Rob Smith on Cash Damming for Landlords: The Interest Write-Off Is Real, the Execution Is Where Most Fail
Rob Smith refinanced his first rental property in 2019 at 2.8%. His primary residence mortgage was sitting at 3.4%. Same borrower, same equity position, rates six-tenths apart. The spread didn't make sense until his accountant walked him through cash damming — a strategy that converts non-deductible mortgage interest into deductible interest by routing rental income through your personal mortgage and paying rental expenses via a line of credit.
The mechanics are simple on paper. Deposit all gross rental income into your personal chequing account. Use that cash to make lump-sum payments on your primary residence mortgage. Open a HELOC or LOC tied to your rental property (or your home equity, depending on structure). Pay all rental expenses — property taxes, insurance, repairs, utilities, mortgage interest on the rental itself — out of the LOC. The interest on the LOC is now tax-deductible because the borrowed funds are being used to earn rental income. Your primary mortgage shrinks faster. The LOC balance grows, but you're writing off the interest at your marginal rate.
"The math is bulletproof. The Supreme Court settled it in Singleton back in 2001. The CRA blessed it in IT-533. But I've seen three landlords blow the deduction in the last two years, and every single failure was execution, not concept."
— Rob Smith, Founder, SmithMan
The first failure mode is commingling. You cannot dump rental income into the same account you use for groceries and gas and then claim a clean paper trail. The CRA's "direct use" test requires that the borrowed funds be traceable to income-earning expenses. If your HELOC is paying for both rental repairs and a new deck on your cottage, the deduction gets denied on audit. Smith's rule: two accounts minimum. One for rental income only. One LOC for rental expenses only. No exceptions.
The second failure is bad rate math. Cash damming works when the LOC rate is close to your mortgage rate. In 2024, most HELOCs were priced at Prime + 0.5%, which sat around 7.5% while fixed mortgages renewed at 5-6%. That 150-200 basis point spread cuts hard into the tax benefit. At a 50% marginal rate, you're saving $375 per $1,000 in deductible interest, but if the LOC costs $750 and the mortgage you're paying down only costs $550, you're losing $25 after tax on every $1,000 cycled through the structure. Smith's threshold: if the spread exceeds 100 basis points, the strategy needs rental expenses above $30,000/year to generate meaningful net savings.
"I tell clients to model it in year five, not year one. The benefit compounds as your primary mortgage balance drops and your HELOC limit grows with home equity. But if rates invert against you early, you're locking in a loss for two or three years before the structure pays off."
— Rob Smith, Founder, SmithMan
The third failure is neglecting the ceiling. You can only dam as much cash as you're actually spending on the rental. If your property generates $24,000 in gross rent but only has $8,000 in operating expenses, you're capping the deductible interest at whatever accrues on an $8,000 LOC balance. For a landlord with low expenses — say, a paid-off condo with minimal repairs — the juice isn't worth the accounting fees.
Smith's breakeven case: $18,000+ in annual rental expenses, stable tenant occupancy, and a primary mortgage with at least five years of amortization remaining. Below that, the structure is technically correct but economically thin.
The strategy doesn't reduce total debt. It shifts $1 of non-deductible debt into $1 of deductible debt while keeping total leverage flat. The win is the tax refund, which for a landlord in the top bracket can run $2,000-$4,000/year on a well-structured property. That's not speculative. It's arithmetic, provided the trail is clean and the rate spread doesn't eat the benefit before it compounds.
