Someone borrowed sechach to use on his succah. The sechach came in relatively good condition wrapped up in plastic and put inside its normal bag. There was a little bit of mold on it but nothing serious. After Succos he did not take it down and return it right away, but rather kept it on his succah for what became a couple of weeks after Succos. During that time there were strong winds causing the sechach to break to some extent and rain which subsequently damaged the sechach more and left a noticeable amount of mold and other damage. The person simply didn’t ‘find’ the time to take it down and return it to its owner (a combination of forgetfulness and laziness). The sechach is still usable however at least one side and a part of it are relatively damaged. How much is the person who borrowed the sechach chayav to pay?
The person who borrowed the sechach is classified as a shoel, and he is responsible for the damage.
If the damage can be fixed, and it is the norm to fix such damage, the borrower is responsible for fixing it.
However, in general, such damage is not fixed, and the borrower will be obligated to pay for the decrease in price of the schach: How much was the sechach worth before, minus how much it is worth now. This sum is very hard to estimate, if not impossible, making it difficult to extract any payment. It is therefore advisable to reach a compromise.
According to the Chazon Ish (Bava Kama 6:3; see also Nesivos Hamishpat 340:3; Shach 95:18, and other commentaries to Shulchan Aruch 95:6), and the common ruling of contemporary poskim, a person who causes damage that stands to be repaired, the damager is responsible for the repair. This applies even if there is no substantial financial damage (the item did not depreciate). The same will apply to a borrower. If the item does not stand to be fixed, the borrower is liable for the financial depreciation alone.