The maximum slenderness ratio for members carrying mainly tension in a roof truss, as per IS 800 (Indian Standard Code of Practice for General Construction in Steel), is taken to be 350.

Explanation:
1. Slenderness Ratio: The slenderness ratio is a measure of the relative flexibility or stiffness of a structural member. It is defined as the ratio of the effective length of the member to its least radius of gyration. Slenderness ratio is denoted by the symbol λ.

2. Roof Truss: A roof truss is a structural framework designed to support the roof of a building. It consists of triangular-shaped units, called trusses, which are connected together to form a stable and rigid structure.

3. Members Carrying Tension: In a roof truss, different members experience different types of forces. Some members carry mainly tension, which means they are subjected to pulling forces along their length. These tension members are designed to resist the applied forces and ensure the stability and strength of the roof truss.

4. IS 800: IS 800 is the Indian Standard Code of Practice for General Construction in Steel. It provides guidelines and specifications for the design, fabrication, and erection of steel structures. It is widely used in the construction industry in India.

5. Maximum Slenderness Ratio: The maximum slenderness ratio for members carrying mainly tension in a roof truss is taken to be 350 as per IS 800. This means that the effective length of the member should not exceed 350 times its least radius of gyration.

The reason for limiting the slenderness ratio is to ensure that the member does not buckle or deform excessively under the applied tensile forces. If the slenderness ratio exceeds the specified limit, the member may lose its stability and fail to perform its intended function.

By setting a maximum slenderness ratio of 350, IS 800 ensures that the tension members in a roof truss are designed to be sufficiently strong and stable, thereby ensuring the overall structural integrity of the roof truss system.