A perimeter drain collects the water that runs down off of the exterior of the basement wall and sends it out and away from the structure. Perimeter drain or not, if your exterior walls are not treated and waterproofed, moisture will still get in. Since you will be excavating for the drain anyhow, that would be the time to treat your walls at the same time giving you a tight seal. Sloping the grading after the excavation is filled away from the house is important as well as gutters on the house. The main objective is to not allow water to pool up against the walls but rather drain out and away.

Sierra, Thanks for the reply. A stupid q- when you say "exterior of the basement wall" do you mean the outside of the house or the inner side of the exterior walls? The reason I ask is all the pics I have seen are of perimeter drains running all around the perimeter on the inside of the basement! Our interior concrete walls are treated with dryloc-so I presume there are no leaks from the walls but since we have not taken down the finished part yet we don't know what's behind it!
Thanks for the tip on grading and downspouts.

One remedy for basements that leak when you dont want to excavate the outside and install a perimeter french drain type system is to do an interior perimeter drain usually coinciding with a sump pump. I was referring to the exterior, but if your interiors are dryloc'd, then i think i would go ahead and see what kind of shape the interiors of the walls are by pulling the paneling and re-coat where necessary. It would be a major expense to excavate the outside of the house, installing the drain and treating the walls if you dont really have to. Just make sure that all the water on the outside has no chance of pooling against the outside wall. The foundation will still get moisture, the dryloc will be a barrier on the inside to keep that moisture from getting in. You still might have issues at the base where the wall meets the floor, so that would tell you that a inside perimeter drain would be necesssary. You might get estimates for an exterior excavation/wall treatment, perimeter drain, as that's really the best avenue overall never giving moisture a chance to penetrate the foundation . If you find that the dryloc is working, then i guess i would go ahead on with finishing the basement, just be sure it's doing it's job.

Thanks for the clarification, Sierra. We have been getting continuous rain since yesterday eve. So far there have been no leaks. So I guess we are doing ok. Will check the wall-floor junction. Is there something that can be done to mitigate that short of installing an interior drain?
Another q I had was how does one control ground water seeping in through the floor? Install a vapor barrier below the floor?
Thank you for your time.
Vinaya

There are coatings to seal the floor but if you are doing any type of glue down flooring, you want to make sure that the sealer is compatible with the adhesives. The sealer acts as a barrier to keep moisture from coming up through the concrete into the flooring. Here's a link. Click on the bottom where it says "dry up wet basements" for an interesting product that works on the inside. There are other sites if you google "basement waterproofing tips"

Here is a link that might be useful: waterproofing

To answer your original question, when is basement waterproofing necessary?

Waterproofing is necessary when there is evidence of water or excessive moisture in the basement or where there is a strong likelihood of water leaking into the basement, but it may not be necessary in all cases.

My Grandfathers farm house has hand laid fieldstone walls and a dirt floor in the basement, yet it the 100+ years since the house was built there has never been any sign of water in the basement.

There are many factors that effect whether or not you will get water in a basement, such as but not limited to:

1. Elevation of the structure in relation to the immediate topography.
2. Type of soil.
3. Height of average water table.
4. How the surface is graded to direct surface water flow and storm runoff.

Just arbitrarily digging a trench around the footer wall to install waterproofing could introduce problems where none exist now. By example, all the soil around my grandfathers foundation has been settling for nearly 100 years so for all intents and purposes we could consider it "Undisturbed soil" and most likely at maximum compaction. If we were to dig a trench around that wall it would loosen that soil and invite surface water to seep down around the basement wall.

You stated that you don't believe anyone in your neighborhood has a sump pump. That is not unusual in some regions where the terrain and soil conditions are right.

I would strongly suggest you stop by your local county engineers office and ask them. They have very detailed maps of the soil and groundwater conditions and they could advise you whether or not ground water is a problem to be considered at your location, and if waterproofing is necessary at your location they can advise you as to what methods will work best. If nothing else, the county engineer is your tax dollars at work and he/she is not trying to sell you anything, whereas I have seen some slick basement waterproofing companies that would go to the Sahara desert and recommend basement waterproofing for the Pyramids if they thought they could get the contract.

There's something to be said about proper compaction practices on a "re-fill", but as always, great advise from lazy pup. Always best to see how it's done in your specific area rather than generalizations.

Thank you, lazypup! That is a really comprehensive explanation. I will check with our county engineer.

When is basement waterproofing necessary?
If your basement is concrete - Always. No matter where you are - whether a dry or wet climate. Concrete may seem solid but it's not waterproof. Even if there are no visible cracks - water will get through it. It's simply in the nature of concrete.

The best way to solve this problem is with 1' rigid Styrofoam, pink or blue, laid right on the concrete floor, not glued to it, use a wide red tape called Tuck Tape to seal all the joints of the Styrofoam, and use a low-expansion spray foam to seal all along the outer edges.

You can then use 5/8' tongue and groove plywood screwed, with Tapcon screws, (designed for concrete but expensive) through the foam and into the concrete floor.
Now you can install whatever type of flooring you would like

The foam is an insulator with provides the thermal break while keeping the warmer room air from reaching the cooler concrete floor and creating the moisture that eventually leads to mould problems.

For the walls use 2' rigid Styrofoam  either pink or blue  use proper adhesive for foam board
- Glue to the walls every 12 inches or so
- Leave ¼' gap all around each board and fill with spray foam to seal boards together
- Tuck tape all the seams  you want no air movement
- You wonÂt need any vapor barrier
- Now you can put up your stud wall 1' away from Styrofoam

This space will give you room to run your wiring, plumbing, etc. Be sure to use mould proof drywall.

If you follow these steps you'll have a worry free basement - free of moisture and mould problems.

Here is a link that might be useful: The site for Finishing a Basement

I would never finish off a basement without waterproofing. We don't get water in our basement, but I would hate to risk all those finished products and then have to add the waterproofing later. Call it insurance!

Don't confuse waterproofing with drainage.

Waterproofing goes on the outside of the foundation walls and the slab should have a gravel layer to break capillary action and a vapor retarder directly under the slab. Insulation at that location or over the exterior waterproofing can help keep the concrete warmer than the dew point to avoid condensation.

If you don't already have exterior waterproofing and insulation the addition of perimeter drains might remove standing water from the basement and a humidifier hooked up to a drain (forget the bucket) will eventually reduce the amount of moisture that condenses on the colder walls and floor. Make sure there is good air flow in all parts of the basement. Be sure the dehumidifier is large enough so it won't have to run full time. Be sure it has a 5 year warranty; they die often. You might need two of them if for no other reason that one can be working while the other is being repaired. You don't want the humidity to build up again even for a day.

I meant to say a "dehumidifier"