Most architects do not have the professional engineering license needed to sign off on a structural design. Typically, if a design is at all unusual, or there is a question about the loads, or a building department wants more assurance about a particular design, a structural engineer is called in to do a review, perform calculations, make possible revisions, and sign off. The architect is responsible for the aesthetics and the structural engineer's responsibility is to make sure the building won't fall down.

In our area we have heavy snow loads. This means that many plans need to be gone over by a structural engineer.

Small simple plans don't usually need this. Large rooms with vaulted ceilings, cantilevers, new building materials can all be things that may require this.

I think many architects employ a structural engineer to check their work. You are right that architects should know how to design a building that won't fall down, but the structural engineer does the math. We had a structural engineer review our plan and design our retaining walls before permitting. Because of the open arches of the carport below our screen porch, the structural engineer recommended and designed a steel frame for the far end of the carport. During the build, we decided to leave the screen porch ceiling open (vaulted) instead of having a flat ceiling. The architect called out the engineer again to see what kind of reenforcements we would need to keep from lifting the porch up if the wind caught it. So, as you can see, the structural engineer does more than what one would expect and archtiect to do.

You are correct, we have the classes in college, we have a test to pass on engineering, but it is fairly basic.
We are required to have a 5 year degree, a series of years of interning and then passing 7 tests. However the education is based on a very wide range of things from engineering, detailing, drawing, modeling, spacial planning, design, etc. Engineering degrees focus 4 years on just that...structural engineering.
However on many houses, architects should be able to design the structure depending on the shape and complexity of the house.
However there is a lot of responsibility and risk involved in engineering, and that is typically why we employ engineers. We shed that risk onto them, however since they work as our consultant, we are both still ultimately at fault should there be issues. A lot also comes down to your building department. They may require a stamp from both an engineer and architect. Most commercial projects we do do require that.
Depending on what needs engineering on your house, it will most likely not amount to that large of an extra charge.

Thank you all for your input. They are very helpful

The issue often hinges on the laws of the state where the project is located. In most states, 1 & 2 family houses that conform to the structural requirements and span tables of the applicable 1 & 2 family dwelling code do not need an engineer's stamp or an architect's stamp.

However, in most states, when the design of a structural element or system is not defined in the code (steel/laminated/ composite beams, trusses, discontinuous lateral bracing, etc), it must be designed and stamped by an engineer.

In some states the design can be done by anyone if the building official accepts the supporting information (manufacturer's tables, etc). There are also jurisdictions where there is no building code for residences.

Therefore, when an architect suggests that there might be an additional charge for a structural engineer he/she is usually anticipating that some part of the structure might be required by law to have an engineer's stamp or, even when an engineer's stamp is not required by law, the architect might still feel the design should come from or be checked by an engineer. The areas where that is often desirable is for structural steel connections, composite steel & concrete, post stressing of concrete slabs on unstable soils, unusual/discontinuous lateral bracing or in earthquake zones, cantilevered beams, trusses not designed by a manufacturer, drifting of heavy snow loads, investigation and repair of existing structures, etc.

A major side benefit is that a structural engineer's errors & omissions insurance will help protect the owner from the cost of property loss or third party lawsuits in the unlikely event that some part of the structure fails.

If the design conforms to the building code it will be safe.

Any variations from the code typically require an engineers stamp.

That means steel beams, glue-lams, etc.
The suppliers of some structural members will supply stamped plans for the application.

Other things require you to find an engineer and draw up the plans.

I see. There is so much to learn everyday, and there is always more involved than a question and an assumption. I see what you are saying, Renovator8 and brickeyee. Thanks

You are right that an architect would be responsible to you for the structural stability of the house but when necessary or prudent the architect usually hires an engineer and passes the cost on to you as a reimbursable expense.