The sign is 4' x 8'. The sign is made of 2' x 6' treated lumber, for the frame, and 2 sheets of Royal Mouldings Limited PVC Board (4' x 8' ft. sheets), for the substrate. The sign is connected together with 3 inch decking screws and construction adhesive. The sign will be mount between 2 6' x 4' treated post cemented into the ground. Because we constructed the sign ourselves the county is asking for wind load proof of 90 mph for a permit to be issued. How can we show our signs wind load?

You have 32 sq. ft. First you need to find out what basis in your area is used for wind speed, it will be in MPH. Then you can use a wind speed to wind load conversion. However, you also need to determine the depth requirements for the posts to determine the tipping point. Just because you have the post size is inadequate, you also need depth requirements. Why don't you ask the local Inspectors for their input. Was a permit required for this sign?

Thanks. We requested the permit (which is required) from the local inspector. The inspector really do not have any specifics to provide us for a guide. It appears that the sign vendors (which we are not) certify that the sign meets the minimum 90 MPH wind load and provide the depth for the post to the inspector. The inspector then inspects that the holes are dug to the depth. Our problem is we do not have the certification from an engineer nor the required depth which the post need to be in order to meet the 90 MPH requirement. We are a church and build this sign ourselves. The sign is 4'H x 8'W x 6"D.

Unfortunately, it's a bit cart after the horse. The permit should have been obtained before the sign was built and installed. I don't know how you can get engineering certification after the sign is done. You might be able to get a PE to donate their time, particularly if a member of your church. I'd also try to Google "engineering signs for wind load" or something like that.