Toilet Pipe runs on outside wall

I am about to install a small en suite toilet to the top floor of a mid victorian terrace house 2up 2 down type. At present the bathroom and toilet are in a small extension on the ground floor. I intend to run the waste pipe through the wall accross the back of the house at an angle over the roof of the extension and then join on to the waste stack at the back. Question is do I have to have some sort of vent where the pipe first exits the back wall or will it run down as described without a problem.

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