You should think very long and very hard before taking out a 40-year loan to buy a house.

And those 50-year loans some lenders began offering a couple of years ago?

Don't do it. Ever.

We know that's harsh. But if we don't tell you, who will?

We feel your pain, particularly if you live along much of the East and West coasts where home prices are high. How does a middle class family without the cash for a big down payment afford a home in Orange County, California, where the median price for a single-family house is $517,000?

Th extra 10 or 20 years won't reduce your monthly payments all that much. You'll pay so much interest to the bank, and your equity will grow so slowly, that your home will never be a good investment, helping you to save and build wealth.

Just look at how much more it costs to borrow $100,000 and pay it back over 40 or 50 years, instead of the traditional 15 or 30 years.

We'll begin with a basic fact about the mortgage business: The longer the loan, the higher the interest rate. Why? Because 40- or 50-year loans are riskier and lenders have to wait longer to be repaid.

With good credit and a suitable down payment, you could probably get a $100,000 mortgage for 15 years at 5.7%. That same loan over 30 years would probably cost you 6%. For 40 years, it would be 6.25%.

Here's what your monthly payment would be for interest and the principal -- taxes, insurance or any fees or assessments are extra -- and the total amount you'd pay in interest over the life of the loan:

• $828 a month for a 15-year loan and $48,992 in interest.
• $600 a month for a 30-year loan and $115,838 in interest.
• $568 a month for a 40-year loan and $172,515 in interest.

There is a certain amount of industry speculation about 50-year mortgages. If they do become common, no one knows for sure what sort of interest they would require.

If they were a quarter-point higher than a 40-year loan, the monthly payment on $100,000 would be $564 -- just $4 less per month than the 40-year loan. But the interest paid would jump to a whopping $238,230.

So, while the difference between a 15- and a 30-year loan is a significant $228 a month, the difference between the monthly payments on a 30- and a 40-year mortgage is only $32 ... and we'll ignore the 50-year loan.

"How much is the payment?" is invariably the first thing a prospective borrower will ask, but another important question is, "How much paid equity are you getting for your money?" In other words, at the end of the first year, how much of the home is yours? How about after the fifth year?

Historically, homes have increased in value, but for the past couple of years home values have fallen in all parts of the country. So building equity, if you can, is totally dependent on how much you reduce the principal every month by making payments. After one year you would have:

• $4,345 in paid equity with a 15-year mortgage.
• $1,228 with a 30-year mortgage.
• $580 with a 40-year mortgage.

The difference is even more striking after five years, when you'd have:

• $24,442 in paid equity with a 15-year loan and paid off close to 25% of the $100,000 you borrowed.
• $6,946 with a 30-year loan and paid off close to 7% of the principal.
40- and 50-year loans don't lower your mortgage payments very much and create home equity too slowly:
• $3,290 with a 40-year loan and paid off a little more than 3%.

There are many emotional and personal reasons for buying a house. But financial considerations should play a big part in the decision, too.

A home should become your foundation for building wealth. When the numbers add up, a home should quickly become one of the most valuable things you own. The equity you create by paying off the loan is a type of savings that you can use to send your kids to college or ensure a comfortable retirement.

The numbers don't add up for 40- and 50-year loans. You don't build wealth because you have to give far too much of what should be your money to the bank. And then it becomes their money.

By Stef Donev

Interest.com Contributing Editor

and Mike Sante

Interest.com Managing Editor

Whether you're buying a home or refinancing an existing mortgage, we have a mortgage calculator that can help you make the right decisions.