Mass Timber Scope of Work: Template and Checklist for Commercial GCs

Mass timber construction is one of the fastest-growing structural systems in commercial architecture — and one of the most technically demanding scopes to manage as a GC. Cross-laminated timber (CLT), glulam beams, nail-laminated timber (NLT), dowel-laminated timber (DLT), and mass plywood panels (MPP) require precise fabrication, tight installation tolerances, extensive moisture protection protocols, and coordinated design between the structural engineer, mass timber fabricator, and every trade that penetrates, attaches to, or sits adjacent to the timber structure. A vague mass timber scope of work will lead to fabrication conflicts, connection detail failures, and costly on-site modifications. This guide covers the complete scope.

Mass timber scopes must define the panel and member types, connection systems, fire design approach, and moisture protection strategy before the fabricator is engaged.

Mass Timber Product Types and Specifications

• Cross-laminated timber (CLT): Layered solid wood panels with alternating grain directions — the mass timber equivalent of structural plywood, but at floor and wall scale. Specify CLT stress grade per APA PRG 320 (E1 through E5 for bending about the strong axis; V grades for shear walls). Specify layup: 3-ply, 5-ply, 7-ply, and panel thickness for each application. Flame spread classification per IBC Section 2303.2: CLT is typically a heavy timber element with an inherent 2-hour fire resistance (per NDS 2018 Appendix E char rate method) without intumescent or sprayed protection.
• Glued-laminated timber (glulam): Laminated lumber beams, columns, and arches. Specify combination symbol per AITC 117 (e.g., 24F-V8 for western species floor beams; 26F-E10 for high-performance roof applications). Define whether appearance grade is Architectural (exposed), Industrial, or Framing — appearance grade drives cost significantly. Specify finish (clear sealer, oil penetrating finish, or left natural for interior exposed applications).
• Nail-laminated timber (NLT): Dimension lumber nailed flatwise on edge — the most cost-effective mass timber panel for floors, walls, and roofs. Specify lumber species (SPF, Douglas Fir), grade, and nailing pattern per design. NLT provides lower spanning capacity than CLT at the same thickness but is significantly less expensive and easier to source from local lumber yards.
• Mass plywood panel (MPP): Similar to CLT but uses structural plywood — compatible with standard plywood supply chains. Specify panel thickness and APA structural panel grade per APA E30 standard. MPP is typically used where CLT is not available within acceptable lead time.
• Dowel-laminated timber (DLT): Laminated panels using wooden dowels for adhesive-free assembly. Specify dowel spacing, species, and panel width. DLT allows the panel to be easily disassembled and is preferred for projects with decarbonization goals.

Connections and Structural System

• CLT-to-CLT connections: Half-lap, step joint, or screwed connections for panel-to-panel joins at floors, walls, and roofs. Specify self-tapping screw (STS) connectors: SPAX PowerLag, SWG Screwfix, or Rothoblaas ASSY product lines. STS screws are engineered fasteners — specify by manufacturer, length, and diameter from the structural engineer's connection schedule.
• GLT-to-CLT and GLT-to-steel connections: Concealed or exposed steel connection plates (half-lap connectors, concealed knife plate connectors, LVL header connectors). Specify the connection system type (concealed vs. exposed — has significant impact on finished appearance for exposed structure). Concealed connections require precision pocket cutting in the CLT panels — this is shop-fabricated, not field-cut.
• Mass timber-to-concrete or steel connections: Post bases, column caps, and shear wall hold-downs anchored to concrete. Specify anchor bolt diameters, embedment depths, and whether hold-down hardware is galvanized or stainless steel for interior or exterior applications.
• Acoustics: Mass timber is denser than conventional wood framing but still requires acoustic isolation at CLT floor panels. Specify the resilient underlayment or floating topping slab system designed for the required STC and IIC ratings. Acoustic design for CLT floors is a specialty engineering scope — confirm whether the acoustic consultant's specifications are included in the mass timber sub's scope.

Fire Design Approach

• Mass timber exposed: IBC 2021 Chapter 23 (Type IV-A, IV-B, IV-C, IV-HT) allows mass timber to be left exposed if the structural member dimensions meet the char rate requirements (NDS Appendix E) for the required fire-resistance rating. Type IV-A requires 2-hour fire resistance for floors and roofs (typically 5-ply CLT or heavier). Confirm the fire design basis with the structural engineer of record.
• Protected mass timber: Where exposed CLT does not achieve the required fire resistance, sprayed fire-resistive material (SFRM) or encapsulation with Type X gypsum board is required. Define whether the mass timber sub or the drywall sub is responsible for encapsulation, and who supplies and installs the gypsum board.
• Char rate method vs. empirical tables: The structural engineer must confirm whether the fire design uses the NDS Appendix E char rate method (most flexible, requires engineering calculation) or IBC empirical heavy timber requirements (prescriptive, limited to specific member sizes).

Tip for PMs: The most common mass timber scope gap is moisture protection during construction. CLT panels absorb moisture during construction, which causes dimensional changes, surface staining, and potential structural degradation if panels are saturated. A mass timber project without a formal moisture management plan — cover all exposed panels, slope for drainage, monitor moisture content, and define a remediation threshold — is accepting significant quality and warranty risk. This plan must be part of the mass timber sub's scope.

Mass timber fabrication lead times are among the longest in construction — typically 16–28 weeks from design completion to delivery. Begin the design and submittal process as early as the project schedule will allow.

Design and Fabrication Submittals

• Mass timber fabrication drawings: shop drawings showing panel layouts, piece numbers, connection pocket locations, opening locations (MEP penetrations), and edge profiles. These drawings are the fabrication instructions — they must be complete and approved before a single panel is manufactured.
• Structural engineer review: all mass timber shop drawings must be reviewed by the structural engineer of record before fabrication begins. The structural engineer confirms that the fabrication drawings match the structural design intent, including connection details and member sizes.
• Product certifications: APA PRG 320 (CLT), AITC 117 (glulam), and applicable chain-of-custody documentation (FSC, SFI) for wood sourcing compliance
• Factory quality control plan: confirmation that the fabricator's QC system monitors dimensional tolerances, moisture content (≤19% MC at time of delivery), and adhesive bond quality (delamination testing per ANSI/APA PRG 320 Section 9.3)
• Moisture management plan: a written protocol for site moisture protection during delivery, storage, and installation — including tarping, drainage board, panel moisture content monitoring frequency, and remediation criteria

Installation and Erection Plan

• Crane capacity and access: CLT panels for a typical floor bay (8m × 4m × 5-ply) weigh approximately 2,400–3,200 kg (5,300–7,000 lbs). Confirm crane capacity and reach for the largest panels. Mass timber erection is a rigging-intensive operation — require the erection sub to provide a lifting plan for each panel size.
• Temporary bracing: mass timber panels require temporary bracing during erection, before permanent connections are made. Include temporary bracing in the mass timber scope — do not assume it is excluded.
• Panel tagging and sequencing: mass timber panels are fabricated with unique piece numbers and must be delivered in the order they are to be installed. Define the delivery sequence in the purchase order — out-of-sequence delivery on a mass timber project halts erection.

Best Practices from Leading GCs

• Hold a design coordination workshop with the mass timber fabricator, structural engineer, mechanical sub, and electrical sub before fabrication begins. MEP penetrations through CLT panels must be factory-cut — field-cutting CLT panels with a circular saw is possible but should be avoided for panels in the structural system. All penetrations must be located on the fabrication drawings.
• Specify the moisture content acceptance criteria for each panel at delivery (≤19% by weight per NDS/APA). Require a moisture content test of each panel using a pin-type or capacitance meter on delivery, and document the readings. Panels delivered above the moisture content limit should be rejected or held pending drying.
• Confirm with the structural engineer whether exposed mass timber will be visible after occupancy or will be concealed. Exposed mass timber requires a higher appearance grade, protection during construction, and a finish treatment — all of which must be in the scope.

Mass timber coordinates with the structural engineer, MEP subs, the waterproofing/roofing team, and the fire protection sub — throughout the entire construction schedule.

MEP Penetration Coordination

• All MEP penetrations through CLT floor panels — plumbing drain sleeves, HVAC penetrations, electrical conduit sleeves — must be located, sized, and confirmed before the fabrication drawings are issued to production. Factory-cut openings are significantly less expensive and higher quality than field-cut openings. Build the MEP coordination deadline into the project schedule at least 4–6 weeks before the fabrication drawing submission date.
• CLT floors require an acoustic and fire topping — typically a concrete topping slab (minimum 1.5" normal-weight concrete per NDS) or a dry acoustic mat system. This topping adds load to the structure — confirm it is accounted for in the structural design. The topping also requires edge forming at panel-to-wall intersections — define who provides the forming.

Weather Protection and Envelope Coordination

• Mass timber must be protected from direct precipitation as soon as panels are installed. Coordinate with the roofing sub to install temporary weather protection (temporary roof membrane or building wrap) immediately following each level of mass timber erection. Do not allow mass timber to remain exposed to precipitation for more than 24–48 hours after erection.
• Exterior CLT walls require a weather-resistive barrier (WRB) and cladding system installed promptly after erection. CLT panels used as exterior walls must be designed with a rain screen cavity — define this in the waterproofing and cladding scopes.

Pre-Fabrication Coordination Checklist

• Structural IFC drawings confirmed — fabrication drawings based on IFC, not design development drawings
• MEP penetrations coordinated and confirmed on fabrication drawings — all subs signed off
• Structural engineer review of fabrication drawings complete before fabrication begins
• Acoustic and fire design approach confirmed — encapsulation vs. exposed mass timber
• Crane capacity and site access confirmed for panel weights
• Moisture management plan submitted and approved
• Delivery sequence plan confirmed and integrated into construction schedule
• Weather protection plan confirmed with roofing sub

Tip for Estimators: When reviewing a mass timber bid, verify that the following are included: factory-cut MEP penetrations, connection hardware (STS screws, hold-downs, connector plates), temporary bracing during erection, moisture management, and the acoustic topping slab forming. Mass timber fabrication bids that cover only panel supply and installation frequently exclude the connection hardware and temporary works that are essential to the structural erection — these can add 15–25% to the base panel cost.