I recently inherited my parent's slab built house that is about 38 years old. The house is in Virginia Beach, VA an area along the east coast with a high water table. Also, over the years the ground in the backyard has risen and is very close to the same height as the slab in places along the back. The good news is that along both ends and in front of the house the ground slopes away from the house toward the street.
The house does have a decent gutter system, which will be in excellent working order when I'm finished with it.
Further information is that the house and it's back yard, back up to a strip center. The water from the roof of the shopping center building drains off in the back, toward our house, and the small paved traffic area behind the building slopes towards our house as well. Our back yard is probably about 40' deep. When it rains a great deal I've noticed our back yard gets mushy and stays that way for days.
At this point I have only visually checked the grade of our back yard, but it appears to slope slightly from the strip center toward the back of the house. This is just my visual impression and I could be wrong. Virginia Beach is over all a pretty flat area, so pronounced easy to discern slopes are rare. During some heavy rains while the slabs were bare I did notice the slab looked darker in places along the back of the house. I did not perform a moisture test Per se but places where plastic buckets set were definitely darker beneath them.
At this point, my plan is to trench along the back of the house turning the corner at each end continuing along each side just past the front of the house, creating a French drain around the back and sides that will convey the water to a downward sloping point past the house.
I have a good understanding of how a French drain works, trench, grown fabric wrapped 4 to 6" perforated pipe, covered with 3/4" gravel.
I know the trench will need to be graded. What I'm unsure of is how wide the trench should be and how deep (depth below surface of the slab). Also how much dirt I should remove along the slab (from a height standpoint) where it is currently close to level with the slab's surface height. I'm thinking about four inches, but I guess I'm thinking that because I'm guessing that is approximately the thickness of the slab.
I am completely open to advice and suggestions for correcting this problem?