Two-Hour Separation Built with the Wrong Board and an Untested Acoustic Change
Six-storey mixed-use building. A 2-hour fire separation is required between a ground-floor commercial kitchen and the residential suites. The wall is 92 mm steel studs at 400 mm o.c., two layers of 15.9 mm Type X gypsum board each side, mineral-fibre batts in the cavity, and resilient channel on the suite side for acoustics, shown as a 2-hour assembly on the drawing detail. On site you find: the crew has installed one layer of 12.7 mm board each side, because the supplier shipped 12.7 mm by mistake and the foreman carried on; the resilient channel is fixed with 25 mm screws at 600 mm o.c.; there is no acoustic sealant at the base or head; a 100 mm cast-iron drain passes through the wall; and three electrical boxes sit back-to-back in the cavity. The specification's wall schedule lists a different assembly number and describes it as 1-hour.
1. What do you do about the drawing/specification conflict and the wrong board before any more board goes on?
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Stop that wall and put the area on hold. Two documents conflict, and the interim rule is that the more restrictive requirement governs: the drawings call for 2 hours, so nothing gets built to the 1-hour schedule until a written RFI answers it. I would issue the RFI the same day, attach photographs and the delivery tickets showing the 12.7 mm board, and get the answer from the consultant who sealed the drawing, not from the supplier or the foreman's recollection.
The board itself is a non-conformance, not a substitution to be absorbed. A fire-resistance rating belongs to a specific tested design - the number of layers, the thickness, the stud type and spacing, the insulation, the fasteners and any resilient channel are all part of what was tested. One layer of 12.7 mm is not half of two layers of 15.9 mm, and adding a second layer of 12.7 mm does not reconstruct the tested assembly either. The channel detail also matters: many rated acoustic assemblies are listed with resilient channel and many are not, and the screw pattern and perimeter sealant are part of the listing.
So: remove nothing until the RFI is answered, but do not cover it up. Mark the wall, record the installed condition, and be prepared for the board to come off so the stud cavity, the penetrations and the channel can be inspected before the correct assembly goes back. Also check whether the electrical and plumbing rough-in was done assuming the finished wall thickness you now have.
2. How do you select and install fasteners for the board and the resilient channel so the assembly is not short-circuited?
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Work from the tested design, but the governing principles are straightforward. The resilient channel is fixed to the studs with screws driven through the channel to the stud, at the spacing the listing gives (commonly 400–600 mm o.c., and always at each stud, not just between). The channel bears on the stud; that is the point of it.
The board is then screwed to the channel, not to the stud. Screw length has to engage the channel by the required amount without the point reaching the stud, because a screw that contacts the stud ties the board rigidly to the frame and destroys the decoupling - the wall may still stand, but the acoustic rating it was designed for is gone, and you will not see the failure.
For the first layer over channel, that means a short screw sized to the channel gauge and board thickness; for the second layer, a longer screw that engages the channel through both layers. Screws go at the listed spacing and are driven so the head sits just below the paper without breaking it - over-driven screws are a common source of both fire and acoustic failure. Two layers means the joints are staggered between layers and the second layer runs are offset from the first. Perimeter: continuous acoustic sealant at the base and head, applied to a clean substrate and tooled, with no gaps behind the channel. If the listing requires it, the sealant goes on before the board, not after.
3. How are the drain penetration and the back-to-back electrical boxes handled in a 2-hour separation?
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Both are holes in a rated assembly if they are left untreated, and a rating is only as good as its weakest penetration.
The drain: the annular space between the pipe and the wall must be closed with a firestop system listed for that pipe material, that wall type and that annular gap - cast iron, plastic and copper each behave differently under fire, and a generic intumescent sealant is not a substitute for a listed system. The installation has to be recorded with the product, the gap size and the depth, because a future inspector will ask. I would also confirm the wall framing around the pipe has not been cut away to make room; if the stud spacing was altered, the assembly needs a design check.
Back-to-back boxes in a fire separation are a recognised weak point: heat passes through the metal boxes, and the wall can fail locally in a fraction of the rated time even though the rest of the wall survives. The typical solution is a listed box protector or putty pad around the back of each box, installed to the protector manufacturer's instructions, plus keeping the boxes offset where the design calls for it. If the assembly's listing does not cover electrical boxes at that location, the boxes need to be relocated or a listed detail obtained. Nothing here is decided by the trades on site - the assembly number on the drawings determines what is acceptable, which is one more reason the 1-hour/2-hour conflict has to be closed out before this wall is finished.