Cash Damming: The Mortgage Strategy Self-Employed Canadians Keep Missing

Here's a sentence that annoys almost every self-employed Canadian who hears it for the first time: the interest on your home mortgage isn't deductible, but the interest on money borrowed to run your business or your rental generally is. Same dollars, completely different tax treatment, and the difference comes down to what the borrowed money was used for.

Cash damming is the strategy that lives in that gap. It's not a loophole and it's not exotic - it's an established structure with real court history behind it. It just demands deliberate plumbing and clean records, which is why most people have never had it explained properly.

How it works, with real numbers

Say you run an unincorporated contracting business with about $4,500 a month in genuine expenses - materials, subs, fuel, insurance - and you also own a rental with $1,500 a month in costs. That's $6,000 a month of expenses you're currently paying out of income, like everyone does.

Flip the plumbing. Pay those expenses from a dedicated line of credit instead, and take the $6,000 of income that used to cover them and slam it against your home mortgage as prepayment.

Watch what happens over a year: your non-deductible mortgage shrinks by an extra $72,000, and a $72,000 balance grows on the line of credit in its place. But that borrowed $72,000 funded the business and the rental, so its interest is generally deductible. Total debt: unchanged. Tax character of the debt: transformed. Run it for five years and you've converted $360,000 of dead, after-tax-dollar debt into interest your accountant can actually work with.

What you need for it to work

• Qualifying income. Sole proprietor income or rental income are the classic fits. Salaried employment doesn't qualify - employees don't have business expenses to fund, and this whole strategy is about funding business expenses with borrowed money.
• A readvanceable mortgage. That's a mortgage paired with a line of credit whose room grows as the mortgage shrinks. Every prepayment opens matching borrowing room, so the conversion runs continuously instead of needing a refinance every year. Not every lender offers one - this is where the mortgage side gets set up right or the whole thing limps.
• Total separation. The line of credit pays business and rental expenses only. Never groceries, never a vacation, not once. Mixing personal spending into the borrowing is the classic way people wreck the deductibility. Separate accounts, full stop.
• Your accountant, in the loop from day one. We build the mortgage structure; confirming the deductions and filing them is your accountant's lane. Every setup we do assumes both of us are involved.

Who shouldn't bother

Purely salaried households - the mechanism just isn't available to you. And anyone whose bookkeeping is chaos: the strategy is only as strong as its paper trail, so fix the books first. The structure will still be here.

Run your own numbers

We built a free cash damming calculator and workbook at mudrickmortgages.com/cash. Put in your mortgage and your business or rental cash flows, and it shows the conversion year by year - how fast the non-deductible balance falls and what the structure is worth in your situation. If the numbers make you sit up, we set up the mortgage side and connect the plan with your accountant.

General information, not tax advice. Whether cash damming works for you depends on your income sources and how the flows are documented - confirm your setup with your accountant or tax professional before acting.