Expert: Dan Griffin - 9/21/2008

Question
Hi Dan,

Hi Dan,

Project background:
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1) More than 15 years ago, right before 'finishing' the basement, we dug 7-8 feet deep around rear, side and front of the house foundation, covered it w/ tar, tar paper and half inch foam. Unknowingly, i did not then fill it to the top with a foot wide of gravel nor did i lay a perf'd pipe 1' below grade to redirect water from back of the house to the side and down to a well. Nowadays, I'm told that Town code prohibits draining out water to sidewalk which runs parallel to front of the house some 30+ feet away.
 
2) Soon after this exterior foundation insulation effort, we built a cement patio (4" thick, 11' W, 28' long) flushed with rear wall of house.  In 20 years of living here never had a flooded basement until a year ago in the Spring when it rained heavy and light, on and off for about 3 days/nights.

A short time after cement patio was built, had to add a cement 'band' (18" W by 8" H) flushed against the flashing all along the rear wall and tapered out so that the rain water would not just sit there. It seems that section all along the wall in back of house, closest to wall sunk somewhat or original contractor did a poor and rushed job. The cement patio starts from one side of the below grade stair way into basement, and stretches 10' past one corner of foundation of the house. Extra 10 feet is the side porch, partially divided/enclosed 1-2' above grade.

We had 2 small termite sworms in one straight line in basement this spring. Conditions favored possibly by flooded basement in last spring. The termite treatment was applied along the rear wall off the house, injected through drilled holes on the cement patio. By law, it can only be applied exteriorly.

I suppose that the foundation wall insulation of 15 years back is mostly useless by now since it has been with soil contact; especially the rear foundation wall which would have soil with the most moisture.  The house is some 120-150 feet from the top of the hill area. On very heavy rains, water cascades off the retaining wall 50' above the house.  But we've channel a good part into 18" square drain that drains with a pipe running diagonally down to the other side of the house.

Proposed correction
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1) replace cement patio because it has various cracks including in the later 'patch' or 'band', this time properly pitched out. After laying the frame, base gravel, and wire mesh raised 2", get the pre-mixed 'drum' truck delivered concrete cement that can be just poured into frame.  But what type is bets?

2) build a 4' wall, cement dropped or cinder block, whose exterior side is flushed with edge of replaced cement patio, some 11 feet away from the house. And the top of that wall would be covered by new cement patio. After applying tar, tar paper and foam insulation to this new wall's exterior side, fill it to top with gravel 1' wide (maybe more), laying a  perf'd pipe 1' below grade to collect water 28' out to the side connecting that pipe to a solid pipe down the side of the house into a well, yet to be dug out. This wall, running parallel to rear foundation wall and 10' past it as explained above, would server to keep water/moisture away from most soil below cement patio.

I believe i have a better concept of what needs to be done than what was done 15 years ago or so. Unfortunately, I'm not in the construction business line of work to know best materials, tools and methods. All i know is what some friends say or contractors respond to when giving an estimate.

Concerns and pitfalls to avoid
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Should some 'posts' (cement or block) flushed against the rear foundation wall and down to 'base' be built to help prevent the same 'sinking' issue with prior cement patio? If so, about 6' apart, using rebar or whatever material that is bendable so that the extended portion of the bars above 'posts' can then be bended sideways parallel to foundation wall and right below bottom of new cement patio?

Cost permitting, should we dig additionally just the rear foundation wall and reinsulate it, this time adding 1'(or maybe 2) of gravel,1 layer of filter fabric next to soil, and perf'd pipe connecting it to new pipe that would run down side of the house?

I thank you very much in advance for any tips and sound advise you can provide in what is most recommendable in materials and methods for:

- new wall
- new cement patio
- rear foundation wall retreatment; extra support 'posts' along rear foundation wall below new patio.

if you think it helps, please also pass this request for help along to other peers at "Allexperts.com" that you know of. I will be participating in as much of the tearing/removal of cement and digging of soil as my frequently muscle-spasm inflicted lower back allows.  

Thanks very much for all your help. I can definitely use it as soon as possible.

Regards,
Santiago

Answer
Santiage, this is quite a letter.  There is a difference between damp proofing and water proofing.

If this was brand new work, here is the recipe for successful damp proofing of a basement.Install a drainage pipe at the bottom of the footing well below finish floor.  Use perforated pipe with the holes down encased in coarse gravel with ground cloth protection to prevent silting in.  This drain pipe would need to flow to daylight, a substantial drywell, or a sump pit either on the inside or the outside.  The basement exterior wall would be treated with a rubber based or asphalt based waterproofing layer applied by spray, roller, trowel, or brush.  The waterproofing layer would be protected by a temprorary junk board - cellulose board or Styrofoam are usual because they are cheap, this sheathing is to prevent rock and clay from scoring through the water proofing layer when the dirt is shoved back in.

Dirt would not normally be compacted in a residential project and it may take mother nature up to two years to "settle".  I would assume yours is finished settling by now.  Patios and any other exteror concrete should have a minimum of 1/4" fall per foot away from the structure.  I see no value in gravel under an exterior slab.  Use rebar dowels into the foundation if your are concerned about more settling, but I doubt it to be necessary.  All grass and dirt areas should fall away from the house a minimum of 6" in the first 10' by code - this is a bit more than a 1/2" per foot.  Avoid planting areas and flower beds next to the house and make absolutely sure that gutters and downspouts get water well away from the building perimeter.  Healthy sod on good soil will shed water to drainage before sending it deep into the ground.  This issue of surface drainage is the number one cause of water problems and can usually be corrected by simply maintaining the builder's original grades.  It sounds as if you grade is not turning the water before it gets to the house, the drainage should move heavy water around the house without ever getting to it.

Moving water in a shallow French drain might help, but the real cure is to have the surface drainage get away on the surface with the French drain below the basement finish floor dealing with water table and deep water issues.