Check that the topping type, thickness, reinforcement, and acoustic mat agree across the structural, architectural, and MEP documents, and that the structural loads use the same topping. Then confirm the timber will be dry and protected before the topping is poured, because a topping placed over wet panels can trap moisture.
What the topping is for
WoodWorks’ expert tip Design Considerations for Poured Toppings on Mass Timber Floor Panels explains that toppings add mass for acoustics, can provide or support a fire-resistance rating where panel gaps exist, can act as the diaphragm on high-seismic projects, and can carry conduit or radiant tubing. Gypsum and lightweight concrete toppings are not durable finish surfaces and need flooring over them; normal-weight concrete can be polished and exposed.
The same tip gives typical thicknesses of 1.5 to 3.0 inches for normal-weight concrete and 1.5 or 2 inches for gypsum. It notes that a 3-inch normal-weight slab adds about 37.5 psf, roughly triple the weight of a 5-ply CLT panel at about 18 psf, which affects seismic loads and foundations. Sources verified October 22, 2026.
Where the topping appears in the set
| Item | Where it appears | What must agree |
|---|---|---|
| Topping type and thickness | Floor assembly types, structural general notes, specifications | The same material and thickness everywhere, including the dead load the structure was designed for. |
| Acoustic mat | Floor assembly types, acoustic report, specifications | The mat product and its position under the topping, and no attachments that bridge it. |
| Reinforcement | Structural details and specifications | Rebar, mesh, or fiber, and whether the topping acts as the diaphragm. |
| Composite action | Structural drawings | Whether the topping is designed as timber-concrete composite, and its shear connectors and shoring. |
| Embedded utilities | Electrical and mechanical plans | Conduit or radiant tubing that fits within the topping and acoustic design. |
| Control joints | Structural or architectural floor plans | Joint layout aligned with the timber beams and girders below. |
| Finished floor elevation | Architectural sections, structural top-of-panel elevations | Top of panel plus mat plus topping equals the finished floor shown. |
Eight topping checks
- One assembly. Compare each floor assembly type with the structural notes and the specifications. A topping thickness that changes between them changes the weight, the acoustics, and the floor level.
- Design load. Confirm the dead load in the structural notes matches the topping actually shown.
- Shoring. If the topping is a timber-concrete composite, check that the documents say whether the panels must be shored until the topping cures, and that the schedule allows for it.
- Penetrations. Check that floor penetrations and sleeves pass through both the panel and the topping, and that firestopping covers the whole assembly.
- Embedded conduit. Confirm conduit or tubing in the topping fits its thickness. WoodWorks notes that embedding electrical reduces future flexibility.
- Joint layout. Compare control joint locations with the beams and girders below.
- Moisture limit. Check that the specifications set a maximum timber moisture content before the mat and topping go down. WoodWorks gives 16% as an example limit.
- Pour sequence. Confirm the topping is not scheduled before the floor is protected by watertight walls or a watertight level above, as the WoodWorks moisture guidance advises.
Moisture is the timing problem
The WoodWorks moisture management guidance warns that concrete toppings and acoustic mats can trap moisture at the top of a mass timber panel, and that drying can take up to a year or longer. It recommends adjusting the sequence so toppings are placed only after the panels are dry and protected. That makes the pour date a coordination item between the structural, architectural, and construction schedules.
Published example: floor elevations that do not match
This finding comes from a mixed-use residential project that was not mass timber. In the published floor-elevation finding, structural and mechanical plans showed different elevations for the same floor area.
On a mass timber floor, the topping and acoustic mat sit between the top of the panel and the finished floor. If one discipline uses the panel elevation and another uses the finished floor, the difference can look like an error or hide one. Confirm which datum each sheet uses before you compare them.
Where Groundbook fits
Upload the architectural, structural, and MEP drawings with the relevant specification sections, and run a cross-discipline review. Groundbook flags potential disagreements, such as an opening shown on an MEP plan but missing from the structural panel layout, with links to each source sheet. Your team decides what each finding means for the fabrication package. Groundbook does not check structural capacity, fire ratings, or the fabricator’s model.