Stem Wall Slab Foundations | The HOW and WHY

[@TheSipherArcher]

I'm new to construction. I inspect finished homes and apartments. My education was poor. So your video has been a tremendous help to me in understanding what a stem wall actually is. Thank you!

[@AddisonHomes]

That's awesome to hear! Before I went fully into construction, I became a home inspector and witnessed many of the issues we now try to prevent in our new construction practices! Let us know if there are other topics you'd like to see covered!

[@billparsons7732]

Excellent presentation.

[@AddisonHomes]

Glad you liked it!

[@realcomedyperrycoalmon9407]

Good presentation of the process!!

[@AddisonHomes]

Thank you!

[@ahmedconstruction624]

Very informative video and years of experience, thankyou for such informative video

[@AddisonHomes]

You're welcome! Glad it was helpful!

[@ericburdette4884]

Awesome video Todd. Great information.

[@AddisonHomes]

Thanks!

[@organogold8]

This was very interesting. Thank you 😊

[@AddisonHomes]

Glad you found it helpful!

[@Trapperjohn1000]

I need this video. I just had plans drawn for a stem wall, slab and pier and beam. I definitely need to learn more about stems. Thanks for this video.

[@AddisonHomes]

So glad it was helpful!!!

[@briandonovan6546]

man im jealous of you guys in the south. our footings here have to be 42" deep and that increases the cost A LOT

[@AddisonHomes]

No doubt!!!! We have it easy in that respect for certain!

[@kirankishore9934]

I agree. Maybe, that is why most homes in the north have basement.

[@backwardsrun]

Really it comes down to being jelous of our wearher

[@richardbeagan5039]

Awesome video! Answereed the questions that wake me up in the middle of the night as i plan on building our home.

[@AddisonHomes]

So glad to hear! We work hard to share what we've learned over our 20 years of construction experience!

[@fordaiffa6022]

very very very clear

[@AddisonHomes]

@@fordaiffa6022 glad you found it clear!!!!

[@tonybrischler5172]

I'm enjoying this series. You give a nice summary of the steps and materials required in a no-nonsense manner. Very professional! We are building outside of Atlanta next spring and our footprint will be about 131' x 72', which includes garages and patios. Given the size it seems like a stem wall slab foundation rather than monolithic is the way to go (crawl space is too expensive.) A few questions, please: how thick a slab would you recommend? Looks like you'll be pouring a 6" slab on this particular house; will that suffice for something as large as my slab? What if I wanted 8" thickness, or would that be overkill? And maybe I missed it; I saw the piping to be "slabbed," but I didn't see the water lines. How did you handle those? Would you be able to share a rough cost of the foundation build up to this point, just as a reference? It's been a few months since this video so hope you post updates on this build! Thanks!

[@AddisonHomes]

Thanks so much for the kind comments Tony! So glad you are finding the information useful. That is our goal. The stemwall slab, in our opinion, reduces any chance of slab heave that could occur with a monolithic slab. Be sure that you have sawcuts or explansion joints in your slab being that size to prevent excessive cracking. The slab in this video will be poured at 4" thick with grade beams poured to 10" thick under any load bearing walls. The 4" slab is perfectly adequate for a residential building but does need grade beams installed under load bearing walls sized based upon the load they carry and your soil conditions. If you wanted additional strength in your slab, I would suggest welded wire mesh and/or higher strength concrete before I would go thicker. We typically keep our drain lines beneath the slab for obvious reasons but run our water lines overhead for access and ease of layout for efficient hot water distribution. They are insulated and buried in the attic insulation. The direct costs were around $36k including the footings, walls, backfill, insulation and concrete slab.

[@tonybrischler5172]

@@AddisonHomes Very helpful, thanks. Todd. Appreciate you taking the time to respond.

[@AddisonHomes]

@@tonybrischler5172 You are so very welcome! Glad to assist.

[@JoseCruz-r7i]

Thanks very informative

[@AddisonHomes]

You are very welcome!

[@cambridgehartley3567]

Crazy how different this is from what I've seen doing commercial construction in Maine. I've seen 6ft footings before and insulation goes under the entirety of the slab.

[@AddisonHomes]

@@cambridgehartley3567 yes- construction details can be significantly different in dramatically different climate zones.

[@reginabrown7532]

I got a better understanding with your videos

[@stephengstokes]

Terrific video and explanation. What adhesive product are you using?

[@AddisonHomes]

Glad it is helpful! 3M Holdfast 70 is what we use.

[@SantiagoRodriguez-mi9zt]

Awesome video, Thank you for sharing it!! Quick question... Here in florida, the plumbing underground pvc pipes cannot be seating under rock foundation, we need sand underneath. Since you are using rock... do you need a soil compaction test before pouring?? thank you!

[@AddisonHomes]

Great question. It depends on the jurisdiction as to whether they require that we bed the plumbing drain lines in dirt or will allow stone. Our plumbers typically lay in some red dirt (in our market) on top of the stone under the pipes to prevent any issues. If we have portions of the foundation that we backfill with compacted soil, we always test compaction.

[@mongorussell]

Thank you for all the informative videos! When insulting the inside of the stem wall, how do you cover the foam? I’m sure a 6” wall will cover most of it.

[@AddisonHomes]

You're welcome! We install a termite flashing on top of the foam edge and under the entire framed wall. The product we use is called TERM and made by Polyguard

[@westtexasdave2140]

If the house is going to have a brick skirt could you bring the brick up 4” above the block and pour to the top of the brick over the block?

[@AddisonHomes]

I don't really follow your points. The best location for the fan is in close proximity to the sources of moisture (tub, shower, etc). If there are multiple sources, install in between them. Regarding makeup air, there isn't a significant need to have air flowing across the sources of moisture and odor - the bath fan is going to pull air from the HVAC supply and/or under the door to the room.

[@webyankee6558]

Do you pour a concrete slab on top of the 4 inches of stone and where does the floor joists go? I was hoping to see a 4 foot raised foundation that was filled to see how it was finished.

[@AddisonHomes]

No floor joists, this is a concrete slab foundation.

[@GiftHorse123]

Was it wise to have the PVC rough plumbing to be in direct contact with the course stone? Maybe should have had a pile of clean sand to lay in the trench then install the PVC on top of the sand. I used this technique in S. Florida, where our monolithic slabs needed to be elevated to meet a minimum floor elevation relative to the crown of the road. The plumbers would create their trenches in crushed rock backfill, install the clean sand, then install the underground PVC.

[@AddisonHomes]

This is typical practice in our market. We don't see much sand used in construction in our market. I know other markets where sand is the predominant fill material. I supposed it depends on what is readily available in a given area.

[@HistoricHomePlans]

I'm a residential designer. This was a great video and answered a lot of my questions. One last question - is there any cost advantage to doing a monolithic concrete pour, instead of his approach, which requires several steps?

[@AddisonHomes]

On this particular foundation, we had some slope across 1/3rd of the foundation. The stem wall slab approach allowed us to adjust for this variation in elevation with a taller stem wall and internal fill material. This would have been difficult trying to approach the conditions with a monolithic slab. For a dead level site, a monolithic slab can definitely be done, but the foundation insulation is then exposed on the exterior of the foundation.

[@AddisonHomes]

Sorry, just re-read your question. To answer your question regarding cost, it can definitely be more cost effective to do a monolithic slab depending on the market, labor availability, etc. Hope that helps!

[@mikeholman4284]

@@AddisonHomes Why couldn't you make it level with a building pad? Planning to build a home myself in Florida, hoping to educate myself. Looking to build an uber reinforced concrete home (1900 square feet). Since I'm doing the work myself, I'll spend money on Owens Corning pink rod rather than steel, fiber reinforcement throughout all concrete. It will be a 2 year project.

[@AddisonHomes]

@@mikeholman4284 We have rolling topography here in the foothills of SC and there are many sites where moving that much dirt around isn't possible due to the proximity of other homes or the topography itself. In this case, the house was adjacent to a nice pond

[@chrisw6734]

Hey. Can you do this in cold climates? Thanks!!

[@AddisonHomes]

Yes, absolutely

[@canonicaltom]

No, any foundation system in a cold climate must guarantee that the concrete doesn't freeze. In this case you would have to add horizontal insulation on the exterior to make a frost-protected shallow foundation (FPSF)

[@squeekhobby4571]

Great information

[@AddisonHomes]

Thanks!

[@erictorbet8104]

For constructing the stem wall, please explain the pros/cons of footings+CMU vs. all one pour for the footing/stem-wall. (The slab is still a separate pour in both cases.). Maybe it's not easy to create that ledge if it's a concrete pour?

[@AddisonHomes]

We’ve never seen a footing and stem wall poured at one time. A poured in place stem wall would typically be formed on a footing previously poured. In our market, this would be more expensive than a CMY stem wall and many poured wall contractors wouldn’t be interested in such a small job. The other alternative would be the monolithic slab where the footings and slab are poured together.

[@AddisonHomes]

CMU- sorry

[@erictorbet8104]

@@AddisonHomes Okay, thank you, good to know!

[@barkleymencer2434]

Yes, thank you for explaining things so well!!! I am grateful to have found your videos!! Keep them coming pls

[@vincentperry3156]

I like the 6" perimeter block on top, Did the inspector let you install verticle rebar after footing was poured . The 1st. video , there was no verticle rebar coming out of the footing.

[@AddisonHomes]

Yes, we typically install the vertical rebar by drilling into the footing and using epoxy adhesive. This is allowed in our area and allows the mason's to more accurately align the rebar with the cells in the CMU block.

[@bjhoneycutt2450]

Great video thank you! I have a garage almost starting to build. 54 by 80. 3 ft foundation, one ft steam wall. In Utah would you recommend same style of installation?

[@AddisonHomes]

This can work but your climate zone and local code will determine the required depth of your footings and foundation. There are also options for frost protected foundations that allow for more shallow footings / foundations in colder climate zones.

[@raylebonville7499]

I found this interesting and also a little concerning because you seemed to suggest that backfilling with dirt would increase the risk of a blow out. I wish to do this build debit free so I'm trying to save money at each step when possible. Dirt is a lot cheaper lol. my wall is only 4 courses high with L blocks on top so a total of 5 this is the tallest portion of the wall . what do you think about a dirt fill of about 3 ft with stone the rest of the way? TIA

[@AddisonHomes]

Thanks for the comment! Backfilling with dirt can increase the risk of damage to the wall when compacting the dirt close to the wall. A couple of thoughts for your project: - to reduce the risk of damage to the wall with relatively small amount of backfill (4 courses = 32 inches), you can stub up some rebar out of the footing every 4 feet or so and fill the cells of the block at the rebar with mortar. This will add strength and keep the wall from pushing if you compact the fill close to the wall. - with only 32 inches of fill, depending on how you plan to compact the dirt fill, stay back a few feet from the wall so you don’t put too much pressure on the wall from the dirt. Your slab will be supported by your block at the perimeter and can easily span 2-3 feet of less compacted fill. You could add some rebar from the L-block back to the well compacted fill for additional support. Hope that makes sense!

[@Wildernesbear]

I'm building my spouse here in NC this exact same way, I've got a question. I'm curious about how yall do your foundation drains, and also what you do about all the water that will be trapped on the interior of the home if it rains before you pour the slab?

[@AddisonHomes]

On stem wall slab foundations, a perimeter foundation drain and waterproofing isn’t required. We’ve never had any issues with water being trapped within the stem wall prior to pouring concrete. Any rainwater that falls prior to pouring the slab just soaks into the soil beneath. If water table was a concern, drains can be added through the stem wall to relieve any water that might come up from beneath.

[@johnmed1972]

Nice video...how do you anchor you sill plates? Thanks

[@AddisonHomes]

Fantastic question! We actually embed them in the concrete slab when we pour. Its a more solid connection to the slab.

[@johnmed1972]

@@AddisonHomes Thanks for you response..just curious how this is done if you have 4" frame wall over 4" block.Are you using a 90 degree strap anchor? Thanks

[@AddisonHomes]

@@johnmed1972 Absolutely - The inside of our exterior walls are actually 5" in from the outside of foundation (1" Foam Board, 7/16" sheathing, 3.5" stud) and we use a standard foundation strap which we lay into the concrete at an angle so the two straps sit under the framed wall.

[@SnakeHandler-g7u]

Cool video, I'm taking NC contractor classes now and am wondering why no dowels/rebar sticking up from the footing into the blocks @6:00 mark?

[@AddisonHomes]

The rebar extending up from the footings aren't necessary when the interior fill isn't over 2-3'. Code requires it over 4' but we typically add it if we are filling more than 2-3' with stone. It would be critical to incorporate when backfilling with compacted dirt fill.

[@garrettp.5018]

Another important detail when doing a stem wall is how the soil was back filled in layers. Ideally it should be 12" layers then a jumping jack compaction (because the skid steer will blow out the walls). This is repeated every 12" until it gets to grade. Is this what your team does?

[@AddisonHomes]

Great point. We actually go one step further and backfill with stone that requires minimal compaction and avoids the issue with too much pressure against the foundation.

[@garrettp.5018]

@@AddisonHomes I noticed that! I should of done that on the side of my foundation that is 6'+ feet on the stem wall. If you do stone do you have to do 100% stone or can you fill some portion with fill and some with stone? You fill with 57 stone?

[@GiftHorse123]

If these stemwalls start to get pretty high, it is required by Code to have interim compaction of the backfill, otherwise, settling can occur. The other thing to consider is installing a floating structural slab if the compaction cannot be fully achieved.

[@tracymankey9314]

I wish you would show the rest of the video of laying the fine gravel down and the styrofoam and the plastic

[@AddisonHomes]

We are working on some videos for this now! Stay tuned.

[@MrLoudog7]

Is there any need for drain pipe and stone under or behind the footing? I see tons of homes in the south with none.

[@AddisonHomes]

No perimeter drain is needed for a stem wall slab or monolithic slab unless there are bulk water issues. No stone is required either, but we often use stone for the backfill inside a stem wall slab.

[@Beloved20223]

Is stem cell wall foundation or monolithic slab more appropriate for south texas?

[@AddisonHomes]

David- It really depends on the soil bearing capacity in your area. If you have expansive clay soils, these may require an engineered slab foundation. Be sure to check with a local structural or soil engineer to be certain. If the soil type is conducive to standard prescriptive foundation types, you should be good with either type of slab foundation.

[@Beloved20223]

@@AddisonHomes Thank you very much!

[@garrettp.5018]

I am also in SC (Anderson) I did a stem wall on my build mainly because I like the features of a slab over a crawl. I didn't weight the two from a cost perspective. Is stem wall slab better value than an encapsulated crawl? I know slab on grade mono is the cheapest foundation (which is why all the production builders do it).

[@AddisonHomes]

The cost depends mostly on the topography of the site and how tall the stem wall and crawlspace walls must be. On a relatively flat site, the stemwall will be less expensive than the encapsulated crawlspace by a significant difference.

[@garrettp.5018]

@@AddisonHomes Thanks for the reply! My property was very similiar to this where about 1/4 of the perimeter was about 6-8 blocks high. I really like my stem wall slab and like the firm ground underneath vs a crawlspace. Also, like knowing no critters are living underneath my feet lol My stem was foundation was also with 4" block and exposed brick sandwiched together.. Why would some foundation experts build it this way vs coming in after and doing the brick on the exterior of a CMU?

[@AddisonHomes]

@@garrettp.5018 if the desired exterior finish is brick, then masons will often lay brick and 4” cmu together as you describe. The brick technically needs to be structural since most of the exterior wall bearing sits on the brick. If the brick was installed outside of the 8” and 4”, it will site beyond the framed wall and require a brick water table cap to terminate. This is, however, exactly how it would be done if the entire building would have brick veneer siding - the brick would sit outside the block foundation and extend up the outside the framed wall and up to the soffit.

[@garrettp.5018]

@@AddisonHomes Okay, So the foundation with the brick and 4" CMU would yield a brick wrap slab only foundation. The siding would protude past the brick. If I wanted a brick roll off I would do the brick as an exterior off of an 8" CMU? This would give the depth to stick out past the siding? What prep has to be done if the brick will be the siding? It would start off of the footers and build up? Then there need to be an air gap between the sheeting and the brick wall? BTW, I emailed your on your business email. Thought this might be the best way to get in contact with you since I am local to Anderson, SC.

[@niveknospmoht8743]

A header block will accomplish the same thing, be stronger, and still give you room for anchor bolt centering.

[@AddisonHomes]

Unfortunately the header block approch prevents the ability for the perimeter insulation which is why we do not use the header block.

[@DavidB-pq1qn]

@@AddisonHomesAre the anchors for the sill plate offset? How did you install them?

[@Dan_Akins]

How much more does (% percentage wise) does increase this cost of the slab? On average, just percent as I know the cost varies.

[@AddisonHomes]

@@Dan_Akins it really depends on the site. If it’s dead flat lot, the cost of a stem wall insulated slab versus a monolithic slab is likely within 1-2%, but the stem wall slab requires a few more steps so it may take a little more time on the schedule. If the lot has some slope, a monolithic slab is impossible without significant site work.

[@PatrickPrice-ny2ie]

New Sub!

[@brenthandley5591]

Why not pour a monolithic footer with a 2 foot stem wall like it’s done in the Pacific Northwest? Is it a matter of the local market not having the subcontractors. I know most engineers say that a mono pour is better for water intrusion and overall strength. Having a block stem wall with a thousand points of failure just seems outdated.

[@AddisonHomes]

@@brenthandley5591 are you suggesting to pour a footing then form and pour a stem wall on top? We could certainly take this approach but it would be more costly than CMU stem wall on the footing and narrow the number of trade contractors available. We would then have to have another crew make a trip to parge or stucco the exposed stem wall edge. I’m not familiar with a way to pour the footing and stem wall in a monolithic single pour. We aren’t relying on the stem wall to resist water intrusion and the CMU wall provides more than enough strength for the foundation. I don’t understand how a CMU stem wall has a thousand points of potential failure?

[@juliojames5986]

This method looks labor intensive, needing serious payback in insulation gain, both heating and cooling . The density strength of foam board also a concern. Great presentation at any rate…thanks .

[@AddisonHomes]

Thanks for sharing your thoughts. We have found that a typical foundation can be insulated on the perimeter in less than 2 hours. The “score board” foam is best since it doesn’t require cutting. Incremental improvement and extreme comfort is what we are after. The compressive strength of most commercial XPS foam is 25 psi. That is 3600 pounds per square foot - well beyond the required soil bearing capacity for a foundation, so we don’t have any issues with the compressive strength of the foam. Thanks again!

[@juliojames5986]

@@AddisonHomes Thanks for the comeback. I was off the wall guessing, an old retired guy, with construction background. 👍🏻😁

[@AddisonHomes]

@@juliojames5986 thanks much! All great conversation and you bring up some great points of clarification!

[@oldowl4290]

Technically speaking, a CMU should be called a Cement Masonry Unit as there is no rock or aggregate in the mix to make them Concrete.

[@AddisonHomes]

They are actually called concrete masonry units. Here’s a link for reference from the Portland cement association. www.cement.org/cement-concrete/products/concrete-masonry-units#:~:text=Concrete%20masonry%20units%20are%20manufactured,they%20enter%20the%20curing%20chamber.

[@coldhandjacinth9069]

Zone 3? Isn't that northern cold climate?

[@AddisonHomes]

@@coldhandjacinth9069 nope - southern mixed humid.

[@craigedelmann6476]

I would never do block and only do a poured concrete wall!

[@AddisonHomes]

Everyone certainly has their preferences. In my view, it comes down to performance, cost, sustainability, and availability of trades to reliably perform the work. In our market, CMU block performs excellent, is more cost competitive, has a lower environmental impact, and we have readily available materials and labor to install. Some of these factors will vary by market.

[@craigedelmann6476]

@@AddisonHomes Well being in the concrete field for 30 years second gen poured walls are cheaper as well as stronger and more durable.

[@AddisonHomes]

@@craigedelmann6476 not sure what market you’re in, but we’ve not found poured walls to be less expensive than CMU. Not sure how poured concrete is any more durable than filled concrete cmu. As someone in the concrete field, how do you the cost numbers in your market compare- poured concrete vs cmu?

[@bradkvanbek7148]

There are better ways to build a wall. ICFs (Insulating Concrete Forms) are easier to work with, already insulated and poured concrete is far stronger than CMUs. ICFs are better than stick framing too.

[@AddisonHomes]

ICFs are a good product but expensive and challenging to obtain a termite bond when using in a high termite infestation area like South Carolina.

[@MeanderLife]

Seems there is a lot of failed logic in your method. Talking about leaving the 4 inch ledge on the stem wall to "support the edge of the slab" and then reducing that ledge with rigid foam board... So in actuality, the slab is poured over the backfill and cantilevered over the perimeter foam board. Then, not tieing the slab to the stem wall with any rebar leaves them both structurally isolated... In the end, the home is either built on a 4" CMU block that is not tied together (like using "beam" CMU block) or is resting on a cantilevered pad... Seems to be a risky trade-off for the longevity of the home.

[@AddisonHomes]

@@MeanderLife interesting analysis but don’t understand how you conclude that the slab is cantilevered. The 4” ledge formed be the 8” cmu transitioning to 4” cmu is reduced by the 1” rigid insulation leaving a 3” ledge. The area inside the stem wall has continuous compacted fill. There is no cantilevered slab area. The framed exterior walls bear on the cmu and a portion of the slab (which is ultimately supported by the cmu ledge and compacted fill beneath) Using header block (beam block?) around the foundation perimeter prevents the continuous perimeter insulation, which is the goal of this assembly. The stem wall and slab are not connected in this approach nor is this needed in this location. There are no scenarios in our location where the stem wall and slab need to be structurally connected. Structurally connecting them might be advisable in a seismic zone or an area of very poor soil quality.

[@ramul-mfume-truth]

What stem wall is best for Florida

[@AddisonHomes]

I would use the same - 8" CMU block with 4" CMU block on top. This facilitates perimeter insulation but be sure to cap the insulation with a termite shield.