Terra-Cotta floor tiles are made with Adobe clay that is than heated to increase it's strength. It is a popular choice for a floor covering because of it's durability. But if not sealed properly can develop stains (mold, dirt, etc.) which are difficult to remove because of it's porosity. An analogy would be it's comparable to a dried sponge (but harder). Once the seal is worn away it will absorb any liquid on its surface. Power washing is one way to clean the tile, but the spray pattern must be very wide to avoid eroding the tile. And it must be kept moving for the same reason. If mold, fungus or algae are the problem first make a bleach solution in a five gallon bucket (1 cup bleach and 4 gallons water). Mop the solution liberally onto the floor and scrub vigorously into the problem areas. Wait a few minutes and either power wash the surface or apply clean water and mop up as much as possible. A floor scrubber works well to, but not with an aggressive brush. The tiles must dry completely. The drying time can be sped up with fans and/or heaters. And depending on your climate should dry in 2 (+/-) days. With a paint roller and brush apply an exterior Terra-Cotta floor sealer according to the manufacturer's directions'. It should be ready for foot traffic between 12 - 24 hrs.
Yes, I like the concrete idea too. However, there are 3 issues to consider: 1) hold it up, 2) hold it down and 3) hold it from rotating.
To hold it up you'll need to pour the 5 footings on undisturbed soil or compacted gravel that will support the load of the roof plus snow load. Minimum snow load is 25 pounds per square foot (unless you live in the deep south it could be less or in the north it could be more). So, 20' x 15' x 25 psf = 7,500 lbs. load. Half rests on house and half on 5 new posts....therefore 3,750 lbs. on posts or 750 lbs. on each post. Soil bearing is typically 1500 per square foot or more. So, a 12" x 12" (or so) footing is adequate.
To hold it down you'll need sufficient amount of concrete to hold it down. Wind uplift is about 20 lbs. per square foot (max.) unless in tornado area or on the coast. So, 20' x 15' x 30 lbs. per square foot = 6,000 lbs. Again, half held down along edge of house and half by the 5 footings = 3,000 lbs. or 600 lbs. by each post/footing. The minimum size of each footing needs to be 2' x 2' x 12" deep, which equals 600 lbs. each.
Rotation is more difficult, but without walls, then the wind will not provide rotation...therefore it's ignored.
Likewise with seismic concerns. Too complicated for this discussion.
NOTE: Make sure each post connector is rated for 600 lbs. uplift and 750 lbs. bearing...at both top and bottom of each post.
The roof connection for bearing to the house should be 3,750 lbs. or 3,750 lbs. / 20' = 188 lbs. per foot. The roof connection for uplift to the house is 3,000 lbs. or 3,000 lbs. / 20' = 150 lbs. per foot. You can find clips at local hardware store for this (house side) connection.
Dryrot, decay and pests occur when posts are in a wet situation, (I.e.: below grade.) Keep all post supports a minimum of 6" above adjacent grade...code requirement. If you can't , and the post has to be below grade, then at least treat it like a fence post: use pressure treated wood.
Use a wood suitable for exterior use...ask the lumber yard salesman for recommendation for wood available in your area.
Good luck. It's not that complicated. I've made it complicated to give you the sizes of footings, clips, etc.
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Gravel will not do you any justice. It is just a cheap way to go deep in ground to get to good bearing soil if it needs to go really deep. Pole barns, for example, are built with 6X6s set in the concrete 4' deep, for a porch roof if it is small enough it won't need 6X6s but 4X4s will suffice if the roof is not too high.