The problem cash damming solves
If you own a rental property in BC, the interest on the mortgage attached to that rental is tax-deductible against rental income. The interest on the mortgage attached to your principal residence is not.
Most landlords end up in the worst-case structure: a big non-deductible mortgage on their Vancouver or Burnaby home, and a smaller deductible mortgage on a rental that already had decades of paydown. Cash damming gradually flips that — moving deductibility toward the principal-residence mortgage without selling anything.
How it works mechanically
You need three things: a rental property with positive cash flow, a readvanceable mortgage on your principal residence with a HELOC component, and discipline.
Each month, you use your rental income to pay down your principal-residence mortgage. Then you borrow that same amount back from the HELOC to pay the rental property's operating expenses (mortgage interest, property tax, insurance, repairs, strata fees).
The CRA's interest-deductibility test is about use of funds: money borrowed to earn property income is deductible. By routing every rental dollar through your principal-residence mortgage and re-borrowing for rental expenses, you create a steadily growing pool of deductible HELOC interest while shrinking your non-deductible principal-residence balance.
What it actually saves a Vancouver landlord
On a $900K principal-residence mortgage and a single Burnaby or East-Van rental generating $4,000/month, full cash damming over 5–7 years can convert tens of thousands of dollars of interest from non-deductible to deductible — typically $4K–$10K/year in tax savings at BC's higher marginal brackets.
It compounds quietly: every year the deductible portion is bigger and the non-deductible portion is smaller.
Where landlords get it wrong
The CRA cares about clean tracing. Co-mingling personal and rental cash in the same account, paying a rental expense with a personal credit card, or pulling HELOC funds for anything non-rental breaks the chain and can disqualify the deduction.
We set our cash-damming files up with a dedicated rental operating account, a dedicated HELOC sub-account, and a monthly cycle the client can run on autopilot. We also coordinate with the client's accountant so the T776 lines up with the trace.
FAQ
Do I need to refinance to start cash damming?+
Not usually — but you do need a readvanceable mortgage with a HELOC sub-account. If your current mortgage doesn't have one, the next renewal is the right moment to switch lenders.
Does cash damming work on a Surrey or Langley rental?+
Yes. It works on any Canadian rental property as long as the rental generates real income and you trace the funds correctly. The savings scale with how big the non-deductible balance on your home is.
Is cash damming a CRA grey area?+
No — it's a well-established strategy built on plain-language interest-deductibility rules. The risk is execution: sloppy tracing, not the strategy itself.