When do you need an engineer?

Many residential projects can be constructed without consulting a structural engineer. State residential building codes give instructions on how many building materials should be used.

For instance:

• Acceptable spans for several lumber species are given in table format for varying joist sizes and spacings. However, if the rooms in your house are approaching 16' in both dimension or you want to save material costs by using framing materials other than sawn lumber, you may need an engineer to design your residential frame.
• The code proscribes (or gives guidance) on how to construct masonry or concrete basement walls. The wall height and soil retention will be limited to 9'. Above which a structural engineer should definately be consulted as the forces involved are quite large.

In many cases, a one or two family house that goes beyond the proscriptions of the building code can be permitted without the aid of a licensed professional engineer. Architects are allowed to design the structure of houses and lumber yards often will provide floor framing plans in order to sell their products.

Architects and lumber yards can provide structural design and in many cases do an acceptable job. As a licensed professional, an architect accepts more financial responsibility for the project than a lumber yard. If rooms are small, upstairs bearing wall align with downstairs bearing walls and the structure is not too complicated, an architect should be able to prepare adequate structural plans. As a structure becomes more complex, the better architects will pass the job on to an engineer, if for no other reason than to avoid the liability.

Lumber yards will produce floor framing plans if you use their engineered lumber products such as I-joists and LVL beams. The biggest drawback to these services is that they do not account for complicated roof structures, nor provide stamped drawings often required by code officials. If your plan calls for brick along a dormer wall, the software used will not easily catch the loads. If your project is an addition, the lumber yard will have more difficulty preparing plans as they do not normally come to the site for the preparation of as built drawings. Also, many jurisdictions will still require the plan be stamped/approved by an architect or engineer.

The decision to employ an engineer to design the structure of your house, like many decisions you will face in building your house, will be influenced by your acceptance or aversion to risk. If you place a high value on the structural performance of your house, you should hire an engineer. There are several examples of how a structure performs well, such as no discernible bounce in the floor joists, no discernible wall vibrations during high wind loads and no long term deflections causing damage to finish materials.

Modern houses have continually grown larger and with this increase in size has come a greater difficulty in obtaining a high performing structure. If you are investing a large amount of money and expect high value from your investment, protect that investment with an engineer that has the training and experience to design your home.

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