Time of Blessing over a Fence

I built a second floor on the roof of my house, part of the area is a balcony. Today, the blacksmith is going to arrive to make the railings. There is still a small section (about half a meter) where I am not going to install railings at this stage (I need to run some more pipes there). Can I already say the blessing for the fence now?

Greetings,
One only blesses when the construction of the fence is completed.

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