DEAR TIM: I am building an addition on my home and realize I must connect the new foundation with the existing one. What is the best way to do this? Is there a way to waterproof the connection? Will this connection become a hinge point in the future causing cracks in the walls? I am very concerned about how to make a connection between the structures that will stand the test of time. Paul M., Orono, MN

DEAR PAUL: I can remember many years ago having these same thoughts when I was building my first room addition. I couldn't figure out how the shallow crawlspace foundation would stay connected to the full basement foundation since they were at different elevations. My geology professors would have frowned at me had they been at the jobsite as I was not thinking through the situation.

Many people fail to realize that a typical foundation wall is, among other things, a beam. You begin to see this if you look at the vertical web in a steel I beam or even a simple wooden floor joist. Understanding this is essential. Steel I beams derive much of their strength by the thickness and height of the vertical web section of the I beam, not the flat part that iron workers walk on as they erect the steel. A foundation wall is no different other than the fact its vertical height is ten or fifteen times greater than a typical I beam.

If you accept this principal, then it stands to reason the foundation will not flex up or down if the soil beneath this "beam" is solid. This is perhaps the most important aspect of connecting the two foundations: making sure you have great soil conditions under the foundation of the room addition.

---

Connecting two foundations and need to waterproof one? Use this Basement Waterproofing Checklist to hire a great professional. I offer a 100% Money Back Guarantee.

---

If the soil under and around the existing house is solid and the house foundation has no structural defects, it stands to reason you will have no connection problems in the future. When new homes are built, it is often a common practice to pour different parts of the foundation at different times and at different heights within the soil profile. The fact that you are installing the foundations years apart instead of days will make no difference.

The masonry foundation or the slab should also contain structural steel. Dual horizontal steel bars 16 inches from the top and bottom of poured concrete foundations help to create a stiff concrete beam. Concrete block foundations can incorporate steel truss fabric that is installed in every other row of the concrete block units. Filling the hollow cores of concrete block foundations with a sand/cement slurry also adds strength. Concrete slabs can contain one half inch diameter steel bars on two foot centers in both directions to stiffen the slab substantially. A residential structural engineer can easily specify the best solution for your particular situation.

You have numerous options with respect to making a physical connection between the two foundations. I have successfully installed steel pins that are epoxied into the old foundation. On other jobs, I have bolted a steel angle iron to the existing foundation. These building components then become encapsulated by the masonry materials that are used to create the new room addition foundation. It is always best to hire an engineer to create a design for this connection. This is especially true if you live in an area with expansive clay soils or an area that is subject to seismic movement.

There are special waterproof connection materials that work well when connecting a poured concrete foundation to an existing one. These flexible waterproofing materials look like weatherstripping on steroids as they have small fins that project from the center of the main strip. The base is epoxied and sealed to the existing foundation and the strip projects into the center of the foundation form. When the wet concrete surrounds the strip, you have a waterproof joint so long as the foundations do not pull apart from one another.

I feel the best way to waterproof new foundations is to apply special waterproofing compounds to the exterior of the finished foundation. There are many different products and systems but all rely on excellent drainage around the foundation. You need to make sure your drainage system can collect the subsurface water and transport it to a low spot on your property or into a sump pit where it can be pumped to a storm water collection system.

Soil stability is everything when it comes to foundations. If the soil quality is suspect, you can ensure a stable foundation by installing piers under the foundation footer. Piers can be made from concrete, steel or wood, but a common one is a simple vertical hole that extends into the soil until it finds bedrock or stable soil. This hole is then filled with concrete

Piers can be spaced at eight or ten foot intervals and resemble table legs. The foundation, which is a beam, simply transfers its load down and through the individual piers instead of the unstable soil in between the piers.

Still Need Help? Type Your Keywords Here: (examples: drywall repair, pilot light goes out, poor lot drainage, crown molding guide, etc.) Your Facebook friends would love to know how this column helped you solve your problem. Type a quick comment of what you discovered here at AsktheBuilder.com. Thanks!

Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. shawn fontenot 19 Jan 2008, 10:00 I'm fixing to make a addition to my home and was concerned on how to connect the new slab to the existing one. I'm tring to prevent the seperation off new slab as the cement cures. AsktheBuilder 19 Jan 2008, 10:21 Shawn, You drill holes into the side of the existing slab and pound in pieces of steel reinforcing bars. Linda 24 Jan 2008, 14:31 We want to build an addition onto our lake cabin. The existing cabin is a log home that is placed on cement blocks. There is no foundation or footings under the log home. Is it advisable to connect an addition that would be on footings or a cement slab to the original without placing the original on footings/ basement foundation? How would we tie the two separate buildings together? AsktheBuilder 24 Jan 2008, 16:28 Linda, It can be done. You need to consult with a residential architect. Vanesa Mann 09 Mar 2008, 16:19 We have bought a home that has an addition (built in 1993). The older side (built in 1949) of the house has a basement that has an exterior part that the house rests on and an interior concrete wall that is about 2ft thick and 5.5ft high with 2ft dirt space between the wall and foundation. The wall has a crack in it (wider at the top hairline at the bottom almost the entire height of the wall) and the concrete is flaking, I also noticed that there are stuffers in the concrete wall (bricks possible other material) The concrete wall where the crack is, is the old exterior wall attached to the addition, behind this wall is a crawl space under the addition. I am wondering if there is actually load on this wall and if it is now sinking and what to do to repair it or if it is really part of the foundation or not? AsktheBuilder 12 Mar 2008, 16:56 Vanesa, You need to have a structural engineer look at this. ShawnN 11 Apr 2008, 11:06 I have an existing floating slab (with reinforcement bars throughout and significantly thicker cement around the perimeter) that I want to build a structure on. There were no metal bolts place along the perimeter to attach a wall to. I want to install a course of blocks around the perimeter before building the wood walls to raise it for better drainage. I was wondering if the mortar is sufficient to use to attach the blocks to the slab or do I need to drill into the slab and add some sort of metal bar to have a better physical connection to the blocks? Thank you! AsktheBuilder 18 Apr 2008, 14:23 ShawnN, You need to talk with an engineer. This is also a code issue. What is the structural component - the brick or the frame wall? You also have all sorts of flashing issues. You should read all of my Brick Veneer columns. Eamon mcgreal 17 Aug 2008, 07:17 Hi I am doing a large renovation to my house and i am putting on a new slab connecting to my exsisting slab. I have to bring in new waste pipes and water pipes through the new slab in to the exsisting house. Do i have to lay the pipes all the way through into the exsisting house when putting in the pipes when laying the concrete slab or can i wait till i have the block work up and am ready to knock through into the exsisting house and if so what is the process. Thanks Eamon randall c 17 Sep 2008, 00:56 i am building an addition to my house 24 x 24 i need to know where to start or how to start digging my footing.      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.