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can i build a 4' x 8' x 5'' sign from expanded polystyrene covered with a stucco coating? Answered

it will be fastened between two posts. it needs to be able to withstand 90 mph wind load?


Expanded polystyrene does not have the structural strength to take that much wind.
Very-sturdy wood (box-type construction), or steel is what you'll need.


the wind load would be 576 lbs total or 18 lb / sq ft. The eps 4 x 8 x 5'' thick would span two 4 x 4 wooden posts and be screwed into the posts with wood lag screws. If one 2 x 4 metal stud thru the middle or two 1'' sq tubes, one across the top and one across the bottom were embedded thru the eps horizontally between the posts, would that do it?

Is the wind pushing the eps against the posts or pulling it away from the posts?

You said 90 mph - it needs a material that can take that. EPS will rip out of screw-fixes, regardless of how big they may be.


Although covering the structural framework with a skin of polystyrene to add detail would work just fine. Not just the EPS by itself, though.


8 years ago

I believe you need to rethink your design.  Expanded polystyrene, structurally,  simply isn't up to the task you have in mind. My figures indicate that a wind load of 90 mph will impose approximately 29 lbs/sq ft. of pressure. Almost 1000 pounds over a surface area of 32 sq. ft. 
Plus, the shear strength of EPS is practically nill. How will you secure the sign to the posts, how will you fasten 40 to 50 pounds of lathe and stucco to the EPS? My guess is that the flexing caused by even light breezes will cause the stucco to crack and fall off in short order, assuming the sign doesn't tear through its fasteners and fall off the posts first.
I would suggest you consider using a sheet of 3/4 inch exterior grade plywood instead of the EPS as the base.  Use 1/2 inch galvanized wire mesh as lathe, attached with fasteners that penetrate the full thickness of the plywood and a fiberglass reinforced stucco mix.
Fasten the completed sign to the posts using stove bolts equipped with large fender washers.