Is your actual water pH 5 or is the mash pH 5
Most grain bills will drop the pH to a desirable level in the 5.2-5.8 range.
If you are starting with. PH of 5 then your grain bill may drop the pH into the 4s which would be too low and you may need to add something to the water to get it up to proper levels.
Visit the water chemistry-brew science thread or go to a water tool like bruin,water or ez water calculator - do you have a water report to start with?
5.2 is a buffer and IMO its really pretty useless!