because the kilns and reactors that make them are only so long. A screen elevation drawn with 18 foot continuous slats will be built with a joint in it, and that joint should be a designed reveal rather than a field surprise. Thermally modified Ash is not available wider than 8 inches, so a design wanting wide boards in a modified product means Accoya or Abodo Vulcan.
Warranty is the other thing worth stating plainly. Modified products carry warranties. Solid unmodified wood does not, and it is not possible to warranty an organic material that responds to its environment. Anyone offering one on solid hardwood is offering something they cannot support.
Priming is a shop operation on a screen, since half the surface area is unreachable once the panel is assembled.
A finished slat screen has an enormous amount of surface for its footprint, and most of it faces another slat. Once the panel is together, a brush cannot reach the inside faces, the slat ends or the bottom of a housing. Coating those surfaces after assembly is not a thing that happens, whatever the schedule says.
J. Gibson McIlvain primes wood in three levels, in both oil-based and water-based systems, and for exterior primed trim the specification should carry a fungicide additive. Open-grained and oily exterior species pull in a great deal of finish, so a primer coat evens out the eventual painted surface instead of leaving a blotchy film. Sanding or buffing after priming knocks back the grain raising that the first coat causes while keeping moulded detail crisp, which matters on a louver blade or a profiled slat edge where a soft, loaded edge would kill the shadow.
Sequence it correctly. Machine, prime including all six faces of every slat and the inside of every housing, then assemble, then finish coat on site. End grain gets extra attention. End grain drinks at many times the rate of face grain and is where nearly every exterior coating failure starts.
A screen is rarely the only exterior wood on the job, and the whole package is easier to control when it is milled together.
Screens sit next to other things. A pergola over a terrace, a courtyard gate, rafter tails on the overhang, a bench along the base, railings at the balcony, shutters at the windows. Those are all in the same category of custom exterior work, planned to fit a specific space, and the ICD exterior millwork group is where they are drawn and built together.
Building them together is not a convenience argument, it is a consistency argument. Same species, same lot, same moisture content, same primer, same profile radius on the eased edges. A screen milled in one shop and a pergola milled in another will not weather at the same rate or take paint the same way, and the difference shows up on the elevation two years later.
Gates deserve a specific note, since a privacy screen and a gate often share a wall. Pedestrian, garden, courtyard and estate entry gates are custom work. Large driveway swing gates are a different animal and require engineering rather than only craftsmanship, given the size and the weight that hinges and posts have to carry over decades of swinging.
Shutters are the other neighbor. Louver and panel styles, with complete historical matching for both style and species where a historic district requires like-for-like replacement. The National Park Service technical preservation standards set the framework those reviews use, and a matched louver profile is exactly the kind of job that gets a knife ground for it.
On the J. Gibson McIlvain side of the house, the linear trimwork, finished cladding and primed trim that surround the screen come from the same floor. Details are at the J. Gibson McIlvain millwork service page. Finished millwork can be stored for inventory control when a site is not ready, and material ships nationwide, regularly to California.
How J. Gibson McIlvain Would Specify This
Start with the sightline, not the gap. State what has to be blocked, from where, at what angle. That single sentence in the specification determines slat depth and orientation, and it prevents the common outcome where a screen at the drawn gap turns out to be transparent from the neighbor's second floor window.
Fix open area next, somewhere near forty to fifty percent for a privacy screen and higher for a trellis. Then choose the pattern from that. Deep fin slats deliver more privacy per unit of wind load than flat slats do, and they let the gap stay generous, which helps drying.
Size slats for flatness rather than for strength. Four quarter stock wants intermediate support past roughly four to five feet of clear span. Heavier sections buy distance. Quartersawn or rift for anything painted or held to a tight reveal.
Species from the millwork palette. Sapele, Iroko, Afrormosia, Teak or Utile for a machined and glued assembly, with Sapele preferred over Genuine Mahogany where the screen is painted, since it holds paint better and comes in a wider range of sizes and longer lengths. Cedar in CVG or STK where cedar suits the building. Accoya or Abodo Vulcan where the design wants a wide, stable, paint-grade board, remembering the roughly 16 foot ceiling on modified lengths.
Housed slat ends, mortise and tenon frame corners, lock miter on any exposed hollow post or cap. Drainage designed into every horizontal surface. Stainless fasteners throughout, slotted where a fastener crosses the grain of a wide member.
Milled to 12 to 16 percent moisture content for exterior service. Primed in the shop on all faces before assembly, oil-based or water-based, with a fungicide additive for exterior work. Gaps stated at a reference moisture condition.
Attachment onto blocking or framing, never onto cladding, with an air gap of an inch or more behind the panel. Structural connections and any guard-related load path are the project engineer's call, and the installing contractor sets and flashes the work. Reference standards for the shop drawings and the quality expectations sit with the Architectural Woodwork Institute.
Send the elevation and the section. Custom knives get ground weekly, the profile library runs to thousands of patterns, and a screen drawn for one specific opening is what this shop is set up to make. Call 800-638-9100 to get a millwork specialist on the drawing, or start at the ICD working process to see how a custom exterior package moves from drawing to delivery.
Frequently Asked Questions
How wide should the gap between slats be for real privacy?
Ask the question about depth instead of gap. Privacy is governed by the ratio of the opening width to the depth of the slat facing the viewer, read at the angle people actually stand. A three quarter inch deep slat with a one inch gap is transparent to anyone a step off axis. The same one inch gap between slats set on edge, with three and a half inches of depth facing out, closes almost completely by fifteen or twenty degrees off perpendicular. Write the sightline you need to block into the specification and let the shop derive the spacing from it.
How long can a hardwood slat span without an intermediate rail?
Deflection is not the limit. Appearance is. A four quarter slat run flat starts to show a slow curve past roughly four to five feet of clear span, and one curved slat in a run of forty is instantly visible in the shadow line. Five quarter and six quarter stock buys real distance, and turning the slat on edge multiplies stiffness in the direction that matters. Where a panel has to reach eight feet or more with no visible mid rail, go to a heavier section rather than a longer thin one.
Can a trellis panel be fastened directly to siding?
No. Fasteners have to land in blocking or framing. A four by eight foot panel in a gust puts a meaningful lateral load on its connections, and lags into siding will pull out. The panel also needs a standoff of an inch or more so air reaches both faces of every slat, which is the most effective cupping control there is, and so the wall behind it can dry. Where the screen sits at a balcony or a guard, the load path and connection design belong to the project engineer.
Which species should be specified for a painted privacy screen?
Sapele is the first call. It holds paint better than Genuine Mahogany, comes in a wider range of sizes and longer lengths, machines cleanly and glues well, and runs around 1,410 lbf on Janka. Specify quartersawn or rift for the slats so the surface stays flat under the film. Iroko, Afrormosia, Teak and Utile are the other millwork palette options. Where the design wants maximum dimensional stability under paint, Accoya is the better answer, since acetylation gives dramatically reduced shrinkage and swelling and the coating is not stretching over a moving substrate. Sapele is not milled 1x8 for exterior use, since that width will not stay stable outdoors.
Why not build the screen from Ipe if it is the most durable exterior wood?
Ipe, botanically Tabebuia spp, along with Cumaru, Garapa, Jatoba and Red Balau, is excellent decking. Those species are extremely hard and carry high oil content, which makes them difficult to glue and hard on cutting tools. A trellis panel or a louvered screen is a glued and machined assembly with mortises, housings and mitered corners in it, so a millwork shop reaches for Sapele, Iroko, Afrormosia, Teak or Utile instead. It can be done where a screen has to match an existing Ipe deck, but the joinery is fighting the material rather than working with it.
Can modified wood be supplied in slats longer than 16 feet?
No. Thermory, Abodo Vulcan and Accoya all top out around 16 feet, and most are sold on metric lengths just under 16 feet, because of the size of the kilns and reactors that make them. An elevation drawn with 18 or 20 foot continuous slats will be built with a joint, so make that joint a designed reveal on the drawing. Thermally modified Ash is also not available wider than 8 inches. If the design needs wide boards in a modified product, that means Accoya or Abodo Vulcan.
Does J. Gibson McIlvain install trellis panels and privacy screens?
No. J. Gibson McIlvain supplies and mills. The shop delivers the panels, frames, slat stock and cap profiles built to the approved drawing, with the joinery machined and the priming done on all faces before assembly. Setting, flashing, blocking coordination and the structural connections are the installing contractor's scope, and field measurement is handled on the buyer's side. The shop drawing should show where blocking has to be so the installer's fastening pattern works.
Is there a warranty on a solid hardwood screen?
Not on solid unmodified wood. It is not possible to warranty an organic material that responds to its environment, and any supplier offering one is offering something they cannot support. Warranties exist on modified products, meaning Accoya, Abodo Vulcan and Thermory. What a solid hardwood screen gets instead is correct species selection, exterior moisture content, quartersawn or rift stock where flatness matters, drainage designed into the joinery and full shop priming.
Sources and Standards Referenced
- Wood Handbook: Wood as an Engineering Material, USDA Forest Products Laboratory (FPL-GTR-282)
- American Wood Council: Codes and Standards
- Building Science Corporation, BSI-038: Mind the Gap, Eh!
- The Wood Database: Sapele
- CITES: Convention on International Trade in Endangered Species of Wild Fauna and Flora
- Forest Stewardship Council
- National Park Service, Technical Preservation Services: The Standards
- Architectural Woodwork Institute
- Accoya Acetylated Wood
- Thermory Thermally Modified Wood
