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Community and Q&A

Wall assembly for a zero lot-line six-story building

MishaB | Posted in Green Building Techniques on
I am designing a small (relatively speaking) infill project on a narrow lot on a main street that will replace an existing townhouse.
We are located in climate zone 6 in Southern Ontario.
The project will be a six or seven-storey building with 4-5 dwelling units.
The structure will probably be concrete columns and floors with some concrete block components, or steel if we will find it makes construction easier.
At approximately 12′ wide, it is a narrow lot. This width makes the sidewalls very impactful in both construction costs and in the area that they would occupy – both of which are significant to the economic viability of the project.
Since the walls are at the property lines, they should also provide fire separation (one or two hours depending on the final height of the building).
The common approach for similar conditions in the area used to be concrete block walls (which were also structural) at the property line with interior insulation. I don’t like this solution because the insulation is interior, the concrete block is exposed and it wastes space; it also would be hard to approve since the City wouldn’t allow exposed concrete block walls.
Solutions with exterior cladding should be possible (they’ve certainly been done in larger zero-lot-line projects) but access can be an issue.
I would be happy for suggestions for an approach that would work well in these conditions, or your experience in addressing similar challenges.
Thank you.

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  1. Expert Member
    NICK KEENAN | | #1

    You really need to talk to a structural engineer, but I'll bite.

    Around here a wood structure can be 4 stories, buildings like that are 2-3 floors of concrete then the rest wood framing. It looks like you're in zone 5, where the R-value for walls is R20. You could get that in a 2x4 wall with polyiso or closed cell spray foam. For a fireproof exterior I'd recommend Hardie board and maybe 5/8" drywall on the interior. Total wall thickness is 5 3/8" which gives a room width of 11' 1-1/4" which is going to be tricky.

    On the concrete portion if you did poured walls you could do some sort of stamped or stuccoed finish without adding much thickness, let's say 1/4". The latest trend is thinner walls with more rebar, let's say 6" for the wall. Three inches of polyiso gets your insulation and then a half inch of drywall. So each wall is 9-3/4" which leaves your rooms at 10' 4-1/2".

  2. Expert Member


    I'm pretty sure Ontario has amended it's codes to allow six storey wood frame buildings. You still need both an architect, a structural engineer, and non-combustable construction on the zero setback wall.

    The typical construction for that type of project is load-bearing steel studs, batt insulation in the cavities, a non-combustable sheathing and exterior insulation. Any non-combustable cladding will work.

    Long before worrying about the detailing of the building assemblies. I'd be getting your architect to run conceptual plans past the city. My bet is what governs this project will be parking requirements.

  3. Expert Member
    AKOS TOTH | | #3

    I was going through a lane way house design on a narrow lot which creates similar challenges. The difference is that in your case you'll need non-combustible construction.

    The idea is to have a structural frame with the walls being non-load bearing. This lets you design the wall with minimal thickness, just enough to run utilities through it, but the all the insulation is outside the wall.

    For example, you can go with 1 5/8" metal studs with 1.5" AFB plus 2.75" of polyiso covered in cement board. This give you a U 0.280 assembly in 5 7/8" wall thickness (you can shave about 1/4" of foam off if you go with higher efficiency equipment).

    This makes sense for any structural wall carrying multiple stories, for one with only roof load, best to go with load bearing 3 5/8 light gauge steel+exterior insulation.

  4. Expert Member
    BILL WICHERS | | #4

    I’m going to suggest an unusual approach: build a steel frame to carry the load, with non-structural walls that could be framed “normally”. With such a small span (12 feet), you could keep all the structural columns in the outer perimeter and have all clear spans inside so no load bearing walls anywhere. Code will probably require a masonry stairwell for fire safety though.

    In a building like this, the perimeter walls are a little like a facade that hangs off of the actual load bearing structure. You could just use steel columns and some girders in a few key places to support conventionally framed floors to avoid the need for floorpan and concrete floors. Such a structure would allow for all kinds of interesting architectural features too (really high ceilings become easy, huuuuge clear spans for open concept “rooms” become simpler, etc).


  5. MishaB | | #5

    Thanks, everyone for the great answers.

    The Canadian building codes don't allow a concrete podium plus 5 wood-frame storeys in the same way that the International Building Code does; at least not as a prescriptive solution.
    But I'll need to look up if we can use wood studs as part of a curtain wall assembly.

    Maybe I should have provided more context: I am the architect for the project. We are in a preliminary phase, but because of the narrow dimensions, it was important to identify as soon as possible what the space requirements of the walls might be.
    We are in discussions with the City. With our affordability crisis and other changes, there's been a change in attitude and small projects have been approved without parking in central locations, though it's not simple.
    Regarding the six-storey wood frame, it is possible but it requires heavy-timber construction and not stick. It's been done here but it's still comparable in price to concrete and has worse acoustic properties. It works well for offices where the structure can be exposed and acoustics are not an issue, but less attractive for res where all the beautiful structure has to be covered for fire protection...

    Akos and Zephyr,
    I really like your approach of a steel-frame structure and non-load bearing side walls.
    It also seems to keep the framing width small enough to allow for more rigid insulation, since I'd like to go over the code-minimum requirement (which is only R-24 for zone 6).
    I am a bit reluctant to do structural steel framing for fire safety reasons. But it is better for sustainability and will make construction and foundation work easier...

  6. MishaB | | #6

    A follow up question: with framing, exterior insulation and cladding, how do you build the assembly at the property line? Do you work from the inside?

    1. Expert Member
      NICK KEENAN | | #7

      Check your local regulations. Here, where the zoning allows for zero-lot-line construction the neighbor is legally required to give "reasonable" access for construction. Varying opinions of what "reasonable" means often lead to lawsuits.

      It's very common to build what's called a "party wall." That's a structural wall that straddles the building line, each owner owns half of the wall and is free to use his half to support a building. See

      This is all super-local, you need to find out what your local law says. The place to start is probably an attorney who handles property law.

      1. MishaB | | #11

        Thanks, DCContrarian,
        I'll look into it. Could make life easier if it's possible to access the wall from the outside.

    2. Expert Member
      AKOS TOTH | | #8

      You can build the complete wall including cladding and tip it into place. Steel construction is pretty light, 4 guys can lift a 45'x8' wall. Does require a bit of precision and care, but not hard. Don't forget all your wall penetrations, they very hard to do after.

      You can even get it prefabed like this and crane it into place in sections. I'm not too much of a fan of this as now you are relying on caulk for the vertical seams between section, but apparently done often enough that way.

      One thing that can simplify your life a lot is if you can build under part 9. For example if you build with mezzanines and tall stories, you can get the same amount of levels for living space with all the extra stories.

      I'm all about efficiency, but in high density construction, there is really no need for it. Per occupant, a building like this is probably many times as energy efficient as single family home. With the high price of sqft, your effort is best used there. For example say a 60' long building, each one inch on exterior wall buys you 10 sqft, that times 3 or 4 stories, is a fair bit of extra living space and extra money at the current price/sqft in Toronto.

      1. MishaB | | #12

        I like the approach of building the complete wall and tipping it into place. I do wonder though if there would be no issues at the seams, especially since the wall sections will also need to cover the floor slab?
        Is that the approach you used in your laneway projects?

        Regarding part 9, IIRC if a mezzanine takes more than 40% of the floor area or has more than 10% of the floor area enclosed it counts as a storey.

        Your point about energy efficiency vs. space efficiency is well taken. We'll definitely need to balance the two.

        1. Expert Member
          AKOS TOTH | | #18

          I've done the full wall section with all the exterior insulation and some of the siding pre-installed. Challenge is to deal with the rim joist intersection bellow as you'll have a seam there, so they'll be have to be some extra flashing there.

          Mazzanines make sense if you have the height but are floor area limited. I've been in a couple of places like this and they are very nice.

  7. ohioandy | | #9

    Misha, I assume you're in Toronto or nearby. The questions that come up for building science with a "skinny house" in a dense neighborhood are absolutely fascinating. I'm reminded of a video I saw recently of a "parking space" house built in Toronto--I thought THAT lot, at 16 feet, was jaw-droppingly narrow.

    At four storeys and only one unit, the design questions were simpler. Still went with a structural steel frame. It wasn't zer0-lot-line, so he had to go with a 14' finished width. No windows on the sides, and limited glazing on front and back. Yikes! The result, however, is super nice.

    1. MishaB | | #13

      Hi Andy,

      I'm familiar with this house! I always like seeing buildings like this on constrained urban sites.
      In our case, the biggest design issue is the interior layout. Since it's multi-res, we have to provide two exits, and we'll likely also put an elevator regardless of code...

  8. Expert Member
    BILL WICHERS | | #10

    Steel is actually better for fire safety than wood. Steel does not provide food for the fire (at least not until you get to crazy high temperatures, but by then you have other problems), it’s a fireproof material. This is why commercial style “non com” (non combustible) construction is pretty much all concrete and steel.

    I wouldn’t use steel studs here, I’d use steel columns, probably square tube. You could probably use columns that would fit inside a 2x4 wall, certainly inside of a 2x6 wall. Then you use some perimeter girders or C channel to define the structure and keep things vertical. You can then set your wood joists on the flanges of the girders or C channels. There is no reason to avoid a hybrid structure like this.

    I am curious though: why are you reluctant to use steel framings for fire safety reasons? I do most commercial work, and we HAVE to use steel for fire safety reasons. If I want to use plywood for a backboard for electrical panels, I have to use 3/4” non-com which is treated with fire retardants, for example. Sometimes you need to apply some fire protection to the steel (we call the blue kind “smurf barf”), which is little like spray foam. Easy enough to do if needed.


    1. MishaB | | #14

      Sorry, if I wasn't clear - I meant steel in comparison to concrete!

      I would be mostly concerned because with the zero-lot-line configuration, and considering how narrow the building is, the whole structure of the building will be exposed to fire from neighbouring properties much more than if it was a wider building with greater separation.

      That being said, I'm not an engineer, I'm just more familiar with concrete than either steel or wood. And steel does seem to make a lot of sense in this case - and I like your suggestions for the structure!

      1. Expert Member
        MALCOLM TAYLOR | | #15


        So much of this is going tone driven by building code and bylaw requirements, I'm not sure how useful any of the advice we are giving is at this stage.

      2. Expert Member
        BILL WICHERS | | #16

        Ah, steel compared to concrete makes more sense! I’m not so sure there is actually much difference in terms of fire safety between the two in the case of your particular structure.

        If fire safety is a big concern, perhaps consider putting in fire sprinklers? You can get some recessed ones that pop out in a fire, this type is pretty unobtrusive.

        Malcolm’s right too, you’re going to need an engineer and some discussions with the city building department before you get to far along with your project.


        1. MishaB | | #17

          Malcolm and Bill,

          You're right that many details will depend on the code, but any construction type, including wood, can be made code-compliant. But the code is mainly concerned with the safe evacuation of the building; how easy it is to bring the building back to use after a fire is secondary.

          BTW, in Ontario, we now need to put sprinklers even if it's non-combustible construction if we are over 4 storeys. Which makes an even better case for considering steel.

          1. Expert Member
            MALCOLM TAYLOR | | #19


            It's not that you perhaps can't use a certain method of construction, or wall type, it's that starting from those details is working backwards.

            You have large decisions about the relationship of the units to each other and to the building as a whole that will dictate things like whether you need an elevator, whether the circulation is common or mostly within the units, and a whole range of architectural ones dictated by the narrow lot around introducing light, privacy, and how the building related to its surroundings, access, and services, garbage and parking. Those are the drivers of the building assemblies and construction type, not the other way round.

            Do you have an architect and engineers involved?

          2. MishaB | | #20


            I am the architect.
            We've resolved the main spatial issues on a preliminary basis and starting discussions with the City. We'll engage an engineer after we know what can be approved.
            I started looking at the assembly of the side walls at this early stage because the solutions for these site conditions are unfamiliar to me, and at the width of the lot, the difference between a 6" wide wall, a foot wide wall and a 1'3" wall can make or break a design scheme.
            For the construction, that will of course be resolved down the road, but it's always good to have an idea of the options in mind.

  9. Expert Member
    BILL WICHERS | | #21

    Misha, structural steel may allow you to use thinner exterior walls. You could potentially box around the occasional in-wall column and use thinner walls since the walls themselves wouldn’t be load bearing. That might be a way to help maximize interior floor space. One of the advantages to steel is that it can carry a lot more load in a smaller area compared to wood, which sounds like it will help you here.


  10. MishaB | | #22

    Good points.
    Based on everything I heard here, I believe steel will be the first option we'll discuss with the structural engineer when we engage one.

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