Terrace repair costs

Hi, Our building was under Govt admin back in 2013. Some residents took it upon themselves, took contribution and repaired the parking stilt pillars and installed a shed on the roof. Subsequently in 2017 a managing committee was formed with elected members, and now major repairs of the building have been taken up. A sub committee was formed due to lack of trust in the MC and its use of funds to overlook the tendering and the repair work. Mid way between the repairs the sub committee (SC) sent a msg saying waterproofing for terrace needs to be undertaken and are quoting approximately Rs 6.5L for the work, per flat contribution comes to Rs. 13,000. Now that external repairs are being done and an overhead shed already exists, so shouldn't the terrace repair costs be borne by the top floor flat owners? Shed impacted the structure positively and was helpful for all flat owners down till the first floor. But this waterproofing will only and only benefit the top floor owners. So are all members liable to pay?