Hi there, I'm trying to modify a roof from a maxis house in the late night expansion but I can't seem to find the way to do it, can somebody help me?

I want to make the roof shorter by one column but I can't remove that border, if I try to remove it it makes a hole like it is some kind of wall...

That "roof" is done with friezes. It acts like a foundation, is 6 clicks high, and can be placed above walls. Friezes were first introduced in Ambitions, and are available even if you don't have that game, as long as your Base game is patched high enough.
To add new friezes, open the cheat console and type in "placefriezes on" without the quotes. Then you can use the foundation tool to make those friezes and modify the roof (it temporarily replaces the tool). When you're done and don't want to make friezes anymore, deactivate the cheat by turning the value to "off".

I don't know if you have figured this out, but if you need it only 1 tile shorter on the long side you might have to build support from the ground up. That is if you want the 1 tile edge on as on the original.

Thanks to both of you! I managed to change the roof with that cheat and I was having trouble because there was no support Thanks guys!