Mass Timber Drawing Review: What's Different

Mass Timber Is Not What You Know

When a design team switches from steel or concrete to mass timber—typically cross-laminated timber (CLT) or glue-laminated lumber (GLT)—the structural system looks familiar at first glance. Large floor panels, vertical columns, moment-resisting frames: the geometry is recognizable. But the fabrication, tolerances, connection details, and field assembly create drawing review problems that don't exist in steel or concrete.

The core issue: mass timber is a prefabricated product with tight manufacturing tolerances (typically ±1/4" on critical dimensions), but it's assembled on site using bolted connections that require precision alignment during erection. Unlike bolted steel, where shims and connection plates absorb minor misalignments, mass timber connections are often pinned or moment-resisting assemblies where slot sizes and bolt hole locations must stack within fractions of an inch. The drawings must account for expected tolerances from the fabricator, field measurement accuracy, and erection sequence—or the project faces coordination failures, connection rework, and schedule delays.

Connection Details: The #1 Source of RFIs

In steel construction, a bolted connection is designed by the structural engineer and detailed on the structural drawings with references to standard connection designs. In mass timber, the connection is often a hybrid: timber elements connected to steel moment connections or moment-resisting gusset plates, or timber-to-timber moment connections using large bolts and dowels. These hybrids create drawing review risks that inexperienced teams miss.

• Inadequate connection drawing dimensions: The engineer shows the timber member, the steel plate, and the bolts, but doesn't clearly dimension the bolt hole locations relative to timber edges or connection plate edges. When the fabricator and erection team receive the drawings, slot locations vary by ±1/2" or more because the reference points weren't explicit.
• Under-specified edge distances: Timber bolt connections require minimum edge distances (typically 1.5× bolt diameter per NDS standards). Drawings sometimes show insufficient edge distance or don't specify which standard applies, creating submittals requesting clarification and delays to fabrication scheduling.
• Missing moment-connection capacity notes: When a bolted moment connection in timber is assumed to achieve moment resistance, the engineer must specify the connection type (welded gusset, bolted moment plate, doweled connection) and capacity notes on the drawings. Vague connection details lead to contractor redesigns during fabrication.
• Unclear fastener material and finish: Are the bolts A325 or A490? Stainless for corrosion resistance, or standard steel? What finish? Timber is hygroscopic (absorbs water)—corrosion protection matters. Drawings without explicit fastener specifications lead to submittals and delays.
• Bearing plate design overlooked: Timber bearing on steel plates concentrates stress. The bearing plate thickness, dimensions, and anchor bolts must be detailed on structural drawings, not left to the fabricator to assume. Missing details = RFI delays.

Dimensional Tolerance Coordination

Mass timber tolerances are tighter than wood-frame but looser than steel. The structural drawings must communicate tolerance expectations to the fabricator, and the MEP drawings must account for typical erection variances when routing systems around timber members.

A structural engineer might specify ±1/4" panel length tolerance from the CLT manufacturer. But when that panel is assembled on site with other panels and connected via bolted assembly, the cumulative tolerance from multiple connections can push a member location to ±5/8" from its nominal position. MEP systems routed 2" away from the nominal timber edge may encounter the timber if erection tolerances stack unfavorably.

The solution: MEP coordination for mass timber projects must explicitly account for tolerance stacks. Mechanical ductwork routed past timber beams needs 3–4" of clearance (not 2") to accommodate realistic erection variances. Electrical conduit and plumbing branches routed parallel to CLT members need similar buffer zones. Drawings without explicit tolerance callouts create field coordination failures.

MEP Routing and Penetrations: CLT-Specific Issues

Cross-laminated timber (CLT) panels are typically 3.5" to 11" thick solid timber. Mechanical, electrical, and plumbing systems cannot pass through them like they would through concrete or steel decks. Every penetration for ductwork, pipe, or conduit requires a floor opening that must be coordinated in advance.

• Floor opening coordination failures: Drawings show floor plan with structural grid but don't clearly identify which systems need floor openings and where. The MEP team discovers during coordination that ductwork, plumbing drain stacks, and electrical chases all need openings in the same floor bay, creating congestion. Redesign during fabrication = delay and cost.
• Inadequate opening sizing: Ductwork penetrating CLT requires openings that account for the duct size plus 1" clearance on all sides (per most building codes) plus rough framing. A 20"×12" duct needs a 22"×14" rough opening. Drawings that show the opening as duct-sized (not including the framing allowance) create shop drawing revisions.
• Opening edge reinforcement not specified: When an opening is cut in CLT, the exposed plies must be sealed or edge-banded to prevent moisture ingress. The drawings should specify edge treatment (plywood blockout, sealant, or blocking) but often don't, leading to submittals and interpretation disputes.
• Fire-rated opening protection: If the floor must maintain a fire rating, openings for MEP systems require fire-rated sleeves or sealant systems. Drawings without explicit fire rating callouts on opening details create RFI loops. Specifying opening details as "coordinated between contractor and MEP" is default RFI language.
• Conduit chases in solid timber: Unlike concrete floors, CLT panels cannot accommodate embedded conduit. All electrical conduit must be routed below the panel (in a ceiling space or in surface-mounted channels). Drawings that don't explicitly address how electrical systems bypass CLT members create field rework.

Shrinkage and Moisture Movement

Solid timber shrinks as it dries. CLT and GLT undergo dimensional changes of 0.1–0.3% perpendicular to the grain and 0.01–0.05% parallel to the grain as moisture content changes from freshly sawn (20–25% moisture content) to equilibrium (8–12% in most climates). Over a 40-foot span, 0.1% shrinkage equals 0.48". Over the height of an 8-story mass timber building, cumulative vertical shrinkage can reach 1.5–2".

Structural engineers account for shrinkage with adjustment factors and mechanical connections designed to accommodate movement. But MEP systems connected to timber members or routed parallel to them must also accommodate shrinkage. Structural drawings should explicitly note expected shrinkage at key elevation changes. MEP review of structural drawings must identify shrinkage callouts and confirm that MEP systems have adequate relief (gap spacing, flexible connections, expansion loops) where timber-to-fixed connections occur.

Common failures: rigid ductwork connections to timber members that fail as the timber shrinks, plumbing connections that separate due to differential settlement, electrical conduit that becomes too tight as timber members move. Drawings without explicit shrinkage notes create field problems that appear months after occupancy when timber has reached equilibrium moisture.

The Takeaway

Mass timber projects require drawing review checkpoints that don't apply to steel or concrete. Connection details must be explicit about edge distances, fastener types, and tolerance acceptance. Drawings must account for realistic fabrication and erection tolerances—not ideal tolerances. MEP systems must be coordinated with the assumption that timber members will shift ±1/2" from their nominal location and that ductwork penetrations require advance planning and opening sizing. Drawings that treat mass timber like wood-frame or steel create RFI cascades and field surprises.