Attach base of a wall

[kathey1]

im modelling a domer at the moment, ive made my walls so that i can do the foot print of my dormer roof so now all i need to do is do a veritcal opening and attach the base of my walls to my roof but for sum reason it wont let me.

does anyone know how to get around this annoying problem ? im guessing its something simple

regards

[kathey1]

found what i was looking for, it on the options above the properties bar