In the midst of a remodeling project we've learned that our subfloor (previously carpeted) is both bouncy and uneven. Our contractor has suggested the addition of 2x6 joists at 24" intervals between the existing girders that ride on top of piers stationed every 5'. The girders are spread 4' OC apart. The current subfloor is 1 1/8" T&G plywood. The thought is to strengthen the system from below and mitigate the bounce.
Now, that said, I discussed an alternative approach with our construction foreman. Instead of installing the extra joists under the subflooring, we talked about installing a new layer of plywood (5/8 T&G) on top of the existing 1 1/8'. These sheets would be fastened with screws and liquid nails. This new layer would then serve as an improved underlayment for the 5/8' Owens unfinished engineered hardwood we are contemplating using. We discussed running these materials perpendicular to the girders to achieve greater stability. We also discussed evening-out some of the dips and unlevel portions of the current subfloor as part of the installation of the new plywood.
So, would this appear to be a sensible and cost-effective solution? It makes logical sense to me. However, I'm prepared to hear any sound advice in agreement or to the contrary.
Fremont Dave

Those in other parts of the country are unfamiliar with a subfloor system that was used for more than two decades here in CA. I have floored over many of these types of subfloor systems. They consist of 1&1/8" plywood subflooring installed over 4x6 beams/girders which are set 4' oc. Some had metal jacks holding the beams up and in theory, one could relevel the subfloor by adjusting the jacks. Others used 4x4 posts. The system had no joists and was used over a crawl space. The problem with these subfloors is/was that after awhile, the plywood could sag between the girders. Also, the concrete pads the posts sat on would rise and fall if the ground underneath got wet, which many did. As bad as this system was, it was used in many, many housing tracks, especially in N. CA. It was fazed out approx. 15 yrs. ago.

It is difficult to fix these subfloors but these are some of the repairs I have performed in the past.
1) To lower a high beam, simply cut down the height of the support posts and attach gussets to the connection between the beam and post. You will need to place lots of weight on the high area to get the plywood to go down.
2) Add 2x6 blocking between the beams 2 ft.oc. But instead of installing the blocking with hangers (which can rattle and are noisy) nail the blocking on a diagonal. I recommend this under bearing walls, cabinet and appliance locations.
3) add more plywood, 1/2" to 3/4". This will sfiffen that floor up and take some of the bounce out of it. You can sand high spots and fill the low ones with asphalt shingles. I've done this quite a few times. Of course this will raise the height of your floor, which you will need to allow for.
4) Finally, do not let the earth under the house get wet, even a little bit. This cause the piers/pads to rise and fall and makes your subfloor uneven again. This I would fix first before doing anything else. The crawl space must be dry.

Having lived 45 years in California and dealing with this sub-floor, we found supporting from underneath the best answer. Raising the sub-floor 5/8" often causes problems with door clearance at front and rear exterior thresholds. To best answer you may be to suggest checking for potential problems and clearances if the floor is raised 5/8". Raising the floor 5/8" also lowers cabinets in kitchens and bathrooms and causing 5/8" height differences at transition areas to other flooring, such as from wood flooring in living room to vinyl flooring in kitchen or bath.
All suggestions offered will strengthen the subfloor so the question is "which way results in the least amount of problems after the installation"?