Is a safe estimate.
Attic wieght limit.
So if your ceiling is unfinished you have some excess carrying capacity up there.
With an evenly distributed live load of 30 psf which the tables show the floor is able to support the total weight on the floor would be about 3 360 pounds.
All weight hanging from the truss has to be added to the existing dead load.
Most trusses are designed for only 10 lbs psf live load for storage unless they are attic trusses like drooplug mentioned.
You ve added about 300 lbs already when you put 6 sheets of plywood up there.
That means you could lay a 2 x 4 ft.
Access to attic storage.
The dead load on the bottom chord of a truss varies with the weight of materials attached to it such as drywall on the ceiling.
As larry said don t overdo it.
Of stuff on it.
Scrap of plywood over the chords and set about 40 lbs.
Make sure items are spaced so that the weight is evenly distributed over the chords.
To simplify you can place 20 pounds of dead load per square foot on your subfloor as long it s supported mid span.
It is vital to observe the weight rating and make sure that you comply with it at all times.
If you want to use your attic on a regular basis or to store large items you may need to enlarge the access opening and install an access ladder or drop down stairs.
If you care about this you will need to add a second layer of floor joists over the originals perpendicular is best to limit thermal bridging insulate then lay floor over that.
Observing the weight rating.
This is because the ladder is simply not strong enough to support any more weight and you are putting unreasonable pressure on the attic access ladders every time you break the weight limit.
A typical load is 5 and 10 pounds per square foot.
If the total weight exceeds the load limit of the truss then the truss is compromised.
Increasing the total weight on the floor to 4 480 pounds however results in a live load of 40 psf which is beyond the floor s load capacity.