How close should I plant silver birch tree's?

Hello I want to plant a row of silver birch trees down the bottom of my garden to cover a distance of around 14 meters. How close together should I them?

Thanks Keith​

 

As a idea, my Silver Birch trees are 60+ feet tall and width is 20+ feet and 30 feet apart but the branches can over lap there may be some that don't grow that big but not sure, i have seen some trees planted 10 feet apart

 

Silver birch are "flimsy" looking trees, I think they are much better planted in a thicket than a row. But perhaps you don't have enough" depth" to do that?

An alternative would be to use multi-stem plants, Silver Birch respond well to that, and grow up to make a nice "clump" in that way.

Might be better to start with the questions:

Why are you planting trees there? Block a view? Wind? "To look nice at end of garden"?

How tall do they need to grow / must not grow taller than?

Are you in a hurry for them to grow? Can you afford "larger" plants at the get-go?