Determining Stem Wall Height
John_Brown
| Posted in General Questions on
How does one determine the height of a stem wall above grade? If one is building a basement, what are the determining factors in assessing how high the stem wall should be above grade. I haven’t been able to understand what the issues are other than what is sometimes described by code. In my case, the site is perfectly flat (not sloped). Assuming I will have no problem with excavation, what are the issues that inform this design decision?
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Replies
John,
1. If you want your house to be wheelchair-accessible, it's easier to design your ramps when the top of the foundation is close to grade.
2. The biggest problem with a low foundation is splashback that rots the bottom courses of siding. As far as I know, most building codes require that the lowest wooden components of your building be at least 8 inches above grade. I'm in favor of higher foundations: 12 or 18 inches is much better.
One hundred and fifty years ago, architects knew the advantages of high foundations. Graceful old mansions were always designed to have a foundation that extends a few feet above grade. Your house will last longer if you learn from the wise architects of the 19th century.
-- Martin Holladay
After he left the farm Dad worked as a carpenter and then later in other construction capacities. Mom had taken architectural and interior design classes after getting her teaching degree; she designed the house I grew up in (and Dad built). So there were often critiques made of other homes as we drove around in Dad's little blue Ford Econoline pick up truck. One oft made comment, that I can still hear in my head: "That house is too low, they should have gone up one more (cement) block."
For a good portion of the construction timeframe for our home I thought I had actually gone too high, in our case one ICF block too high. However, that view slowly changed as we got the backfill in, siding on, started towards hardscape installs etc. Eventually I came to the conclusion that I did get it right; Mom and Dad would have been pleased.
Anyhow, other things to consider on foundation height off grade are basement windows/wells, particularly if you think you might want egress, and whether it's comfortable to have someone staring right in your window. On the latter there is a slight feeling of security if a first floor bedroom is a bit above grade.
If your site is perfectly flat, you would do well to use some of the dirt you get from excavating to build up the area around the building slightly so as to have the grade slope away from the foundation to direct water away.
The IRC code allows wood sheathing and siding to be as close as 6" to the ground (R317.1.5: http://codes.iccsafe.org/app/book/content/PDF/2012%20International%20Codes/IRC/Chapter%203-Building%20Planning.pdf) but framing members need to be at least 8", and more is better, as Martin wrote.
Thanks, Michael.
-- Martin Holladay
John,
If there are no impediments to digging a full basement that may influence the decision, the prime considerations are what relationship you want your house to have to the surrounding landscape and what type of space you would like the basement to be. The height isn't a decision you take in isolation, but rather as part of the larger design.
The closer to grade a house is the more direct the relationship between the inside and the area around it. You can step directly onto patios or gardens. With a higher floor level this relationship has to be mediated by decks and stairs. Neither is inherently good or bad, they just create different experiences and possibilities.
The higher a basement protrudes above ground, the more opportunities you have for windows and doors linking it to the exterior. The spaces seem more like conventional rooms than basement ones. Unlike considerations of the height of the main floor, increasing the grade of the basement has no downside I can think of.
Malcolm,
You're right, of course, that "The closer to grade a house is the more direct the relationship between the inside and the area around it."
As I'm sure you know, climate matters. In Hawaii, a house with such a direct relationship to the outdoors may be welcome. In Vermont, a sliding glass door leading to a patio is likely to be blocked by a 4-foot snow drift for several months each winter -- and the occupants might prefer a roofed porch that is two feet off the ground.
-- Martin Holladay
Maybe it goes without mentioning, however the invert height of your drainage line to the septic tank is quite important. Also often on a flat lot having a plan as to where your footing drains can find daylight should be considered when planning a first floor elevation.
John,
As Scott and others have said there are some practical considerations to the height of a basement wall and the grade of the slab.
One is that a completely buried basement usually means 9'-0" concrete walls. Under our code that triggers a requirement for the foundation to be engineered.
And, of course, the deeper you go the more likely you are to encounter the water table.
its interesting that "The IRC code allows wood sheathing and siding to be as close as 6" to the ground but framing members need to be at least 8""
i have a sloping lot and i don't want to do a tall concrete or block stem walls above ground if i don't have to. suppose i pour the concrete stem walls to 8" or so above grade then do the rest of the stem walls to floor level with wood, is that OK? sounds like it is right?
for the stem wall close to the ground would you use pressure treated 2x6s and pressure treated plywood cladding? then fake stone cladding to finish it off? seems like this would be quick, cheap and easy. thx
Steve,
That's a common approach to building on sloped sites. Where the steps are in the forms should be looked at with an eye to how it visually affects the elevations of the house.
You don't need to use PT lumber or plywood anywhere where you wouldn't use it if the forms weren't stepped. In other words, just the sill-plates and studs that are in contact with the stem-walls. These should also be separated by a sill-gasket as a capillary break.
Before using a stone cladding look carefully at how the wall behind should be detailed, as reservoirs cladding like stone can be problematic.
thanks for the reply but i didn't understand the last sentence. i was thinking the Faux Stone Veneer which i think glues onto stuff, but not sure how i stick it to the tyvec?, i guess i need to look into that
ok, i guess like this https://www.nsvi.com/specs/001a2Lr.jpg
Steve,
Reservoir-claddings are those that can take on moisture - like stone, brick, and some concrete based siding. That moisture can get driven into your walls when exposed to the heat of the sun, and cause your sheathing to rot.
Here is a good summary of the problem: https://www.buildingscience.com/documents/information-sheets/reservoir-claddings
Are you building in Thailand, or is this for a house in the US? The advice you are getting is assuming the latter.