Answered by Sidi Omar Mahmood
Reading the Fatiha behind the Imam
Answer:
With regard to reciting your fatiha after the imam’s fatiha, it is recommended to wait until he is finished before starting your recitation of the fatiha, even if that means you will end up reciting a bit over his next sura, in the case where you know that the imam recites very quickly, or does not leave much room for recitation after his fatiha and recites short suras, then there is no blame in beginning your fatiha during the recitation of his fatiha. This will never nullify your prayer.
Allahu a`lam
forgive me.
wassalaam
-Omar