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Every well-built house starts with a solid foundation, with level and true structural and load-bearing walls and accurately calculated point loads.

Then someone renovates it. They cut out joists - or even beams - and compromise the structure. They make "educated" guesses about structural members, or about what the current load on a wall is, or decide that some other wall can take a lot heavier load.

For a time, you might not even notice, but the house is affected. And it might even undergo a second renovation - with more of the same changes made to the structure. What you end up with are cracks in the plaster, buckling drywall, windows and doors that stick, or worse - a house that leans, with floors that sag and slope. It may even fall down.

Trust the experts

A lot of people want to renovate their houses by taking down walls to create an open-concept floor plan. But what walls can you take down safely? I don't know how often I've been asked by people thinking about a renovation, "How do I know if it's structural?" The truth is, you don't. A good contractor does. An architect does. A structural engineer does.

Older houses have pretty straightforward structural layouts that are not hard to figure out. The exterior walls support the perimeter, a beam supported by posts that you can see in the basement runs down the middle of the house, and the joists all run in the same direction from side to side.

You can expect that the load-bearing walls are the outside ones, as well as the walls that are directly above the beam in the basement. In the construction, the load was taken right down to the footing at the base of the foundation wall.

That doesn't mean you can assume that your home has a solid structure just because it's older. I've seen some older houses that have terrible footings - sometimes they're just rubble or fieldstone foundations that are crumbling and falling in.

In newer houses, the floor plans are more complicated and structural plans are far more sophisticated.

Beams in the basement are not just run down the middle of the house; sometimes a basement beam is installed to carry a point load only, and the real bearing takes another direction entirely on the second floor.

Cantilevering - not that common in older homes - is used more and more today to carry loads in different directions, and these loads have to be carefully engineered.

Material costs are rising and builders often reduce the size of beams and posts to the minimum required dimension, and rely on a complex web of load-bearing members to do the structural work.

It's just not that simple to figure out where the bearing is any more, or how much additional load you can add to any structure.

Exterior walls can have so much glass in them they don't function as primary load-bearing walls. In open-concept designs, there are beams where walls used to be.

There are flush beams you can't see because they are inside the floor systems that carry loads where you think there are none.

Complicated designs

What do you think happens when complicated designs like this meet with an underground, unqualified contractor?

You house may look great in the end, but you never know what's underneath - and unless you are experienced, you won't know what to look for. I came across one bad renovation where another contractor had cut out three triple 2-by-10s to run some new ducting to an addition. I'm surprised the house didn't fall down!

Recently, I was doing a renovation on a house that had started out as a bungalow and had a second floor added several years ago. When I removed the drywall and exposed the beam that held up half the second floor, I found the 14-foot, 10-inch steel beam sitting on crumbling brick with no more than two inches of load bearing. That was scary.

I've seen beams that are just barely sitting in their beam pockets. Where there should be four to six inches of beam sitting firmly on the concrete or steel plate or wood post, I've seen as little as half an inch, on a cracked and weakened concrete block.

It's not easy to catch because the beam pocket is always either covered with drywall or - in the case of a basement beam - cemented over.

Get an assessment

If you're planning any renovation that might have an impact on the structural integrity of the house, you should get a proper structural assessment by a qualified engineer before you start.

You have to know exactly where the structural components of your house stand before you go forward with a major renovation.

It helps if you have the plans of the original house and the plans for any subsequent renovations, but don't assume that the house was built according to those plans.

They are a guideline only, and an engineer's assessment is still necessary. The money you spend is well worth the comfort of knowing what you've got, and that you did it right.

You may get bad news and find you need to beef up your footings before you can finish your renovation. You may find - scary but true - that your house is in danger. The previous renovations may have compromised the structure to the point where you'll have to put your plans on hold until you have properly resupported your house.

Count yourself lucky. Better you know now and fix it before you spend thousands of dollars on a renovation and the house falls down.

Mike Holmes is the host of Holmes on Homes on HGTV.

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