How much to reserve per rental unit for capital replacements

A property investor we work with owned 8 units in a 25-year-old apartment building. He had been funding his CapEx (capital expenditure) reserve at $60 per door per month for five years. The account held $28,800, which looked adequate.

A roofing contractor walked the building and quoted $48,000 to replace the original asphalt shingles. The reserve covered barely more than half. He paid $19,200 out of operating cash.

The roof was not a surprise. Asphalt shingles carry a 20- to 25-year expected life. The building was 25 years old. The reserve was underfunded because $60 per door was never calculated against the systems it was meant to fund.

What a CapEx reserve is for

A CapEx reserve is money set aside each month to fund major, non-routine replacements: roofs, HVAC systems, water heaters, appliances, windows, and plumbing. These are not repairs. A repair restores something that still functions. A CapEx replacement retires a system at the end of its useful life.

Because every major building system has a known typical lifespan, the monthly reserve per door is calculable, not a guess. The formula is: replacement cost divided by useful life in years, divided by 12. A $7,200 roof with a 20-year life needs $30 per door per month. A $5,000 HVAC system with a 15-year life needs $28. Add those across all systems and you have a funded reserve. Most investors pick a flat number instead.

Why flat rates come up short

No formula behind the number. A figure like $60 per door is an average across properties of different ages, conditions, and climates. Your building may carry systems that are 5 years from replacement or 15. An average tells you nothing about what your specific building will need or when.

Building age not accounted for. A flat rate treats a 10-year-old building the same as a 30-year-old one. If the original HVAC units are 20 years old, every year without replacement is a year closer to a $4,800-per-unit bill. The reserve should reflect the actual remaining useful life of each system in your specific buildings.

No record of system ages. The calculation depends on knowing when each major system was installed and last replaced. Most investors do not maintain a system log. When a system fails, it is treated as an emergency rather than a scheduled replacement that was overdue.

Reserves pooled across all properties. Investors who run one reserve account across multiple buildings pool risk across properties with different ages and replacement schedules. A recently renovated building ends up subsidizing an older one. When the older building needs a roof, the reserve looks healthy, but the money was never allocated to that specific property.

What a calculated reserve looks like

Here is a per-door reserve calculation for a small multifamily unit with all systems at the start of their useful life, at current replacement costs.

System	Replacement cost	Useful life	Monthly reserve per door
Roof and gutters	$7,200	20 years	$30
HVAC (heating and cooling)	$5,000	15 years	$28
Water heater	$1,100	12 years	$8
Kitchen appliances	$2,500	10 years	$21
Windows and exterior doors	$4,000	25 years	$13
Plumbing and electrical	$4,500	30 years	$13
Total per door			$113

This is the baseline for a property with recently replaced systems. On a building where the roof has 5 years remaining instead of 20, the roof reserve alone rises from $30 to $120 per door per month, because the same $7,200 is now divided by 5 years. For a 25-year-old building with original systems, a fully funded reserve commonly reaches $175 to $200 per door per month. The $60 flat rate covers roughly half of what a mid-age property actually requires.

Why this matters

Cash demands arrive at the wrong time. When a major system fails without a funded reserve, the cost comes from operating cash. A single roof on an 8-unit building can absorb six to nine months of net operating income. A year when two or three systems need replacement can eliminate annual cash flow entirely.

The Profit and Loss report becomes misleading. A property contributing $60 per door per month to reserves shows a higher net income than one contributing $113, because the lower number is a smaller line item. The books do not record deferred capital spending. When the replacement arrives, it hits as a cash bill that was invisible in prior-period reporting.

An investor using the Profit and Loss report (P&L) to decide whether to hold, refinance, or sell is making decisions on incomplete numbers if the reserve has been underfunded.

What proper reserve accounting looks like

For the real estate clients we work with, every property has a CapEx schedule: a list of each major system, its installation date, estimated useful life, and current replacement cost. The monthly reserve is calculated from that schedule, not from a rule of thumb.

When a system is replaced, the schedule is updated and the reserve is recalculated at today’s costs with a fresh useful-life clock. The reserve for each property sits in its own sub-account, separate from operating funds.

The $19,200 out-of-pocket shortfall in the opening scenario is the direct cost of running a flat rate for five years instead of running the calculation once.

Best practices for rental investors

• Build a system log at every acquisition. Record the installation date and estimated useful life for the roof, HVAC, water heater, appliances, windows, and plumbing. Update it when replacements occur.
• Calculate the per-door reserve from the system log, not a general guideline. The right number depends on the remaining useful life of systems in your specific buildings.
• Keep reserves in a separate account per property. A pooled reserve makes it impossible to know whether any individual building is adequately funded.
• Recalibrate the reserve once a year. Remaining useful life decreases with each passing year, and replacement costs shift with the market.
• Factor the calculated reserve into every acquisition analysis. If a property requires $150 per door per month in reserves and you model $75, the projected return is overstated.

Three questions worth asking

If you own rental property, three questions for whoever handles your books:

1. Is the monthly CapEx reserve based on a system-by-system calculation, or is it a flat rate applied the same way to every property?
2. Does each property have its own reserve account, or is the reserve pooled across the portfolio?
3. Do we have a log of when the roof, HVAC, and water heater on each property were last replaced, and does the current reserve reflect the remaining useful life on each system?

If the reserve is a flat rate, the per-system calculation has not been done. The correct number depends on the age and condition of the systems in each building you own.

Get a second look

If you want to know whether your reserves match what your buildings will actually need, send us a property list and any notes on system ages. We will review the calculation and tell you whether the number in your books is adequate, or whether there is a gap worth correcting before the next replacement arrives.