We want to build a 20 x 40 cabin on a 100yr, flood plain. It will be elevated. How many 6x6 posts do I need?

Question

Is there a site where I can get info. on beam spans and what size joists to use depending on spans and such? Thanks

Answer 1

The first site you ought to be looking at is the FEMA site, to see exactly what they will (or will not) let you construct there. Depending upon where this 100 year flood plain is located (inland waterway, coastal, etc.) the rules change a bit. And, recently, FEMA updated their FIRMs (Flood Insurance Rate Maps), so you better check and see what changes there may have been to this (construction) site.

Also, you should check with your state's EPA, along with the County you live in. There may be rules governing storm water management that may keep you from building anything, even an elevated structure, in this location.

While checking with FEMA, it is also a good idea to check with the Corps of Engineers to determine whether or not this is classified as a wetland area. If it is, you are out of luck; or out a lot of cash to have a wetland demarcation study done.

If you are allowed to build there, hire a good P.E. or S.E. to design the structure. The wood will have to be treated lumber, and the treating process breaks down the cells of the wood and weakens it. You will probably be required to submit "Engineered Plans" using the Fb and deflection characteristics for the grade and species of lumber used, with adjustments made for the "damage" done by the treating process used.

Proceed with caution; don't trust any answer you get from any unofficial website; and keep in mind that FEMA can change the classification of the lot, or parts of it, if you provide proof that it is above base flood elevation. You can obtain the forms for requesting a Letter of Map Revision ("LOMAR") from the FEMA website. Then all you have to do is hire a licensed surveyor and an engineer to provide the documentation you'll need, and hope they don't determine that FEMA was wrong, and even more of the property than you thought is in the floodway.

Answer 2

I have to concur with Electrical Inspector - carefully research the site before building anything on it with the federal government and your insurance group.

A structural engineer (SE), professional engineer (PE), or architect will help you adequate engineer and size the members to be used for the cabin.  It may be helpful to approach one of these individuals as they can help you with researching answers with the appropriate agencies.