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Is it advisable to go for a ready mix concrete for roof slab only (for relatively small residential construction)? or it is better to stay with the traditional route of concrete mixing in the site.

As always, pros and cons would be great from getting educated on this?

I had same type of confusion, my architect suggested this - if your house design has different levels, meaning in same floor if there are any level differences, RMC wont help. Contractor needs to do the shuttering work for all level and call for the RMC, which is big task.

Also, in case of moderate RMC providers use a lot of "dust" sand, lower grade cement etc etc. You will have to get it from trustworthy guys. However on the good side, there is a good quality control in case of RMC, which generally go missing is manual process of mixing.

I have decided to go with manual because of different levels.

I too had the same confusion. We used RMC for the foundation. We were not 100% sure of the quality, so, we asked the engineer to use manual mixing, but unfortunately, our house is a 60x40 site in a fully developed area and there are no empty sites nearby. So, the engineer calculated the amount of sand, jelly required to cast the entire slab in a day and we finally had to revert to RMC, as there was no space to keep the required material. The engineer felt strongly that we should cast the entire slab in one day for the sake for homogeneity. So, we finally decided to continue with RMC for the slabs.

I have the same confusion and finally deciding to go for RMC for my roof slab. I am planning with complete cement. I spoke to a few RMC guys including Ultratech, Lafarge, RCC. The quote is around 4300 for complete cement without GGBS plus additional charges for pumping, ranging from 6k to 10k for any thing less than 30 Cubic Meters. My engineer also suggested to use complete cement mixture instead of the GGBS mix. The ultratech people are not giving their composition mixture. I am feeling that it would atleast come with some quality. But lot of people including my contractor are saying it would not be good, they mix chemicals, use rock dust sand instead of river sand and is expensive etc for a small site(30*40). However it is coming to around 5k more for the RMC.

I finally went in for manual mixing of the roof slab concrete. The main reason behind somehow it is still not very convincing and after reading in some places that the normal RMC is not water proof and some forum members had bad experience. You need to specifically ask them to add water proofing at a cost of 150 Rs per cubic meter. It is okay if it is not the final roof and if you are building some thing on top. Another friend of mine also had similiar water proofing problem. So went ahead with manual mixing.