I haven't looked through all the books for it, but I noticed that in the CrTe guidelines in AM5 they only address creating terram substances from scratch and don't address repairing existing structures/items made from Te. I am thinking in particular of the precedent of the CrAn ritual spell that repairs books from Covenants. Does anyone have house guidelines for Terram? Or are there guidelines buried in some other book I should be looking at? That would do something similar....
Why I ask is that our Magi have discovered an old roman ruins that is a suitable location for a covenant and instead of creating a Mystical Tower or some such I thought it would be interesting to reestablish/recreate the original roman building using a CrTe ritual. Is this even possible? We do not know the original layout of the building but a few pillars remain.
You can use the ritual that allows you to look into a location to see how the villa looked like. I can't remember if in 5th this is a Quaesitor only ritual, though. Also, I am sure that there must be quite a few tractati on the order on how a roman villa used to look like. If yours is special in its layout for some reason this might not work, but this is likely to be an infrequent problem.
On your question, sure. is see no problem with what you suggest. Go for the CrAn guidelines. remember the +2 stone increment, though, or you will end up making repairs with mud.
IMS we have made repairs using MuTe instead: turn stone into mud for sun duration. Fill in the gaps and then the mud turns back to stone. Easy and can be done by unqyualified labour force acting under the instructions of a Mason
Stone buildings are expensive in this era because stone is a) hard and b) heavy. Thus it is hard to work into suitable shapes and to move. If your magi have significant ReTe to move stone it's a lot easier to get stuff from the quarry.
Consider the expense in moving enough stone to construct a small tower from a quarry 2 miles away. Now consider how easy it is to do with ReTe. Even if you can only transport 1/20th of the stone each day without taking multiple fatigue levels it's still not a big expense. So your magi is a little tired for a few minutes each time he casts, so what? It's not like you'd be in that much of a rush. If you were you'd be building in wood.
Working the stone is even easier, PeTe cuts it into managable blocks, which can be made into more complex shapes that masons usually bother with. So you can have interlocking blocks that are more stable and stronger than usual. This needs a Finesse roll to get right but even if you mess it up a mason can simply finish it off. This is a lot less effort than having him cut the whole stone.
"So you want me to accompany these obviously shady characters who are rumored to be wizards and therefore demon worshipers to a place way out in the wilderness to build what they claim is a retreat for scholars, though why scholars would want to live dozens of miles from any possible patron is unexplained. Yeah, that'll be triple my usual wages, paid up front for a month at least and you leave enough stuff here that my widow is compensated if you slit my throat in the woods."
Careful, you're walking on dangerous ground here. There's some ... enthusiatic ... disagreement onto the nature of Perdo and whether or not it can be used to cut things or if that's the domain of Rego.
So, instead, what you want is a Muto Terram spell. Render the Whole Fragile MuTe25
This spell modifies a sizable portion of a cliff-face such that the lines of fracture form a neat, 3D grid with each portion the size of a normal building stone. The focal point for the fractures is the part of the cliff touched, and striking it firmly with a hammer during the duration of the spell causes the cliff to shiver appart into the convenient lumps. Some of these inevitably crumble when the spell ends as they weren't good stone to begin with, but it's still a good start.
(Base 3 - slightly unnatural change, +1 stone, +1 touch,+1 part, +3 size)
For those who still wish to use Rego, some Flambeau have made a sport of this, using Finesee-controlled variants of Wield the Invisible Sling to target the weak point. At least one limestone quarry in Britain is believed by the locals to be inhabited by curiously helpful faeries as a result, and a minor conflict between the Flambeau and the faeries collecting the offerings the peasants have left is brewing.
It's also had bits broken off it by a madman with an axe and so is clearly Perdo. ::shrugs:: I find it unsatisfying that I can cut stone with ReTe but not flesh with ReCo, and that people seem perfectly happy for PeCo effects to be neat and efficient, but require PeAnythingelse to be messy and ruin the substrate.
To my mind, Pe can destroy bits of a thing, cutting cleanly through something but with material gone at the end of it. Re can cut cleanly through a thing, but with no material gone, just displaced and possibly in bits. Likewise, ReCo should, on its own, be usable to tear a person limb from limb. It should just be remembered that anything affecting a living creature is always orders of magnitude ahrder than affecting the dead remnants, and that a high finesse roll is also needed. To me, this is part of the point of the Ars Magica magic system - cleverly using your resources and skills to achieve your goals in your own way. As pointed out on the BerkList, Ars Magica has a minimum of 50 ways to magically skin a cat, though some are more convoluted than others.
Ok, I get the idea that the stone doesn't become more perfect, but is the paradigm idea of perfection something that doesn't apply to buildings? Is a building (or any manufactured good by extension) beyond the realm of platonic forms? Anyone have a cite? If they are not, then it must be that buildings, and not just their component materials, can exist closer or further from their platonic state of perfection. This implies Creo to me. I would assume using Rego could certainly do it too, but I think they are both just different ways of skinning this particular cat. My understanding of "emulating a craftsman" is that the magic emulates what a craftsman does to an object, as opposed to changing the object itself. I'll have to review craft magic when I get home.
IIRC, there are Creo spells that make building more perfect (square room squarer, circular rooms more circular, etc) in the Hermetic Geometry section of TMRE. So there should probably be a way to use Creo to repair buildings.
You might want to use "Heal a XXX wound" guideline, increased to Structure target, translating wound levels to amounts of degradation in the building.
Maybe check the item damage rules from City and Guild, too.