Like dimples on a golf ball (350 to 375 from what I understand), it would at the very least reduce skin friction, enabling the roof to fly farther once detached.

As for lift, I am not an aeronautical, but if you add curvature to the top surface, do you not increase lift, at least up to a point?

Mike McCann
MMC Engineering

The uplift at edges is caused, primarily, by the air flowing off the wall creating a low pressure region, before the flow reattaches to the roof. The code does not really contemplate whether the roof surface creates laminar or turbulent airflow, and whether the air is smoothly attached to the roof or not, this phenomenon exists. Even lower local pressures occur where vortices are formed in this unattached flow (faster airflow equates to lower pressures, hence the typical airfoil shape.)

There could be local effects due to roof texture or shape which are not accounted for in the code/standards. I would expect harsher design demands with the proposed irregularities. Wind tunnel testing would probably need to be done to validate such a design.

Thanks Mike & TXStructural.

I was hoping for an answer that said yes, more turbulent flow on the roof would reduce the airspeed and therefore likelihood of it detaching, so that I could reduce the structure but .. its better to know the answer than not.

Cheers

Creating turbulent flow that affects the roof membrane system is not a good idea. The fastening of a roof membrane system against uplift does not contemplate the potential for fatigue from many cycles of load as would be produced by turbulent flow. It's tough enough to fasten a roof membrane system against uplift without adding more issues to the mix.

Thanks Ron, I'll stick with the standard guidelines.