Low cost reimbursement for concrete roof

We are building a single-family house with a general contractor. In addition to the house, the construction of a masonry storage room is also contractually agreed. This was originally supposed to have a concrete ceiling. However, after consulting with the site manager and a friend who builds carports, we have subsequently decided to omit the concrete roof and cover the storage room ourselves together with the wooden roof of the carport.
What can we reasonably propose to our GC as a cost reduction for the roof?

Here is the exact agreed scope of work:
The contract includes the storage room of the carport (not the carport) with the following specifications:
The storage room is specified as follows:
a) with a concrete ceiling (welded, edge covered with zinc sheet analogous to the main roof)
b) light under-plaster with mineral top plaster and leveling coat
c) masonry made of sand-lime brick
d) including earthworks with concrete slab and screed
e) internal drainage with roof drain
f) sand-lime brick walls provided with lime-cement plaster inside
h) the reinforced concrete ceiling of the garage receives a professional slope construction and is sealed on the top side with UV-resistant bitumen sheets or suitable plastic membranes
i) execution with concrete base according to statics and screed
j) with side entrance door and window; execution

Thus, the affected items would be a, e, h. We would then carry these out ourselves as described.
The roof area measures 14.875 m². The agreed total price is €8,000.00.

For the ceiling ~ 40,-€ / m² depending on the span.. + the sealing + edge covering

An update for all interested to read: I have considered one third = €2,666.67 and asked our site manager for a proposal. He offered €2,100.00, we agreed on the middle = €2,380.00 gross.