Vacancy and capex reserves: the math that keeps you solvent

Most rental 'cash flow' that disappears doesn't vanish into bad tenants or bad markets — it vanishes into two line items the owner never budgeted: vacancy and capital expenditures. A rental that clears $300/month on a spreadsheet with zero vacancy and zero capex isn't cash-flowing; it's borrowing from a future that always collects. Reserves aren't pessimism. They're the price of the asset, paid monthly instead of all at once.

Vacancy: a guaranteed expense on a random schedule

Every rental sits empty sometimes — between tenants, during make-ready, during the occasional eviction. Budget it as a percentage of gross rent: 5% (about 18 days a year) for strong markets with sticky tenants, 8–10% for average situations, more for student rentals and rough neighborhoods. The subtle part: vacancy actually lives at turnover. A tenant who stays four years at slightly-below-market rent usually beats maximizing rent with a move-out every 18 months, because each turnover costs a month of rent plus make-ready. Retention is a vacancy strategy.

Capex: every component is a countdown timer

Capital expenditures are the big, rare replacements — roof, HVAC, water heater, flooring, appliances. The honest way to budget them is a component schedule: replacement cost divided by remaining life, summed. Do this once and it will change how you shop for properties.

• Roof: $14,000 ÷ 25 years = $47/month
• HVAC: $9,000 ÷ 18 years = $42/month
• Water heater: $2,000 ÷ 11 years = $15/month
• Flooring: $6,000 ÷ 12 years = $42/month
• Appliances: $3,500 ÷ 11 years = $27/month
• Exterior paint/siding, windows, driveway, plumbing and electrical allowances: ~$75/month combined
• Typical single-family total: $200–275/month — regardless of what the rent is.

How much cash to actually hold

1. Baseline reserve per property: 3–6 months of full carrying costs (mortgage, taxes, insurance, utilities) — this absorbs vacancy shocks and evictions.
2. Plus a capex floor: at least the single biggest near-term component. If the roof is at year 22, that's a roof's worth of cash, not a percentage.
3. Fund it monthly: transfer the vacancy percentage and the capex number into a separate high-yield savings account. Across a portfolio, one pooled account works — four properties can share reserves that would overfund each individually.
4. Adjust for age: a 2015 build can run near the bottom of the ranges for a decade; a 1962 house with original systems should run 1.5–2x the schedule until the big items are replaced.
5. Replenish before you spend: after a reserve draw, cash flow refills the account before it returns to your pocket.

The bottom line

Vacancy runs 5–10% of rent; capex runs $200–275/month on a typical single-family — before the property earns you anything. Budget both as monthly expenses, hold 3–6 months of carrying costs plus your biggest looming component in cash, and underwrite every purchase with these numbers in the model. Rentals don't surprise owners who do this math; they just occasionally invoice them for exactly what the schedule predicted.

The reserve budget at a glance

Why owners skip this — and how to make it automatic

Nobody under-reserves out of ignorance alone; they do it because the money is invisible until it is urgent. The fix is mechanical, not motivational. Open a separate high-yield savings account per portfolio, set an automatic transfer for the vacancy percentage plus the capex number on the same day rent lands, and treat that transfer with the same seriousness as the mortgage draft. The behavioral trick matters: money that never touches your checking account never gets spent on a kitchen remodel at home. After two or three years the account holds five figures, earns real interest, and converts the scariest events in landlording into logistics. A roof failure with $18,000 sitting in the reserve account is a contractor scheduling problem; the same roof with no reserve is a 24% APR credit card balance, a forced sale, or a hard-money loan against your own equity. Same shingles, wildly different outcomes — and the only difference was a recurring transfer set up years earlier.

Reserves also change how you buy. An investor holding proper reserves negotiates repairs from strength, waits out slow leasing seasons without panic-discounting, and can say yes to the occasional genuine bargain that requires closing in two weeks. The reserve account is not idle money — it is optionality earning 4%, and over an investing career the deals it lets you take (and the desperate sales it lets you avoid) are worth far more than the interest. If this article changes one behavior, make it this: tonight, open the savings account, name it after the property, and schedule the transfer. Everything else in rental investing is easier to fix later than an empty reserve fund in a bad month — and no spreadsheet insight, however clever, has ever replaced one. Reserves are the least glamorous line in real estate and the most reliable predictor of who is still a landlord in ten years.