Replies

1. 
   DanH | Apr 20, 2010 07:25pm | #1

   I think the customary thinking is that if you have the perimeter piped well then there's no need for anything in the middle -- that basically the water infiltrates horizontally and not (so much) from the bottom up.

   But I don't see how additional pipes could hurt, and they would provide for a better radon sucking system should that ever be needed.

1. 
   Homewright | Apr 23, 2010 05:02am | #11

   Ever heard of hydrostatic pressure?

   that basically the water infiltrates horizontally and not (so much) from the bottom up. 

1. 
   DanH | Apr 23, 2010 09:03am | #12

   Like I said, probably unnecessary, but won't hurt.  The perimeter drain handles the water coming in horizontally, and that's the route for 99% of the water entering.

   Hydrostatic pressure can be a (minor) factor, but in all but the most obstinate soils "water seeks it's own level" and the perimeter drains act like pool scuppers to drain away any water infiltrating from below.

2. 
   davidmeiland | Apr 20, 2010 09:09pm | #2

   Assuming you are doing footings and stemwall first

   and then filling and pouring a slab, you should be fine without perf pipe. One key is using gravel as fill, and hopefully you used drain rock and not crushed rock. Drain rock will not wick nearly as much moisture up from grade towards the bottom of your slab. Using sand or crushed rock as fill really changes the equation. Another important piece ismaking sure all your drains easily flow to daylight. Give the water an easy way to go out instead of collecting and seeping upwards. 

   I built a house about three years ago not too far from you, with similar concerns about water under the slab (in this case because the house is on a rock shelf with a lot of upslope exposure. I poured footings and walls 24" high, filled with 18" of 7/8" drain rock, laid Stego Wrap down and taped it (including around all the various pipes and conduits coming up), laid 2" XPS and rebar over that, and poured the slab. I'm pretty sure that water trickles across the ledge right under the house, but the slab has stayed very dry. We also did a very thorough job of getting footing drains in at or below the level of the bottom of the footing.

   Oh yeah, pipe your roof water well away too, don't just dump it next to the house. I see that all the time.

1. 
   bd | Apr 22, 2010 10:15am | #7

   Drain rock

   So, David, what is "drain rock"? Are you talking about pea gravel or something else. Never heard the term.

   Ed

1. 
   davidmeiland | Apr 22, 2010 10:28am | #8

   Around here

   drain rock is a graded product from a rock crusher. They crush a load and run it thru a 7/8" screen so that stuff larger than 7/8" is kept out. They run it again thru a 5/8" screen so that stuff smaller than 5/8" falls thru. During this second screening they rinse the rock to get the fines out of it. The result is a fairly clean product that's uniform in size and it's somewhat angular because it's a crushed product. It works well as an underslab base because you can compact it fairly well, although it doesn't compact the same way as driveway rock, which is 5/8" and everything smaller, including a lot of fines.

   What you might find elsewhere as drain rock is river gravel that's graded to 3/4" - 1" size. More rounded and very clean, may not compact as well. 

   The idea is that water can easily descend thru drain rock, and there aren't any fines to support root growth. 

   You need to work with whatever's locally available and stick with standard/best practice for your area.

3. 
   Homewright | Apr 20, 2010 10:58pm | #3

   Speak of the devil (groundwater)

   I've been a couple of months into dealing with groundwater issues in gumbo here in Texas.  I'm managing a room addition where our slab couldn't even be prepped until we drained the area.  This involved removing two feet of topsoil to be replaced with a red clay/sand mix for compaction.  The water table is so high on this site water was wicking up through the slab and soaking the carpets of the office next door to the addition site.  And the office is uphill.  After cutting away the top and bringing in 15 trucks of fill and packing it in, I had the excavator cut two 3' deep trenches in thirds sections across the footprint of the addtion slab.  We took concrete block and rammed them into the bottom of the trenches then filled to the top with 3/4 limestone.  This first line of blocks were set for pitch toward the collection point, a buried plastic 55 gallon barrel for a sump pit.  The hole it sits in is lined with six inches of gravel and the barrel itself was drilled in the lowest quarter and floor for seepage collection.  Next was socked, perforated pipe down the center with another layer of blocks holding it in place and providing protection from crushing.  Another layer of limestone to the top of the second layer of block locked it in.  Perimeter drains were cut below the footings with the same socked and gravel above and below configuration with the same pitch to the sump.  The last line of defence was gutter drainage into the same sump.  The sump runs several times a day as a norm in the rainy season but at this time is still dropping the water table at this particular site so it's cycling a little more for now. 

   It largely depends on the specific site what is and isn't necessary.  This property I'm working on has a neighbor who needs a boat to get out of his driveway anytime it rains even moderately.  The sump empties into a ditch I cut through the natural flow of the land to empty into a creek across the road.  So far it's keeping up with our periodic rains and has stabilized our addition site.  I did another drainage system at the office uphill and have been able to keep the carpets dry while diverting all the runoff from the concrete and roof.  I reclaimed a 15 years abandoned septic tank, then punched holes in the walls and floor to act as a wick for groundwater.  A  sump pumps it out to the ditch feeding into the creek.  A couple of strategically placed drains in the parking lot slab where I could water drill to pipe out the water into the septic has proven to greatly divert the water that was previously causing flooding issues.  I guess the gist of the story is each situation is going to differ somewhat and how you approach it needs to address the core issue and not try to bandaid over something needing open heart surgery.

4. 
   Scott | Apr 20, 2010 11:10pm | #4

   Rather than pipes under the slab, have you considered any of those drainage plane products that sit up against the concrete wall? There are various types, but they all create some sort of fibrous or mesh barrier that allows the water to fall before it can create any hydraulic pressure against the concrete. Those around here with high water table issues report good success.

   Of course, once the water falls, it needs to find good perimeter drains. Be sure to use lots of drain rock, covered in felt. I would use perforated PVC, not the corrugated black stuff.

1. 
   wadswob | Apr 21, 2010 01:35pm | #5

   Thanks all.  Scott...are you talking about using the drain mat on the walls or under the slab?  We are planning to use it on the walls, but I noticed in the brochure for the stuff (Platon) that they also suggest using it under the slab.

2. 
   wadswob | Apr 21, 2010 01:36pm | #6

   Thanks all.  Scott...are you talking about using the drain mat on the walls or under the slab?  We are planning to use it on the walls, but I noticed in the brochure for the stuff (Platon) that they also suggest using it under the slab.

5. 
   junkhound | Apr 22, 2010 12:37pm | #9

   Seattle, basement...wow, you must really be building it yourself, have not seen any builders putting in basements, <G>

   Anyway, own house in Renton and son's in Kent have basements.  Own house from day one, son's i dug out.

    Both drain to daylight, but high side is 8 ft below surface level.

   BOTH have both external footing french type drains AND Both have 4" gravel with 6 mil poly over pea gravel under the basement floor with 4" perf pipe on 10 ft centers. External is 1"-1/2" graded rock.

   External and internal all tied together , on own house 'daylight' is another 40 ft downhill with drain pipe/gravel to about 2 ft below bsmt floor level - which is also the high water mark of the pond in winter.   

   Put in lots of drain pipe, you wont be sorry, sorry only if you dont.

6. 
   DanH | Apr 22, 2010 09:41pm | #10

   And for under-slab drainage you want the drainage down several inches, not right below the slab.