Reply to Expert views needed: Building an addition on piers in DC on a semi detached house

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Reply to "Expert views needed: Building an addition on piers in DC on a semi detached house - can it be done?"

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[quote=Anonymous]I just did this. DC has a strong tree preservation law, you can't disturb the roots of a mature tree. In order to expand my footprint I had to build on piers. Helical piers to be specific.  I've also talked to neighbors whose house was built on fill -- this is in Palisades/AU Park -- and in order to hit bearing soil they had to go down 40+ feet, they used helical piers as well because they don't get much more expensive the deeper you go.

The guy who installed my piers seemed to be quite busy.

Here's where your friend is half-right: DC has what they call a "prescriptive" building code. That means there are certain ways of doing things that are prescribed in the code, if you do it exactly that way you don't need an engineer. So the code will say that the prescriptive way is to build a foundation with footers that are X inches thick and Y inches wide and you're good. If you don't do it the prescribed way, you can hire an engineer and have him design a custom system for you.

Prescriptive is always going to be cheaper -- you don't need an engineer, and the contractors you work with are going to be familiar with the technique. With a custom design not only do you have to pay the cost of the engineer, but the contractors are taking on risk that they don't do it right, and they are going to pass that cost on to you in their pricing.

Where your friend is right is that a crawlspace is the prescriptive method. Piers are a custom method. But they are allowed and an experienced engineer should have no problem with them. [/quote]

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