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New Floor on a existing shoebox in Montreal

New Floor on a existing shoebox in Montreal

New Floor on a existing shoebox in Montreal

I have been reading this forum for a couple of years now and the information has been exceptional. I have been working on residential structures in Montreal for the last 3 years and I have a case where I think we could have a constructive conversation.
English is not my native language, hopefully I’ll be able to make everything clear.

Existing situation:
An existing Shoebox (one-storey building), built around 1930. The foundations are made of rubble bounded with mortar (typical foundation from that era in Montreal).
The building is 20 ft width with a 1-storey building on one side and a two-storey building on the other.
The existing wood joists are supported on the rubble foundation (and on a central axis). We also have 10 inches hollow concrete blocks acting as the fire protection between buildings.
As usual with these type of foundations, the mortal is so brittle that it crumbles with a slight touch.

The project:
To add a floor on the existing building and build a new roof.
The basement won’t be used as a residential area, so I can take space there with new structure without causing a problem.

The challenge:
I don’t want to add another 12ft of hollow concrete block on the existing rubble foundations without making reinforcement. Obviously, safety is my first concern, but I’d like to find a solution that would be economical.

Initial Ideas:
I haven’t made calculations yet, but this is where I am leaning right now.
New Strip footings
1: I’d poor a new strip footing (inside the basement) next to each rubble foundation.
1.1: I’d build a bearing wall on this new strip footing to support the existing floor joists.
1.2: I’d build a bearing wall on the first and second floor to support the new joists.

By doing this, can I assume that the live/snow/dead loads from the floors and the roof are not supported by the rubble wall?

Rubble foundation reinforcement
2: I would like to repair/reinforce the rubble wall (even if in reality it will be less loaded than initially). Is there an economical way to do that? By pouring a concrete wall next to the existing foundation? If so, is there a good way to bound the rubble foundation to the new concrete foundation? Do I have to use a special concrete not to cause chemical reaction between the old mortar and the new concrete? Should I link my new strip footings to the reinforcement layer on the rubble foundation?
2.1: My client is going to sell the building after the project. There is no way I can leave the rubble wall as it is right now. Even if I could prove (and I can’t) that it’s strong enough, the look of the wall will create a lot of problems. So I have to reinforce it.

I’d like to hear ideas about this project. Am I on the wrong path? Any improvements I could make? Suggestions?

Thank you very much in advance


RE: New Floor on a existing shoebox in Montreal

Repairing rubblestone foundation walls requires an extremely talented mason with significant experience doing so. I have seen inexperienced contractors cause more problems with these than they've fixed.

Reinforcing it isn't exactly an option. The only repair option you would have is to re-point it as required. Keeping in mind that these walls were never intended to be covered by vapour barriers, moisture barriers, insulation, etc. and the lack of air movement once these things are installed actually cause deterioration of the mortar joints faster than if left exposed.

I'm sorry I don't have a better option for you. On one project we did here, we supported the new framing off of discrete columns that all landed on new foundations located inside of the existing. All walls for the new construction were steel stud to save weight.

RE: New Floor on a existing shoebox in Montreal

I would not discount the capacity of the rubble stone foundation walls. If they are in good condition, they can support quite a bit of load. The rubble stone foundation walls in my house are approximately 2'-6" thick (no footings) and support A) up to 26' of field stone wall above, 2'-6" thick to underside of roof, B) all the dead, live and snow load of the 2 storeys of floors plus attic above. They have been doing so for 150+ years, with only minor maintenance performed over their life span. I have seen on other projects where talented, experienced masons have done an excellent job re-pointing deteriorated rubble stone walls and replacing stones as necessary. I have also had to go in and deal with the disastrous aftermath of 'do it yourselfers' trying to work on their own rubble stone walls, after the emergency personnel (fire/ambulance/coroner) left the site.

I have also successfully specified reinforced concrete up against the sides of rubble stone foundation walls to reinforce them. I would consider this, especially if you need more footing area for the bearing capacity of the soil.

If you are not worried about losing space upstairs with the new walls, your approach if fine. With a 20 ft. width, in my experience (I worked in Montreal for a few years) I would likely be building a concrete block, or perhaps lighter (wood/steel stud) wall, [with appropriate fire/sound rating] directly above the rubble stone wall. The house I lived in Montreal was 4 storeys (divided into 2 units), supported on rubble stone foundation walls. The condition of the rubble stone walls is key, and rehabilitating them and using them likely could result in a more cost effective solution that maximizes space.

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