Hi to all,
I am waterproofing my home’s foundation. This is an old house with a granite foundation. Have Delta MS exterior membrane and will use this as well as Henry brand tar on the exterior. I had an engineer look at this and he said that the basement had been dugout and that the foundation only needed to be dug out to the new interior wall height (i.e., the top of the new interior walls). In some cases, this would mean only digging down a few feet (3 or less) on the exterior. He also suggested a french drain at the base of the exterior excavation.
Knowing that engineers are not always the most well informed from a building standpoint, I would appreciate any feedback regarding both the depth of exterior excavation and the french drain suggestion.
Replies
That might be true if the only water that ever exists in your soils is in the upper three feet - say from rain that never penetrates any deeper and no high groundwater levels.
But that would be a rare case. Don't know where you are or what sort of soils you have.
In general, the primary thing to do in waterproofing is to give the water someplace to go. That means a perimeter drain at the base of the foundation wall leading to daylight or to a sump or drywell.
various people have different definitions of what a french drain is, but his description is close to my understanding of it, a sloppy half azsed measure when one does not want to do it right, or a surface catch basin employed when the real drain tiles have failed due to age or tree roots clogging them. It can help for drying up a soggy lawn or souppy driveway, but not to waterproof a foundation wall.
Another way to put this - if you only dig down three feet to add drainage and waterproofing, any water that makes it's way past that, or that comes up from below, will still enter the lower two thirds of your cellar wall.
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piffin,thanks for the response. what type of drain should i use?zac
Perforated pvc, scedule thirty is what we use. You can also get a flexable 4"drainage line that is perf, but is much thinnner walled. We do use it occasionally where there are lots of twists and I can use prime backfill. After you digand lay the line in, you need to surrond it with clean gravel and use fabric to keep the fines from clogging it again, or you can buy the kind with a fabric sock over it.
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Piffin has it right; the tar and membrane should be thought of as a raincoat...not a drysuit. If there is sufficient hydrostatic pressure from the outside, water will go through it somewhere.
You need to dig down below the footings. Then, if you are on bedrock or if the ground is not highly permeable, you need to make sure the footing on the downhill side has exit drains through or under it so that any water that comes up under your slab can find an easier way out than by squirting up into the basement. One 4" BNQ per 15 linear feet of footing is the general rule I use.
Then you lay the French drain. Place geotextile in the bottom of the trench and up the trench wall first, and then lay a 12x12" bed of ¾" washed gravel and a 4" perf'ed and filter-encased flexible pipe laid on top of that. If you have major ground water to deal with, use 6" pipe instead of 4". The top of the drain pipe should be flush with or slightly lower than the top of the footing. We then fill on top of the drain with another 24" of gravel before folding the geotextile over the top and filling with clean sand to within 6" of the top of the trench. Garden earth goes on last, usually over another layer of geotextile to protect the sand from contamination for as long as possible.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
dinosaur and piffin,thanks for the help. it all sounded a little weird to me but the structural engineer insisted that that was all that was needed. i live in atlanta and we get LOTS of rain. so, after digging down 3', i thought how can this possibly be right? that's when i decided to ask the experts. and sure enough, the engineer is wrong and i would have blown a gasket after refilling the 3' only to find i still had a wet basement!z1
dinosaur and piffin,a coupla more questions. where do i get geotextile? can i use a pvc plumbing pipe and drill holes into it to make a more substantial pipe than using perforated black landscaping pipe? if so, how big should the drill holes be? if not where do i get the pvc that piffin mentioned? what is BNQ?thanks again.zac
I cal my lumberyard for any and all of this, but often get the textile fabric from the dirt guy who brings the gravel. Lay the pipe with the holes down at four and eight o:clock
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You can get small rolls of geotextile in any garden center; but for your application you need the 9'x100' roll sold by commercial suppliers. Check your lumberyard or (shudder) the local big box.
The rigid pipe we use here in Québec is white pvc and quite robust. It's called BNQ which is an abbreviation for Bureau de Normalisation du Québec (that's the standardization office for the provincial government). It's available in sched. 40 or 80 for most residential applications. We use this type for all septic weeping fields and for some French drains. The holes are about 15mm dia. on 8" centers perfed at 12,3,6, and 9 o'clock IIRC
The flexible pipe we use is black; it resembles an oversized vaccuum cleaner hose being pleated. It is perfed with slits and is encased in filter fabric like a large sock. I find this is quite adequate for any but applications where you know you're going to have rubble in your backfill that could crush it. You shouldn't have any of that if you go with the method I've described. You must protect the gravel surrounding the encased pipe from organic matter infiltration, though; if you don't the filter fabric on the pipe will clog after a few years. That's why the geotextile. If you can't get geotex, use two layers of heavy weeping-field paper or a 4"-thick layer of straw between dirt backfill and your drainage sand and gravel.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Some pictures.
SamT
That another view is a great demonstration shot.I use builder's wire to tie the gutter drain up using a staple in the rim joist or sheathing. Then nip it off after backfill is done.
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Thanks for the photos. Have gotten down to the bottom of the foundation and am ready to put the drain in today.
Excellent advice. Just make sure when you call for fill you call for "washed stone", not gravel, to bed your drain tile into. In most places in the US, "Gravel" is kind of a generic term for sandy fill mixed with rocks of all sizes and can be known as "road base", "bank run", "boney stuff", "light fill" and a host of other names. You want washed, crushed stone.
regards,
jim