Wage labor, usually.

Despite the widespread use of slave labor in Roman society, unskilled slaves were overwhelmingly used for tasks that were either considered ‘demeaning’ (like domestic servants), did not require any real precision (like mill grinding and monocrop farm labor) or needed a constant application of labor (like mines). The Romans recognized that people work better when offered carrots rather than sticks - some slave who only barely cares if he lives or dies isn’t going to put much effort into aligning the brick with the mortar properly unless you watch him like a hawk - which is more labor you have to put in. Manual labor for construction is not a task that requires a doctorate, but it is a task where you have to do it right the first time, or you waste everyone’s labor and effort.

Construction, furthermore, is only intermittent work in most places. If you own a bunch of slaves, you don’t stop paying for their food and shelter when they aren’t working - if you want them to be profitable, you’ll have to find other work for them to do the rest of the year. And at that point, it’s probably not less profitable to just have them do that year-round instead. You could, potentially, have your slaves as a traveling construction crew, but travel is not only uncertain and expensive, but offers opportunities for unmotivated workers (like slaves) to simply… slip away, and choose to no longer be one of your workers. Even if you try to hunt them down. Even just transporting building materials from Point A to Point B includes a lot of very dangerous unsupervised time - perhaps something you’d trust a household slave with, but not one of the faceless slave numbers on your business ledger!

Funny enough, it would be more likely, if anyone was a slave on the job, that it would be potentially a few of the skilled positions. Skilled slaves were often given more trust and responsibilities precisely because they were offered more ‘carrot’ than ‘stick’ - payment, privileges, and the possibility of freedom for a decade or two of labor were on the table. Skilled slaves were thus less likely to run away - and unlike free wage laborers, especially skilled ones, wouldn’t (or rather, couldn’t) demand more pay at the prospect of being dragged from one of the empire to another - very handy if you’re a small construction firm going from place to place, and hiring local labor for most tasks!