Does the footing for an addition always have to be at the same depth as the house's?

[roolger]

Hello all!

I'm building a 20' x 25' addition to a duplexed house with a full basement... the access to the addition coming from the basement. The plan is to convert a basement window into a doorway, which would lead to steps up about 4' of elevation, to a concrete slab floor. The building will have ICF foundation walls, sitting on a wide footing.

Obviously the new footing and foundation around the stairs needs to be at the same depth as the house's foundation. My question is, once the floor height is at the concrete slab (about 5' out from the house), can the foundation and footing step upwards also? Or does the entire footing need to be at the level of the house's footing?

The house is built on heavy clay soils, with no hydrostatic pressure as the grade on one side of the house is flat, and the other side drops down a slight slope to below the level of the house's footing. The house is well built with a great deal of gravel around the foundation perimeter. I'm aware that the water table around the house will be altered, and the lower house drainage might drain the supporting clay soil of an addition's higher footing. But this is heavy clay soil we're talking, and it's never dry 6" below grade.

Any input is much appreciated.

Rodger

 

[pwood]

anything is possible with a soils report and engineered plans! Welcome to the show.

 

[mark handler]

Does the footing for an addition always have to be at the same depth as the existing house footing?

No

Make sure the existing ftg can withstand and new surcharge fron the addition

 

[tmurray]

Hi Rodger, you should be able to step the footings as per 9.15.3.9 (assuming this is a part 9 building) until you get to the desired height.

 

[north star]

[ & ]

tmurray,

What is a Part 9 building?.....Is **roolger** asking about Canadian codes,

or something else?

[ & ]

 

[tmurray]

Sorry, I was assuming Canada since that is where his profile says he is from.

Part 9 buildings are buildings that do not exceed 600 square meters in size and are not Assembly, Care/Detention, Medium or High Hazard buildings. So, it would include houses, small offices, small retail, small low hazard, etc.

 

[roolger]

Yes a Part 9 Building, and in Quebec. Thanks all. tmurray, the code book I have is an outdated version of the Ontario Building code and it doesn't have a 9.15.3.9 section. Can you summarize it for me?

If it matters, or helps narrow it down, I'm presuming the clay soil to have a 1500lb/sqft load-bearing pressure. My footing design will be based on this, minimum 6" thick, and minimum 16" wide, as per CodeCheck 403.1.1.

 

[tmurray]

9.15.3.9.Step Footings

1)Where step footings are used,

a) the vertical rise between horizontal portions shall not exceed 600 mm, and

b) the horizontal distance between risers shall not be less than 600 mm.

 

[roolger]

How about a slab on grade for this addition? The stairwell would need full depth foundation, but the rest of the addition... can it be slab on grade? As long as the soil is well compacted around the stairwell, it seems fine to me... but unsure what the code says.

Hopefully some of you are still following this thread...

 

[mjesse]

can it be slab on grade?

If it's attached to the main structure, your foundation will need to extend below frost-line (unless engineering proves otherwise)

 

[roolger]

Ok thanks mjesse.

 

[tmurray]

How about a slab on grade for this addition? The stairwell would need full depth foundation, but the rest of the addition... can it be slab on grade? As long as the soil is well compacted around the stairwell, it seems fine to me... but unsure what the code says.Hopefully some of you are still following this thread...

It would still need a 4' frost wall (or whatever you use for local frost depth). Basically the only time you can do a slab on grade with no frost wall is for garages that are smaller than 55 square meters (~600 sq. ft.) and are only one storey in height (see 9.35.3.3).