Building out a basement

January 3, 2011 10:12 AM

Dig a basement under an existing house? I have a crawlspace underneath an attached 100 year old house that I want to dig into a basement. What Do I need to do to make it happen?

Firstly, you are not my engineer. I'm in Montreal, Quebec Canada.

Basically there is nothing but a crawlspace underneath my house, it's be excavated underneath in the front because of water and electrical service upgrades before I bought the house. So there is about 4.5 feet deep hole in the front of the house.

The centre beam has settled and even rotted in some places because it was laid directly on the dirt 100 years ago. There's also some crumbling of the footing in the back of the house. We're currently digging a shallow tunnel (above the level of the footings) in order to access the areas that require repair as well as so we can replace some old water piping etc.

The soil is very hard packed and clay like. About 3 feet down it's slate with some sort of harder blue rock. The footings are directly laid on that rock.

I imagine I will either need a block wall filled with concrete, or a solid wall with rebar inside. Metal centre posts and a poured concrete floor.I think the best thing to do is bolster the joists with temporary columns, dig down 8 feet, replace bad beams and put in metal posts. After that we would dig towards one wall and build a wall somehow and finish with a concrete floor.

What I want to know is how I should go about this process. What kind of engineer needs to be involved and what kind of work is to be involved? Can I just find a structural engineer to make my plan up and then execute it myself? Do I need some sort of city permit? Does anyone have any recommendations for finding a reasonably priced engineer to prepare plans? PS I'm aware I'll be paying for a load of concrete or two.

FYI my neighbour has done a partial excavation and has a pseudo basement. He know's what's going on and supports the idea. He has not mentioned ever needing a permit for the work he had done.

posted by Napierzaza to home & garden (10 answers total)

I cannot speak to the need for a permit in Montreal, but I would be very surprised if one was not required. You could do the work without a permit but you would be liable for any death or injury that would result, might get caught and have to pay big fines as well as have to restore the property to the condition it was in before you started.

Other than that, the only problem I see is the very real possibility of the work collapsing in on you while you are digging and the house coming down as a result.

If you are still looking to go forward, contact a knowledgeable and licensed civil engineer who practices in the Montreal area. Pay to have him look at the situation and advise you. He is going to advise you against doing it, but it will be up to you to decide.
posted by Old Geezer at 10:31 AM on January 3, 2011

Very likely you will need permit/s. You might even have to have the house jacked and moved, then returned to the spot once the basement is dug. Even if you figure a way not to have to go that route you will undoubtedly have to have it inspected for structural integrity after the fact (and perhaps prior to make sure it can withstand the process).

No idea about your neighbors situation, perhaps with a partial/pseudo basement he avoided the permit process in your location. However, if it is at all like where I live he either did get a permit, or just had the work done even though he should have.

Contact your city clerk's office (or equivalent) and ask them they will give you a much better answer.
posted by edgeways at 10:36 AM on January 3, 2011

Here is the (probable) contact info you need to answer your question:
posted by edgeways at 10:45 AM on January 3, 2011

Who knows about Canada?

I've seldom seen permits needed for repair, though yours sounds extreme. It also sounds a tad dangerous, unless you feel really, really competent in these matters, legalities aside.

If you can afford it, a reputable contractor with a civil engineer would be the way to go, IMO. House moving companies do house lifting and foundation work all the time. (I am in Vermont, USA and there are two lift/foundation projects going on with a mile of me.)

Fix it with a checkbook, if you can. (Incidentally, I had a quote last year on my carriage barn foundation, including lift, fix, and some wall straightening. About $20K US dollars. 4-6 week project. Turnkey. By a pro. With all the right tools. (I didn't take it, but that's what the deal was.)
posted by FauxScot at 10:51 AM on January 3, 2011

It's going to be hard to give specific advice without seeing the house. I've dug out a couple crawlspaces but in BC where the houses were on sand/silt for as far as you could dig.

You should pull a permit; converting a crawl space into a basement is going to be a renovation and not a repair.

Excavating the rock is going to be hard work; especially considering the limited head room. I'm not actually sure how you would do that so I'll ignore it.

First thing I'd do is stabilize the house. Replacing that beam would be job one and if possible I'd use a large enough beam that I could free span the space (IE: no internal supports; just the external foundation.) This will make the work going forward easier and give better space utilization when you are done. You might need to go with a steel beam though.

After that was done (or while doing it) I'd install a good set of external stairs. Ideally they project perpendicular from the external wall but a large landing at the bottom of a set running along side the house would work too and take up less space. These stairs do two things: they allow you to easily remove dirt and rock from the future basement and they tell you exactly what you have down to the bottom of your future basement floor.

Once that is done excavate inside and out down to the footing of you current foundation. On the outside you want a wide enough excavation to work comfortably. It sounds like you have mostly rock however if their is any risk of the excavation collapsing on workers in the hole you'll need to brace the unsupported material.

When the foundation is exposed you can work your way around the house a couple metres at a time installing temporary support; removing the existing foundation wall; excavating to the desired depth of foundation; and installing a new foundation wall. At this point the only good thing about having to deal with solid rock appears: you probably don't need to pour a footing.

Both times I did this we used concrete block to construct the new foundation wall. However if I was to do it today I'd use concrete poured into insulated concrete forms. It would have all the advantages of easy installation in tight places as block does but provide a better end result.

Once the foundation is complete you remove the remaining fill from the interior of the basement and then get the floor placed and finished; a job well worth playing professionals to do IMO. It cost me around $2000 to get a 24'X28'X5" floor placed and finished smooth including services of a 31m concrete pump. Strongly consider installing radiant floor tubes in the floor; radiant floors go a long way to eliminating most people's objections to basements as living space.
posted by Mitheral at 11:57 AM on January 3, 2011

I was watching a holmes on holmes episode where he was dfixing a contractors mess up on exactly what you asked. His recomendation was not to do it but since it was done anyway he had to fix it.
posted by majortom1981 at 1:06 PM on January 3, 2011

Find a good civil / structural contractor, who does foundation work.Get him to look at it and report on what needs to be done. He can probably recommend an engineer. Get a second opinion from another good civil / structural contractor. If the don't agree, get a third.

Do not cheap out on the design, if the design is not done well, it costs 10x to fix it in the field.

You are touching structure. You need a permit. If you touch electrical, you need a permit. You touch plumbing..etc. Talk to your city, they will tell you what you need. This system is there to protect you. They will come out and do inspections at various stages in the work.
posted by defcom1 at 7:32 PM on January 3, 2011

Vindaloo's wife posting.

Also in Mtl, also own a 100 year old house - 1920's semi-detached duplex w/ a poured concrete foundation, & unfinished 7' basement. Our neighbour spent six months this summer w/ a general contractor excavating and rebuilding their foundation, to convert it into livable space.

Excavating & rebuilding a foundation is not a DIY job. You will need permits and special insurance, to cover damage to your own & your attached neighbour's property. I recommend using a specialised foundation contractor, such as Bisson, Henault & Gosselin or K2 Construction. (My neighbour regrets not using such a firm).

It will be extremely expensive. The neighbours' spent over 140K. Engineering firm estimates ranged from 80K to 120K. These quotes did not cover all the extra work such as plumbing, electrical, window/door installation, new stairwells, floor, etc.

The work is risky. Neighbours experienced major settlement issues. We had issues on our property, with staircasing masonry cracks + water infiltration from a foundation crack.

Excavating through rock is an extra cost.

From a real estate perspective, it's unlikely money spent gaining basement living space will be recouped selling your property.

I would recommend not excavating, and focus on rebuilding/repairing the foundation footings, supporting the house's structure, and adding a french drain/sump pump system. Again, the engineering firms mentioned above plus company's dealing in smaller repairs such as Allen Foundation could offer recommendations.

Good luck!
posted by Vindaloo at 9:43 AM on January 4, 2011

Excavating the rock is going to be hard work; especially considering the limited head room. I'm not actually sure how you would do that so I'll ignore it.

And yet, this might be the crux. Your clay is Champlain sea glacial era sediments -- found everywhere along the St. Lawrence and Ottawa valleys. Its interesting stuff, and likes to creep and is the reason so many of the iconic Montreal walk-ups are so crooked.

Your carbonaceous shales and limestone is real bedrock -- Ordovician carbonates of the Trenton group. Excavating through this is not digging, it's mining / quarrying and will be awkward (to say the least) to do in that kind of space, with a house overhead. Get thee to a civil engineer, but my feeling is that you should just be happy your footing is poured on bedrock, and give up the idea of excavating a full basement. I'm surprised your neighbour managed to do this, actually.
posted by bumpkin at 3:18 PM on January 4, 2011

The shale is not so difficult to remove so far, but like I said we have not gone deep. I'm not sure why my neighbour got away with his project. He said he paid something like 20k for some guys to do it. But they don't have a lot of floor space because they dug a few feet from the wall, poured blocks of concrete.

Clearly we're going to keep going ahead with the repairs that we need to do for the foundation.
posted by Napierzaza at 4:16 PM on January 4, 2011