Building With CLT and Glulam

The Panel You Set Is the Ceiling They See

A concrete frame gets forgiven twice: once by the screed, once by the plasterboard. A CLT soffit gets forgiven by nothing. The panel a crane sets at nine in the morning is the ceiling a client stands under two years later, raked by light from a rooflight at exactly the angle that finds every scuff. That single fact reorders the job. Your erection gang is doing finish carpentry while wearing gloves rated for steel edges, and nobody following them will cover their mistakes.

Everything downstream inherits it. A slung load that swings into a wall panel leaves a bruise that no filler matches, because crushed fibre takes stain at a different rate than sound fibre and the repair reads as a pale ghost forever. A boot print pressed into wet dust on a floor panel dries into a grey shadow. Chalk lines on the face, crayon marks for cuts, a dropped box of screws on the deck — ordinary framing habits, all of them, and all of them permanent on an exposed surface.

Cost lands somewhere unfamiliar too. Damaged mass timber is rarely structurally condemned; it is aesthetically condemned, and the fix is a replacement panel cut on the factory's lead time rather than the site's programme, with the floor above waiting on it.

Appearance Grade Is Decided Before the Truck Loads

Grade governs what you are allowed to be annoyed about later, and it is fixed before the truck loads. CLT declared to EN 16351 (Timber structures — Cross laminated timber — Requirements) or ANSI/APA PRG 320 (Standard for Performance-Rated Cross-Laminated Timber) carries an appearance classification for the outer lamellae, typically split between a visual-quality face and a non-visual or industrial face. Glulam declared to EN 14080 (Timber structures — Glued laminated timber and glued solid timber — Requirements) or manufactured to the North American structural glued laminated timber standards carries appearance classes running from framing through industrial and architectural to premium, defined in the appearance specification rather than the strength one.

Read what those classes actually permit. A visual grade is not a promise of clear timber; it permits knots within stated limits, face patches, filled voids, and colour variation between lamellae cut from different logs. If the architect's rendering shows uniform blonde spruce and the order says industrial face, the argument arrives on site, aimed at you, months after the specification settled it.

The trap that catches crews is orientation. Appearance grade usually applies to one face only, and panels are stacked for transport in whatever order suits the load, not the erection sequence. Every panel schedule needs the exposed face marked, every panel needs that mark readable while it is still in the stack, and the banksman needs to be reading it. Flipping a panel to make a lift easier exposes the industrial face of a soffit permanently.

Placing Joints Someone Will Stare At

Joint positions are a design decision that the setting-out drawing makes and the ceiling then advertises for the life of the building. Panel width is limited by road transport, so a floor gets divided whether anyone plans it or not; the only choice is whether the divisions land somewhere deliberate. Joints aligned with a glazing mullion, a lighting run or a partition line read as intent. Joints falling a hundred millimetres off those lines read as an error, and they read that way from every seat in the room.

Detail type changes the visual weight as much as position does. A half-lap shows a single line. A surface spline shows a strip of different material and often a double row of screws. A butt joint connected with structural screws from the topside shows almost nothing from below, which is why it earns its cost on exposed soffits even where a spline would do structurally. Settle this early, because it drives panel widths, and panel widths drive the whole grid.

Openings compound the problem. Stair voids, riser shafts and rooflights interrupt the layer direction, and a panel cut hard against an opening can leave a narrow remnant strip that is awkward to lift, awkward to fix and visually obvious. Working the grid so openings fall within panels rather than across joints usually costs less material than fighting it afterwards.

Rigging for a Surface, Not Just a Load

Lifting is the stage where damage actually happens, and most of it is decided by gear choices made before the crane arrives. Screw-in lifting anchors are quick but leave holes; on any panel with an exposed soffit those anchors must land in the top face, which constrains rigging geometry and sometimes forces a spreader beam where a two-leg sling would have done. Round slings need softeners at every arris, and vacuum lifters need a clean, dry, planed face to seal against — the dust that ruins the seal is the same dust that grinds into the finish.

Landing and stacking do the quieter damage. Panels set down on concrete pick up grit, and grit under the next panel scratches the face beneath, so bearers go under every stack with the exposed face protected and covered. Site cutting, where it cannot be avoided, happens off the finished deck with the offcut supported, because a panel that breaks free at the end of a cut tears fibre on the underside. Erecting a plate in driving rain and leaving it open for a week is a decision about the finish, not only the programme.

Glulam Reads Against Every Straight Line Near It

Glulam in the open competes with every straight line near it. A beam sized for strength, and even for code deflection, can still read as sagging above a level window head, because the eye compares the two edges directly and detects a deviation far smaller than any limit in the standard. Where the beam runs above a long span of glazing, the perceived defect is the difference between two lines, and it grows with span.

Precamber answers this, and it has to be ordered rather than wished for — manufactured into the member, with the direction marked, since a cambered beam installed upside down is a defect on delivery day. Deflection limits themselves vary by jurisdiction: EN 1995-1-1 (Eurocode 5: Design of timber structures — General — Common rules and rules for buildings) works in instantaneous and final deflection with a creep factor tied to service class, while the National Design Specification for Wood Construction and CSA O86 (Engineering Design in Wood) set their own frameworks. The locally adopted code and the project specification govern.

Long-term behaviour matters more here than on a covered frame. Creep under sustained load continues for years, so a beam that looks acceptable at handover can look tired later, particularly in damper service conditions. Check final deflection rather than instantaneous, and treat a marginal result as a reason to go up a section.

Connections That Have to Look Deliberate

Connections on an exposed frame face two acceptance criteria, and the second one has no tolerance written down. Concealed connectors — slotted-in plates, proprietary concealed beam hangers, glued-in rods where the design permits them — cost more and demand better setting-out, but they leave the timber reading as timber. Surface-fixed angle brackets and top-flange hangers are faster and, on a visible face, become part of the architecture whether anyone intended that or not.

Fastener discipline decides how the rest looks. Self-tapping structural screws to EN 14592 (Timber structures — Dowel-type fasteners — Requirements), or the equivalent qualification in the local code, go in at specified angle, spacing and edge distance, and on a visible face they also go in straight: a line of forty screws that wanders is a defect the client will point at even after the engineer signs it off. Templates are worth their setup time, head finish is a decision to make at order stage rather than at the box, and overdriven heads crush fibre that holds dirt, darkens first, and cannot be sanded flush without cutting deeper into the face.

Water Signs Its Own Work

Water is the only site hazard that signs its own work. End grain wicks fast, so panel ends, site cuts and unsealed CNC pockets absorb far more than a face will, and timber that has taken water at the end grain dries unevenly and checks. Floor panels are worse, because a fresh deck is a tray: rain collects at every upstand, sits, and drives moisture through the top lamella into a soffit somebody will look at.

Drainage belongs in the temporary works. Drilled drain holes at low points, sealed once the roof is on, are ordinary practice; so are factory edge sealers, taped joints, temporary membranes and, on longer programmes, a full weather-protection roof over the working level. Delivered moisture content is stated in the manufacturer's documentation — the site job is stopping it from rising, then letting it fall back slowly before the building closes.

Steel plus water plus tannin produces the stain everyone recognises. Black marks bloom around ferrous fixings, scaffold contact points and steel offcuts left lying on a wet deck, and they penetrate rather than sit on the surface. Brightening treatments sometimes lift them and sometimes only fade them. Non-ferrous or coated temporary fixings, plus a rule about what may rest on an exposed deck, prevents a class of damage that no repair fully undoes.

Concrete Tolerance Meets CNC Tolerance

Two tolerance regimes meet at the base of the first panel, and they were not written to agree. Concrete works to tolerances of the kind set out in ACI 117 (Specification for Tolerances for Concrete Construction and Materials) or an equivalent national specification; CLT is a CNC product held to the dimensional tolerances of EN 16351 or ANSI/APA PRG 320, tighter by a large factor. Setting a precision panel on an imprecise slab without measuring first transfers every hollow and hump into the frame.

Survey the substrate and pack it. Level the bearing line with shims or a proprietary levelling system, check with a laser before the first wall panel goes down, and grout or dry-pack afterwards where the design calls for it. A base left a few millimetres out shows up four storeys later as a wall that will not close on its neighbour.

Movement continues after erection. Cross-lamination restrains in-plane shrinkage, but timber still moves substantially perpendicular to grain, and platform framing stacks that movement at every floor-to-wall junction, pulling at facade fixings, risers and door heads. Where the design allowed for it, the allowance has to survive contact with site — a hard-packed joint meant to compress becomes a split later.

Every Hole Belongs in the Machine File

Every hole in an exposed panel should exist in the CNC file before the panel is machined. Cutting a service penetration through a finished soffit produces a torn edge, unsealed end grain, and a permanent record of the coordination failure behind it. Sprinkler drops, socket positions, riser openings, downlights and duct crossings all have to be resolved by the model freeze, which lands far earlier in a mass timber programme than trades expect.

Route services where they can be reached instead: raised floors, a dropped bulkhead zone, chases in a non-exposed wall face, or a designed screed zone above the panel. Where a late penetration genuinely cannot be avoided, it goes through engineering review before a drill touches the panel, because a hole interrupts layers that were carrying load and the layer positions are invisible from the face. Follow-on trades need that explained at induction, in writing — a fixer who has spent a career shooting into plasterboard soffits will treat a CLT ceiling as a nailing surface otherwise.

Char Depth Is Finish Depth

Fire design on exposed timber consumes material from the surface you are trying to protect. Where the design relies on charring — the notional charring approach of EN 1995-1-2 (Eurocode 5: Design of timber structures — Structural fire design), or the equivalent provisions of the National Design Specification — a defined sacrificial thickness is doing structural work in a fire and is not spare material. Sanding a stain out of a soffit removes some of it.

How much timber may be exposed at all varies by jurisdiction. In the US the International Building Code's mass timber construction types set exposure and encapsulation requirements, and the locally adopted edition governs; elsewhere the national fire code and the approved design strategy decide. Encapsulated areas can look identical to exposed ones on a drawing while carrying a lining requirement that no follow-on trade may defeat.

Remedial sanding beyond a light finishing pass therefore belongs on the engineer's desk: the question is whether the residual section still satisfies the fire case that got the building consented.

Protection, Records and What Counts as Repair

Protection is a programme item with a cost, and building it into the erection sequence beats bolting it on after the first complaint. Floor panels take a sacrificial covering as soon as they are landed and screwed — breathable protection over traffic routes at minimum, full coverage under wet trades. Scaffold standards and hop-ups get softened where they bear on exposed timber, and wall arrises take corner protection until handover. None of it survives without someone whose job includes replacing it after each trade moves through.

Record what arrives and what happens to it. A photograph of each panel on landing, a damage register keyed to panel numbers, and an agreed threshold for acceptable variation turn a subjective argument at handover into a documented one. Manufacturers publish repair allowances for their products — plugged holes, sanded patches, colour-matched fills — and the permitted extent is a contractual matter to settle before the first repair.

Final finishing usually belongs to a specialist pass once the wet trades are done, and its timing is the last thing left to protect: sand too early and the deck picks up dirt again, sand too late and the drying stains have set.