How Drawing Review Caught 47 Stack Misses on a Denver Apartment

47 Stack Misses Repeated Across a 280-Unit Building

A developer's preconstruction team was reviewing a 280-unit, five-story multifamily building. One error in a typical unit repeats on every floor. A misaligned plumbing stack in one unit type becomes a field conflict in every copy of that unit.

The set had eight unit types, mirrored and stacked. Waste-stack locations on the plumbing plans did not line up with the slab penetrations on the structural plans for several types. Demising walls also cited different UL assembly numbers on different floors.

They had three weeks before the GMP. The estimator needed a count of coordination issues before the contingency was set.

Helonic Counted 188 Field Locations From 8 Unit Types

They uploaded the full set and focused on repeating units and stack alignment through all floors.

Helonic listed 620 issues. Forty-seven waste-stack centerlines on the plumbing plans missed the slab penetrations on the structural plans. Across the repeating types and five floors, that was 188 field conflicts that would have meant core drilling or a slab change.

Fire separation used three different UL numbers for the same demising wall on different floors, 34 inconsistencies in all. Mirrored units added 89 clashes at shared plumbing walls.

See multifamily drawing review, Colorado plan review requirements, how multifamily drawing sets get reviewed, fire-rated assembly coordination, stacking analysis, and the Portland apartment review.

$210,000 in Core Drilling Kept Out of the GMP

The 47 stack misalignments went into the GMP risk register at an estimated $210,000. That is the cost of core drilling five floors, rerouting waste lines, and re-inspecting firestopping at each penetration.

They made stack alignment a condition of the GMP, so the design team owned the core drilling instead of the contractor carrying it as contingency.

Once a typical unit was corrected, the same fix applied to every copy. Review after the correction covered eight unit types, not all 280 units.

Stack misses like these are conflicts between two disciplines. Across 1,000+ drawing packages Helonic has reviewed, about 8 in 10 had at least one coordination conflict between disciplines. See what Helonic found across 1,000+ drawing reviews.

IBC Fire Partitions and the 34 Demising Wall Inconsistencies in Denver

Denver permits buildings under the Denver Building and Fire Code, which adopts the International Building Code and International Fire Code with city amendments.

The IBC requires walls between dwelling units to be fire partitions. They are generally rated at one hour, with a half-hour rating allowed in some sprinklered construction types. Each wall is built to a tested assembly, usually a UL design number, and that number is what the inspector checks in the field.

That is why three UL numbers for one demising wall matter. The framer and the drywall crew build the assembly on the sheet in front of them. If two floors cite different designs, at least one of them is wrong, and the inspector finds it after the board is hung.

Every stack that crosses a rated floor also needs a tested firestop system. A stack that misses its slab opening gets cored, and the new hole needs its own firestopping and inspection. That is part of the $210,000.

“In multifamily, one plumbing stack error becomes 280 plumbing stack errors. Helonic found 47 misalignments across eight unit types before we locked the GMP. That's $210K we didn't have to bury in contingency.”

Denver Apartment Drawing Review: Questions and Answers