terrasse sous le toit

Bonjour a tous,
étant résidant au dernier étage d'un immeuble de 1920, j'aimerai aménager au dessus d'une pièce de 5.5m X 5m   une terrasse.
le problème c'est que je n'ai  pas de poutre pour poser mon sol

ayons des poutres de 450mm X 25mm de 9 m sous mon plancher (bois)
 je me demandai si c'était possible de faire supporter le poids de la terrasse sur une des poutres et le mur de 50cm  
la poutre poura t'elle suporter le poids?
si non quelle solution vous me conseiller merci de votre repense
                                                                                                                                                                   
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                                                                / ________5m_______:
                                                               III                                 II
                                                               III                                 II
                                                               III                                 II                                         
                                                               III                                 II                                        
                                                               III                                 II                                           
                                                               III ________________II_                                                                             
                                                               III   [P]           [P]          [P]                                 
                                                               III                                                                              
  
  III = mure de 50 cm 
  II  = poteau  
 [P] = poutre