Frost line consideration for walkout basement

[Buleeek]

Hello,
What is your opinion regarding the bottom of new foundation for the basement walkout stair? Creating a walkout for the basement will affect the frost line. Should I start counting the required 36" from the bottom of the stairs? In addition, the existing house foundation should not be undermined. Thanks for sharing your experience.

 

[lexpatrie]

Answer depends on a sketch of the condition.

 

[Buleeek]

See attached. The owner wants to access the ex. basement from outside.

 

[dik]

Can you use extruded polystyrene rigid insulation?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Buleeek]

dik,

I think so. Can you point out where exactly it should be installed ?
Thanks,

 

[PEinc]

A different problem: Will there be a floor drain at the bottom of the steps to prevent water from running into the basement?

http://www.PeirceEngineering.com

 

[Buleeek]

PEinc,

Yes, drainage provided.

 

[Enable]

Did this exact setup at my old house 10 years ago, which my brother still lives in. Zero issues so far! Frost line should be measured from whatever the outmost protective surface is, which will obviously change throughout the structure. What we did was as follows:

A) heat-trace the drain at the base landing slab. Where this comes into the home may dictate your other elevations. You may also have a weeper along the top of the footing, which you'll need to modify.
B) place 4" of XPS underneath the base landing slab extending 4' (horizontally) underneath the new walk-out stairs
c) place 4" of XPS underneath and upside the back of the retaining wall 4' since frost acts in all directions. You have options for placement depending on how much you plan to over-excavate

Naturally that was based on the local frost depth of around 4' from ground cover. If you have a 36" typical frost depth then you can use 3" of XPS.

 

[Buleeek]

Enable,

Thank you for your help!

 

[structSU10]

for more specifics ASCE has a shallow frost protection guide for the requirements for insulation, details, and minimum depths based on where you are located.