Rental cash damming in Canada
The most under-used legitimate tax strategy available to Canadian rental property owners.
Direct answer
Cash damming converts non-deductible personal mortgage interest into tax-deductible interest by paying all rental property expenses with a line of credit, while directing 100% of rental income against your personal mortgage principal.
It relies on paragraph 20(1)(c) of the Income Tax Act, which allows interest on money borrowed to earn income from property to be deducted. It requires a readvanceable mortgage with separate sub-accounts so borrowed funds can be traced, a dedicated rental bank account, and an accountant. It does not reduce your total debt. It changes the tax character of the interest.
On this page
The problem it solves
In Canada, interest on your personal mortgage is not deductible. Interest on money borrowed to earn income is. Most rental property owners have both kinds of debt and never think about which dollar is which.
Meanwhile the rental generates income which you use to pay the rental’s expenses. Money in one pocket, out the other. Perfectly normal, and quietly inefficient, because you’re using after-tax-relevant income to cover expenses you could have borrowed for deductibly, while your non-deductible personal mortgage sits there earning the bank interest you can’t write off.
Cash damming reroutes the plumbing. Nothing about your total wealth changes on day one. What changes is which interest is deductible and how fast the expensive debt disappears.
The mechanics
Think of it as a dam with two channels. Money that used to flow one way now flows the other.
- Every rental expense is paid from a line of credit: property taxes, insurance, repairs, condo fees, property management, utilities, and the interest on the rental’s own mortgage. That borrowing is for the purpose of earning rental income, so the interest on it is deductible.
- Every dollar of rental income goes at your personal mortgage as a principal prepayment. Not into a savings account. Not toward the rental’s costs. Straight at the non-deductible debt.
- The line of credit readvances as your personal mortgage principal falls, which funds next month’s rental expenses. The cycle sustains itself.
Over time, non-deductible mortgage debt shrinks and deductible line-of-credit debt grows. Your total debt is roughly flat. Your tax bill isn’t.
The structure you need. This is the part that fails
Cash damming is not a product you buy. It’s a routine you run on top of the right structure, and the structure has to be built before you start. Retrofitting it is painful and sometimes impossible.
- A readvanceable mortgage with separate sub-accounts. Not a single all-in-one account. CRA tracing requires that borrowed dollars for the rental never mingle with personal spending. One sub-account for personal, one for rental expenses. Never crossed.
- A dedicated rental bank account. All rental income in, and nothing else.
- A record-keeping habit. Every transaction, every month. If you can’t demonstrate the trail years later, the deduction is at risk.
- An accountant who has done this before. Not optional. I build the mortgage structure; your accountant confirms the treatment and files it correctly on Form T776.
The most common failure I see: someone reads about cash damming, uses their existing HELOC for a rental repair, pays for groceries from the same HELOC two weeks later, and has now contaminated the tracing on the entire balance.
The numbers
| Item | Annual |
|---|---|
| Rental income redirected at personal mortgage | $26,400 |
| Rental expenses paid from line of credit | $15,800 |
| Additional deductible interest generated (year 3, average balance ~$40,000 at 6%) | $2,400 |
| Tax saving on that interest at 43% | $1,032 |
| Personal mortgage principal retired ahead of schedule | $26,400 |
Illustrative only. The tax saving compounds as the deductible balance grows and the non-deductible balance shrinks. The larger effect, in most files I’ve modelled, is not the annual tax refund. It’s the years knocked off the personal mortgage by redirecting rental income at principal instead of letting it circulate.
Risks and rules you need to hear
- Total debt does not fall. You are changing its character, not eliminating it. If that makes you uncomfortable, this strategy isn’t for you, and that’s a legitimate position.
- Tracing failures are the number one killer. One mixed transaction can taint a balance. Discipline is the entire game.
- Variable rate exposure. The line of credit floats with prime.
- Rental vacancy interrupts the income leg while expenses continue.
- CRA can review historical years. Records must survive scrutiny long after you’ve stopped thinking about it.
Getting started
The sequence matters. First, confirm with your accountant that cash damming fits your situation and your rental’s expense profile. Second, put the mortgage structure in place, usually at renewal or refinance, since that’s when it costs nothing extra to build. Third, open the accounts and start the routine on the first of a month.
If you have a rental property and a personal mortgage and you’ve never had this conversation, it’s worth thirty minutes. If it doesn’t fit, I’ll tell you in the first ten.
Written and reviewed by Derrick Johnston, Mortgage Agent Level 2, BRX Mortgage Inc. (FSRA #13463). This page describes a tax strategy in general terms and is not tax or accounting advice. Implementation requires a licensed accountant. Interest deductibility depends on your specific facts and on the Income Tax Act as applied by CRA.
Straight answers
Frequently asked questions
Is cash damming legal in Canada?
Yes. Cash damming relies on paragraph 20(1)(c) of the Income Tax Act, which permits the deduction of interest on money borrowed to earn income from a business or property. The structure is well established. What determines whether it survives review is tracing: you must be able to demonstrate that the borrowed funds were used for income-earning purposes, which is why the account structure matters more than the strategy itself.
What is the difference between cash damming and the Smith Manoeuvre?
Both convert non-deductible interest into deductible interest using a readvanceable mortgage. Cash damming uses a rental property or business: you borrow to pay its expenses and use its revenue to pay down personal debt. The Smith Manoeuvre borrows against home equity to buy income-producing investments. Cash damming carries no market risk, which makes it the more conservative of the two.
Do I need a rental property for cash damming?
You need an income-earning source whose expenses you can pay with borrowed money: a rental property or an unincorporated business. Without one, there is nothing to borrow for, and no basis for deductibility.
How much can cash damming actually save?
It depends on your rental expenses and your marginal tax rate. If a rental generates $14,000 a year of deductible expenses and you are in a 43% bracket, the conversion is worth roughly $6,000 a year in tax, while the redirected rental income accelerates the payoff of your personal mortgage. Over a full amortization the combined effect is commonly six figures.
What are the risks of cash damming?
Three. First, your total debt does not fall as fast as it appears. You are shifting it from non-deductible to deductible, not eliminating it. Second, sloppy tracing can cause CRA to deny the deduction retroactively. Third, the line of credit is variable rate, so rising rates raise the cost. This strategy requires an accountant, not just a mortgage agent.
Next step
A 30-minute call tells you whether there's money on the table.
No application, no credit pull, no pitch. Bring your current mortgage balance, your renewal date, and a rough list of what you owe elsewhere. You'll leave the call knowing your options and what each one costs.