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Question
I have extensively searched the internet for answers/tips on how to attach a new basement foundation to an existing (exterior) house/basement foundation. Finally I came across an article that satisfied me in terms of securely attaching and sealing the new foundation to the existing foundation. Here is the question/answer pertaining to that article (I have omitted some parts of the article).
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Expert: Bruce E. Johnson - 2/7/2009
Question
QUESTION: I would like to build an addition on my home with a full poured basement and two stories. I am wondering what the best method is to join the new foundation walls to the existing foundation walls. We live in a cold climate where frost is a concern although the basement will be heated.
Answer
Hi again Stella, okay, cool. If your existing foundation is a basement also then you will need a vertical column of dowels inserted into the existing wall. Each dowel will correspond with the horizontal steel of the new foundation wall. This will include the footing steel also. The joint where the two foundations meet should have a bentonite expansion joint of 1/2" if available or a 1/2" joint of standard expansion material. After the pour the expansion material is cut out to a depth of one inch one each side of the wall. This joint is then packed with a foam backing rod and caulked with a urethane caulking. Then a strip of peel and stick ice and water shield rubber flashing is applied over the corner joint (one half on the existing foundation, one half on the new foundation. This will give you a waterproof joint that should never need replacing. You also do an interior caulk joint with the backing rod and urethane. I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
I am wanting to do a somewhat similar project on my house as well. However, my side-by-side basement plan is only for a possible future additon to the house. This is why I have chosen to do a full basement with footings, slab, and 6' to 7' high walls (concrete).
The existing house foundation/basement sticks out of the earth approximately 2' to 2 1/2', which means it is actually only about 6' deep into the ground. So I should only have to dig down 6' or 7' to get to the existing house's foundation footing (plus depth for crushed rock for new footing, etc...)
I am wanting to bring the new foundation up to about 1' below where the floor level of the inside of the house is (which is 3' to 3 1/2' above outside ground level). This is so I can set floor joists on top of the new foundation and have them attach to the exterior of the house as to be at the same level as the interior floor level of the inside of the house. The reason for this is so I can knock out part of an existing upstairs exterior bedroom wall/window and install a patio door. Thus being able to walk out of the patio door on to patio/deck. YES, I said deck!
My plan is to build this structure in a way as to initially serve as a patio/deck (30' long x 16' wide). With a walk-down entrance (from outside) down into a basement-type room which will accomodate a hottub and other entertainment. The patio/deck would be 12" floor joists covered with 1" plywood, covered with waterproof deck material.
Windows will be factored in to new foundation walls before pouring concrete. However, I have no initial plans of cutting a doorway from existing house's basement foundation wall through to this new basement. I am guessing that fire codes may ask for exit/egress locations for such a structure. There are 2 existing basement windows that already adjoin the two structures (one of which I would choose as the location to cut in a future doorway as to join one another as a full/single basement.
Quite a solid stucture for just a deck and underground hottub?? Well, this is to accomodate a future addition to the upstairs of the house as well. The footings, foundation, floor joists, floor, etc.. would all be in place.
house new foundation
+++++++++------
+ <+> - ) = existing basement window
+ +) -
+ + - <> = future patio door
++ +) ----
+ + <stairway to under deck. 10' wide to
+ +--------- install hottub
+ +)
++++++++++
I think some of my concerns would be as follows:
* moisture and humidity in new basement due to hottub.
-waterproofing the concrete interior walls,floor and ceiling?
-possible extra ventillation needed?
-drainage for the floor in the event of water spills
(plumbed in to existing house plumbing?
* weeping tile for exterior footing of new basement(not sure if existing house has weeping tile).
* waterproofing both exterior and interior walls of new basement.
- in waterproofing both sides of the interior/exterior concrete walls, does this allow the concrete to breathe or does it need to?
I guess I'll cut this long and detailed queston off now and just end by saying that I will obviously be taking my plans to a local engineer and contractor. But I was so pleased with your answers in the above-mentioned article, that I figured whatever suggestons you may have for me I would hope to hear something (hopefully) very similar from my local engineer and contractor.
Thanks for the opportunity you have provided everyone with in asking questions and getting answers. Hope to hear back from you.
BTW - I too live in a very cold environment up here in Canada!
Hi jason, okay, I think I have it straight. You want to build a basement foundation next to your existing basement foundation, put a waterproof deck above it and a hot tub inside it. With that in mind I have a few comments to add to what you have written.
First of all, your existing basement wall is approximately 8' high or there abouts. Therefore if you cut that height down a foot to accommodate 12" floor joists you will probably want to make your new foundation a foot deeper than your existing one otherwise you will only have 7' of headroom in the new addition and in a room that size it will seem too low, especially if you are tall. 7' is an acceptable height for bathrooms and kitchen soffits but for entertainment areas etc. 8' or higher would be preferable.
Yes you would need to waterproof your exterior walls, on the outside, to stop moisture from penetrating through. You obviously would require a vapor barrier under the slab and a full blown roofing system on top of the floor joists. Besides waterproofing, you will need a footing drain installed around the perimeter to keep water from building up around the addition.
I don't usually recommend interior waterproofing because if the exterior is done properly you shouldn't need it.
A floor drain in the new addition would be nice especially if you have a hot tub down there. Your existing plumbing is probably deep enough that you should be able to tie into it at the proper eleveation to allow proper drainage. Or a sump pump could be installed if your finished elevation is lower than your existing sewer lateral.
As far as moisture inside the room upon completion. Yes you will need to address this issue either with air conditioning or a dehumidifier. The humidity should be maintained around 40-50 percent. You can go lower in humidity but much higher and you will begin to see signs of moisture accumulating on the surfaces over time. Your best bet would be a small heat pump unit that will give you heat in the winter and cooling in the summer plus dehumidify the air all the time. The airconditioning system should probably have some outside air added to it, most air handlers are equipped with a 3-4" port to duct to an outside air damper for make up air or you can duct an outside air duct to the return air duct or air handler plenum. If it is only one room you could probably just vent through the floor joist to the outside with a simple vent to allow outside air into the room.
As far as egress goes, one exit should be all that a room this size requires. Remember also that a walk down or walk out stairwell from the outside will need retaining walls on either side, footing drains and a drain in front of the door to keep the stairwell from filling up with water during a rain storm or winter snow melt or a roof structure/ cellar door covering. I hope this information helps. Please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
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| Comment | Just wanted to say thanks for the well-described answer to my questions. The information was a great help in my deciding on the right contractor for this project. I had to haggle with the city to allow me to go ahead with my plans, however they finally agreed once their engineering department had a look at my drawings and the detailed description of how I wanted to attach two separate foundations together (thanks to much of the information contained in your response). I was originally only wanting a crawlspace under the deck, as it seemed to be a waste of space not to utilize this area underneath, but once I looked at the square footage of my backyard and wondered where I might put a hottub in the future, it dawned on me that maybe I could go much deeper than just a crawlspace and use the area under the deck (rather than on/or inserted in to the deck) for a hottub. The option will always be there in the future to join the two "basements" together, but for now I save a bunch on land taxes as it is not considered living area. SWEET! Not much incentive to add value to your property when the city wants you to pay more taxes for doing so. I have never understood this concept. Thanks again for your insight and the service you have provided for those of us who are not all that experienced/educated in the construction field!!! | ||
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Building Homes or Extensions
Answers by Expert:
Bruce E. Johnson
Expertise
I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.
Experience
Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.
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