Why are my dogs pooping on the patio? How can I get them to stop?

About 6 months ago I bought a house with a nice fenced back yard. There is a staircase going up to the covered back porch and underneath is a concrete patio. The patio is covered (by the upstairs porch) and has "walls" made of lattice with roses and climbing vines. I had planned to create an outdoor barbeque/party area, but the dogs have adopted it as their bathroom! They non only poop on the concrete, they pee there too. I suspect Dumplin, my rescued female terrier/bully mix (who HATES water) is the instigator. She won't even walk on the gr if there's a speck of dew on it. Of course Dagwood, my rescued boxer/bull mix, follows her lead in everything. The route from the back door, down the stairs and to the patio is all covered (i.e. dry). Could it be that, because I haven't furnished this patio area or spent any time there, that Dumplin thinks it's a space up for grabs? Since I haven't established it as a living space, maybe she's claiming it as a bathroom? Anyway, whatever the case- how can I get them to STOP? I don't want to make them feel bad about pooping outside, which is where they should poop, but I don't want the m to poop on the patio. As a side note, I know the family who lived here before had several small dogs who used the patio as their bathroom. Are my dogs just picking up on that? AArgh! Help!