Wet pipe system piping is permitted to be installed level. Only dry pipe and preaction system piping is required to be pitched to drain and tie-in drains on dry pipe and preaction systems shall be pitched as well. Sections 8.16.2 and 22.214.171.124 - NFPA 13, 2016 edition.
Sprinkler Drain piping not being required does not necessary mean it shouldn't be done. If running a long horizontal distance before exiting (with most drain piping being 2" or smaller) a sloped drain would be a logical idea. On the Plumbing side, a 2" waste/rain water pipe would be sloped 1/4" per foot. Looking at it with a non-code perspective lens a 1/8" per foot slope is a reasonable idea since this is relatively "clean" water, I say that knowing people will disagree with that statement. Especially if you've ever been dumped with sprinkler water like I have. It will only aid the system in the drain time wise. Drain Time normally isn't a factor unless you're the installer who's waiting to fix a leak. Also an air relief valve at the highest point of the system aides in the filling and draining time.
I work with fire alarm systems. I can't tell you the number of times I have had to go on site to fix the fire alarm system because of false alarms on the sprinkler system.
Most of the time, the problem was air in the sprinkler system that had not been bled out.
Please install the air relief valve mentioned above, and make sure people use that valve when servicing the system. It will save me having to explain it to owners, and will save the owners 2 (that's two) service calls because air is in the sprinkler system after servicing.
European Norm EN12845 on 17.1.8 indicates minimum slope values only on dry and preaction systems (branches at least 0,4% and distribution 0,2% towards the drain valve).
Wet system is indicated only as "drainable" and not specific slope is given.
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