Sounds like a strength check for the floor. And the floor should probably be taking 10.
Do you mean the floor makes the Str check or the heavy object? Either way, what would the Str check even be?
Use the table on this page
DCs to Break or Burst Items
In most of the adventures I've run where this comes up, it's a pretty simple formula. Example: "If more than ### lbs. is on the bridge, then it collapses." Just fill in the number and you're set.
I can see four options. Either use the burst mechanic or use the hardness and damage mechanic. Potentially, both could be used together as option 3. Or follow Shadowborn's advice for the easiest option.
With the burst mechanic, set a DC to burst for the surface and a DC modifier based on mass. Any round that you roll above the burst DC the surface gives way.
With the damage mechanic. Decide on the surface's HP and hardness and the heavy object's damage potential. Repurposing the falling rules might suit, in essence using potential energy rather than kinetic energy to do the damage. Every round the damage is rolled, hardness reduces the damage applied and the rest is taken from the surface's remaining HP. When it reaches the broken condition, you may choose to add some effect, such as reducing hardness, to speed the complete failure of the surface.
With both mechanics, you could have gradual damage with a catastrophic failure any round, or any round after the broken condition is applied.
Really, it depends on what you are trying to simulate with your homerule game mechanic.