My BI asked me if I’ve ever put diapers under footings. I replied that , since I never had kids I’ve never put a diaper on anything…but what did he mean? He considered that as we put bitumen on the outside of the foundations, and vapour barrier over the ground in the crawl/basement, why don’t we have a membrane wrapped around the footings…the place where water is likely to wick up the wall. I said I couldn’t undertake to put bitumen on the underside of the footings, but something like Tyvek, plastic VB, or even the wrapping from lumber lifts might probably work there ok.
And that I’d run it by “the guys”…
All the best…
To those who know – this may be obvious. To those who don’t – I hope I’ve helped.
Replies
Piko.
Diapers? Soiled or not? Would it make a difference? What is the joke here?
Now, you've helped me in the recent past. Help me understand this.
Mark, nailer01
I expect you'll see from the other posting that there is a serious point to this. Sure, it sounds humourous - daiper - but that was the BI's mode of speaking. Essentially - wrapping a waterproof membrane around the footings to stop groundwater from wicking thru said footings into the basement...All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
Piko
I get it now after reading the thread. Sorry, after six beers your post struck me a funny.
Mark
What he's asking makes a lot of sense- though probably not applicable for most residential projects.
In a high water table area, you've got to create a complete waterproof enclosure, or a "bathtub" as we call it in commercial work, to keep water out. This usually occurs when the water table itself occassionally rises above the level of the basement slab.
The easiest way to do this is to use a membrane under the slab and footings, which gets tied into the wall waterproofing system to create a system with no openings or seams. Typically this is done by pouring a "mudslab" (a 2" thick layer of concrete to give a solid working surface), and then installing a material like Grace's Pre-Prufe membrane which is placed with the reactive side up. When you place concrete footings and slabs on the Pre-Prufe, a chemical reaction occurs that "welds" the membrane to the underside of the concrete, creating a waterproof barrier.
The membrane gets wrapped up the sides of the footings as well, and is tied into the wall waterproofing, which is usually a material like Bituthane with protection board over it to protect it during backfill.
There are other things to be concerned with in situations like this, such as having enough weight in the structure above to keep it from lifting due to hydrostatic pressure if the water table rised too high. You also need to consider providing relief drains that will allow the basement to flood if water gets too high up the walls. If you don't do this, the water pressure could push the walls in.
I recently completed an office building with a 4-story underground parking garage, which was located around 100 yards from a major river. The lowest level of the garage was +/- 2' above the mean high tide level of the river, and well within the 100-year flood level. We had 4' thick mat slabs to create enough mass to prevent uplift, and drains around 4' up the walls to allow water in. This was in addition to a massive dewatering/pumping system. If the pumps failed and the water rose high enough, the garage would flood rather than collapse. Really made you want to avoid parking on the lowest level of the garage during a hurricane ;)
Bob
I agree - in some instances this is necessary. In 'normal' cicumstances what you've described would be overkill. For the purpose of my Q then, essentially, you agree in principle. Personally, I've yet to be utterly convinced - I've not seen anywhere 'normal' yet that would benefit...All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.
Bob,
You are right on the money. We had a house we were going to design on a lot where there was a high water table but the clients wanted to go ahead and build a house with a full basement anyway. We looked into the exact system you just described minus the relief drains (interesting that aspect was not brought to our attention by the reps or the engineer). The rep for the company that offered the waterproofing casually explained that they offer no warranty on their system whatsoever. That kinda drove the nail in the coffin for that project. Although, I really can't blame the company, too much liability and variables. I think maybe it's not practical for residential use unless in extreme conditions.Also, since you are basically designing a boat hull for a basement, and as you said, you are resisting enormous hydrostatic pressure, the engineer was proposing essentially one big footing for a slab. If I remember correctly it was like 12" thick with #4 or 5 bars e.w. along the top or something (it was a while ago).Anyway, between the added cost of the foundation/boat hull plus the 35,000 waterproofing for the 5,500 footprint, there was nothing in the end but a dead deal and more lot searching."I'm your huckleberry"
Interesting that they didn't offer a warranty. Grace offers a 20-year warranty on the stuff we used. They send their rep out several times during the course of the installation to inspect, and give you written reports on their findings. If everything is OK, you get the warranty.
Bob
WarningIf anyone does this they need need to take care of the ground electrode system.NEC now requires a "ufer ground" for new construction "where available". That use 20 ft of rebar in the footing.If the foot was wrapped then it would not function as GE. Make sure that they is acknowledge by the inspector and alter methods allowed or you might end up having to burry a 20 ft long chuck of concrete for for the GE.
We used two grounding triads outside of the building footprint, bonded back to the structural steel frame.
Bob
Which version of NEC is that? And, is the ufer ground connected in parallel to the ground rod driven into the soil or what?
The ufer ground as been allowed for a long time.But starting with the 2002 version it is required. I don't have either the 2002 or the 2005, but I know that there was some changes in the wording.If I am not mistaken that the 2002 made the Ufer mandatory and the 2005 revised to only require it was available.Now the code requires that any and all electrodes of the follow types be used.a) metallic water pipe that extends at least 10 ft underground.b) metal frame of a building where "effectively grounded" (whatever that means).c) a conrete encased electrode - 20ft or more of 1/2" or bigger steel rebar encased in concrete in or near the bottom of the foundation or footer - a Ufer ground.d) a ground ring.A home, before the use of ufer grounds, only the first and now many don't even have that.However, with a metallic water pipe as an electrode a seconard electrode is also required. My guess is because of the possiblility of it being converted to plastic sometimes in the future.Now that secondard electrode can be one of the above, but if you don't have one then a "made electrode" is required.A made electrode can be other metallic pipe and underground tanks (except gas), a plate electrode, or a ground rod.Most common in residential applications is a ground rod.And if the ground rod is the only electrode then either it needs to be tested and have less than 25 ohms OR a another ground rod is needed.So pre-ufer common home ground electrode systems where.a) metallic water pipe and ground rodb) plastic water pipe and two ground rods.With a ufer ground you only need the ufer ground and the metallic water pipe if it exist. That is based on the NEC, however it may be modified locally and/or inspectors might have their own interpretations.d)
Here in NC we use NEC 2005 as printed. I do not know if local inspectors/inspection departments choose to forgo certain aspects of the code nor have I read the paragraphs you are referring to, however for single family homes the only ground used is a ground rod. There is one jurisdiction (township) that requires 2 ground rods, but then they want all kinds of extra stuff not in the codes the state has adopted. It makes the houses more expensive.
Here is another aside - do you know about the conductivity of concrete?
"Here is another aside - do you know about the conductivity of concrete?"I don' have any specifics, but I know that it is conductive. That is why GFCI's are required in unfinished basements.The two things that you have going for you is that concrete is hydroscopic and that you have a very large surface area for contact with the ground. While a small area probabably has a fairly high resistance you have lots and lots of of resistors in parallel and thus a much lower resistance.
Edited 2/3/2006 9:01 am by BillHartmann
A good read: The Ufer Ground
More: Eleven practical tips for grounding substations.
Contains a brief note on conductivity: http://www.scott-inc.com/html/ufer.htm
2x10's w/ cheap plastic would have the same results.
Have used Fastfoot on a couple of jobs and I really like the stuff. We use 2x4s held up with normal steel stakes. After the pour the lumber goes into the framing pile. Beats the heck out of packing around and storing 2x10s.
Some concrete guys have said we're catching on as they've used reinforced poly for the same thing.
Beer was created so carpenters wouldn't rule the world.
I lined my forms with 6 mil black poly, not because of a high water table, but rather because the ground is extremely dry here. The plastic ensures that the concrete in the footing neither gains nor loses water. It hydrates under the exact same conditions as the stuff in the plastic test cylinder.
-- J.S.
This company swears that "rising damp" is a contributor to wet basements.
They sell a roll of form liner that prevents this, and is the form itself, saving lumber.
I used it on the last house, but I'm unconvinced. 2x10's w/ cheap plastic would have the same results. Here is a link...
http://www.fab-form.com/products/fastfoot/prevention.html
THanks - I'll go back to my BI with that...All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.