Mold risk for a multifamily building
How to check mold risk for a multifamily building: score the building, then treat units separately. Stack position, shared systems and party walls make one address several risk profiles.
Step by step
Score the building address to get the shared factors
Regional moisture load, terrain and drainage, construction era, cladding and roof form are properties of the building, and they apply to every unit in it. That is 55 of the model's 100 points established in one lookup, before any unit is considered.
Split the stack by position
Top-floor units carry the roof and its flashing detail. Garden-level and basement units carry the ground, the grading and whatever the terrain module said. Middle floors carry neither and are usually the driest in the building. A single building score, applied flat across a stack, hides the two ends where the problems actually are.
Map the wet stacks and the shared systems
Plumbing runs vertically, so a supply-line failure or a waste leak on one floor shows up on the floors below before anyone on the originating floor notices. Identify which units share a stack, because they share a failure mode. The same is true of common extraction ducting and of any shared HVAC.
Check where extraction actually terminates
In multifamily this is the most common systemic defect. Bathroom and kitchen extraction ducted into a common loft, a shaft, or a soffit rather than to outside air moves moisture from one unit into shared structure. It is a building-wide problem presenting as a series of unit complaints.
Run occupant intakes across the stack, not one unit
The intake is worth up to 20 points of contribution and is the only input from inside. Run across several units it does something a single intake cannot: it separates a unit problem from a building problem. Three units on one stack reporting the same thing is a stack; three scattered units reporting it is the envelope or the ventilation.
Log humidity on both ends of the stack
One logger at the top, one at the bottom, left for a season. The difference between them tells you whether you are managing a roof, a ground condition, or a whole-building ventilation problem — which is a different capital decision in each case.
Why one address is several risk profiles
The model scores an address. In single-family housing that address is the building and the building is the dwelling, so the score maps cleanly onto what someone lives in. In multifamily it does not.
Roughly half the weight — climate, terrain, era, cladding, documented history — genuinely is shared and applies to every unit. The rest is not. A garden-level unit against a wall with no capillary break and a third-floor unit in the same building do not face the same conditions, and averaging them describes neither.
So the operationally useful output is the building score as a baseline, plus a per-unit adjustment for stack position, orientation and system exposure that only someone who knows the building can make.
What changes as a result
Capital gets spent on the systemic defect rather than on the loudest complaint. Extraction terminating in a shaft is one job that resolves a pattern of unit-level reports, and it will not surface from a maintenance queue organised by complaint.
Turnover inspections get targeted. The units worth opening up between tenancies are the ones at the ends of the stack, not the ones in rotation.
Disputes get a building-level answer. When several residents report the same thing, having already established whether it is a stack, an envelope or a ventilation problem is a materially different position from investigating it under pressure.
Cost at portfolio scale
One building is one address, so the free check covers the shared half of the picture at no cost regardless of unit count.
Paid reports are per property — $29, or $79 where a disclosure exists, which for rental stock usually it does not. Most owners buy the paid tier for buildings that surfaced high, not for the whole holding.
Volume arrangements for portfolio owners are agreed directly rather than published as a self-serve tier.
Questions people ask
- Does each unit need its own score?
- The model scores addresses, so a building gets one score covering the shared factors. Per-unit differences — stack position, orientation, system exposure — are judgements someone who knows the building layers on top.
- Which units are usually worst?
- The ends of the stack. Top floor for roof and flashing, ground and below for terrain and grading. Middle floors are typically the driest.
- One unit reports a smell. Is it a building problem?
- Run intakes on the units above and below it before assuming either way. A vertical pattern points at a plumbing stack; a scattered pattern points at ventilation or the envelope.
- Does the disclosure module help here?
- Rarely. No transaction generates a statutory form for a rental, so the largest module is usually inert and the score rests on climate, terrain and building data.