ᐅ Reduced cost reimbursement for concrete roofing

We are building a single-family house with a general contractor. In addition to the house, the construction of a masonry storage room is contractually agreed upon. Originally, this was supposed to have a concrete ceiling. However, after consulting with the site manager and a friend who builds carports, we have decided to omit the concrete roof and cover the storage room ourselves together with the wooden roof of the carport.

What kind of cost reduction could we reasonably propose to our general contractor for the roof?

Here is the exact agreed scope of work:
Included in the contract is the storage room of the carport (not the carport itself) with the following specifications:
The storage room is specified as follows:
a) with concrete ceiling (welded waterproofing, edge covered with zinc sheet metal similar to the main roof)
b) lightweight base plaster with mineral topcoat and leveling paint
c) masonry made of calcium silicate bricks
d) including earthworks with concrete foundation slab and screed
e) internal drainage with roof drain
f) calcium silicate brick walls finished inside with lime-cement plaster
h) the reinforced concrete ceiling of the garage will have a professionally executed slope and will be sealed on the upper side with UV-resistant bitumen membranes or suitable plastic sheets
i) execution with concrete base according to structural engineering and screed
j) with side entrance door and window; execution

The affected items would be positions a, e, and h. We would carry out these ourselves as described.

The roof area measures 14.875 m² (160.1 sq ft). The agreed total price is €8,000.00.

For the ceiling approximately €40 per m² (about $4 per ft²), depending on the span... plus waterproofing and edge covering

An update for everyone interested to read:

I calculated one third = €2,666.67 and asked our site manager for a proposal.
He offered €2,100.00, and we agreed on the middle ground = €2,380.00 gross.