ENGINEERED FLOOD VENTS

I believe that you only specify the number of permanent flood openings, and the square inches of the openings on the flood cert. You must provide a copy of the engineer's letter with the certificate.

I thought I had read that one inch of flood opening=one square foot of enclosed space.

We very rarely see any engineered openings in my area of practice, so I really don't see this very often.

Yes on the certificate we report the opening and whether or not they

are engineered vents. My problem is that I am trying to help out

a client (ahead of construction) figure out how many engineered vents

he needs to install to cover his foundation size. As usual, when you

try to help someone out in issues related to FEMA you end up sticking

your neck out to liability issues...The engineered vents only cost

$45.00 each - which is a very helpful item if I could just figure out

how many applies to his site.

I also question "1 square inch of net free space for 2.424 square feet of

enclosed space. FEMA says you need 1 sq. in. per sq. ft..

To use an engineered opening, one must have an understanding of the rate of flood rise as it is one of the variables in the equation for engineered flood vent sizing (See Fema Technical Bulletin 1-2008). This is an engineering exercise. The rate of rise may be available in the FIS for the area in question, but more than likely it will require additional engineering/hydrologic-hydraulic analysis above the information found in the FIS.

Basically, a 5 ft/hr rate of rise is a fast rate typical areas such as, but not limited to, narrow constrictions in a flood plane or areas prone to flash flood. I wouldn't expect such a rapid rise in a broad flood plane.

Anyways, look at the FIS if available and see if you can find the rate of rise from it, otherwise, consultation with a civil engineer competent in hydraulics would be necessary.

Be also aware that the engineered flood opening design and calcs may be copyright protected.

Also, the typical non-engineered opening will qualify providing it meets the criteria of 1" of opening per 1 square foot of enclosed area.

It is when the need for less than 1"/1 sq ft of opening is necessary, or where a special, engineered covering is applied, that engineering calculations will be required as part of the elevation certificate.

Hope that helps.

> I also question "1 square inch of net free space for 2.424 square feet of
>
> enclosed space. FEMA says you need 1 sq. in. per sq. ft..

1" per square foot is for non-engineered openings, which is very typical.

If you have a situation where a building has less than 1" per square foot, then an engineering analysis may well qualify for a lesser opening amount as "engineered openings".

See Fema Technical Bulletin # 1-2008 for a thorough discussion.

Thanks for your reply. This is what I suspected. It is an engineering calculation

and based on my very limited understanding, our Flood Insurance Study does not

give data (on the face of the document at least) sufficient for the civil engineer

to make that calculation. The bottom line, then, is that the client needs to see

a civil engineer who specializes in hydrology and is willing to come up with

a rate in his particular location. You have helped me more than you know!

Maybe the insurance company will be willing to be a good neighbor to my client
!
and grant him the engineered specs 😉 Not me!

Tom Coady

I would double check with the engineer, but it seems like they gave the specs.. The vent apparently has 98 square inches free space (stated 238 square feet flood it's good for). Figure the square footage of the structure and determine how many vents using the 2.424.

if you want to help your client, have him talk to the flood plain manager in his jurisdiction. They may refer him to the building department. A qualified engineer could help as well.This is beyond the scope of surveying in my opinion and could open you up to liability you dont want.

> I would double check with the engineer, but it seems like they gave the specs.. The vent apparently has 98 square inches free space (stated 238 square feet flood it's good for). Figure the square footage of the structure and determine how many vents using the 2.424.

That's only true if you know the rate of rise is less than or equal to 5'/hr.

Tom wants to know how to determine what the rate of rise is for a particular flood plane so that he can know how to apply the design vent criteria. The vent being used was apparently not designed specifically for his project site/flood plane.

I have worked on projects where Smart Vents were specified:

http://www.smartvent.com/

They have the following certification
http://images.smartvent.com/images/uploads/codes_and_certs/icc-esr-2074.pdf

They may be able to just be utilized by their square foot rating. I don't pretend to be highly knowledgeable on this product but I seem to recall they cost around $250 apiece.

Not sure why FEMA would not provide the rate of rise. That information is essentially part of determining the location of any particular flood event. Seems to me the taxpayer has already paid the engineering bill for the flood study and should be entitled to the information needed to comply with the regulations. I would make a few calls for the client and try to get the info.