Here's my tip. As you're building your house/structure, put up all the walls like you would. This is all BEFORE doing the roofs. Now, where you want the covered porches, also enclose those with walls. Your house should now be all walled in. Now, place your roofs (since it's all walled in, it's much easier). After you're happy with your roofs, simply delete the walls where you'll place all the railings. Then put up the fencing you'll be using for the railing and then using the MoveObjects on cheat, place the columns on the fencing. Presto, a covered porch. This method ensures you don't have to place extra columns just to get the upper floor tiles on.
Edit: One more thing. After you've put up the fences, place your stairs if you've built on a foundation. Then place your columns.