Roof is common area, shared by all residents. You can sell separate portions, you cannot exclusively own roof rights.

You are at liberty to sell first floor of your house 

2) terrace forms part of common area for benefit of all flat owners 

3) you cannot retain ownership of roof 

You may make a partition of the proeprty in order to get the property legally partitioned by a partition deed duly registered, then both the floors become separate units.

This will enable you to sell the separate units separately.

In the partition deed, you may invoke a clause to retaining the roof rights or terrace rights over the floor of the floor which is the top floor. 

If you do not want to partition the floors by a registered deed, then you can sell a  portion of the property consisting the floor which you are desirous of selling it now by a separate deed and the buyer can apply for sub-division of the same subsequent to purchase, on his name. You can include a clause in the said registered deed that the terrace/roof rights are retained by you. 

Dear Sir/ Ma'am,

Yes you can sell the 1st floor area of your possession to any body of your choice as you are a free hold owner of the property.

There is no law which can debar you for getting it sold first thing and next if there is any common passage of use it shall remain under the law of the easement for the use of the first floor by the occupier of the the first floor. Please go ahead with the agreement to sell with the some advance and after a stipulated time of the agreement go for the sale deed, the ground floor owner can not object on any ground for this sale.It is your title and you are exercising the right to sell the property as per your sweet will and sound mind with out any hindrance and coercion from other third party.
It is also advisable to mention in the contract the provisions relating to the use of terrace to avoid any disputes in the future.

Yes you can retain

- As per rule, when a person buy an flat , then technically he buys two things,

1. First is the constructed part of the building where that purchaser after becoming owner will actually reside, and

2. The Second is a proportionate share of the land on which the property is built, and this share of land allotted to the flat buyer is known as an undivided land share or UDS.

- Hence, the details of the undivided share of land. must be mentioned in the sale agreement, and further the same details should be mention in the Sale deed ,at the time of registering the same .

- Further, as per rule, common area is not a part of undivided share, and legally it cannot be included as part of the floor area. 

- Hence, you can sell the first floor without roof right , but legally he will have undivided share in your property as well , as i mentioned above. 

You can connect me , if further suggestion needed. 

You do all that. You have to Specifically state what portion are you selling what would be the consequence etc. In the sale deed.