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Zero lot line duplexes

C

Code Neophyte

Silver Member
Joined
Oct 17, 2009
Messages
271
Location
Central Missouri
Would you allow - provided zoning does not prohibit, of course - two "stacked" duplexes to be built side-by-side and connected on a zero lot line (so that it has the appearance of being a four unit apartment building - two units over two units)? I don't see that it's specifically prohibited by code?

Btw - we're on the '06 codes...
 
You would be in the IRC rather than the IBC - and depending on local requirements might not require a design professional - and would be allowed to use 13D rather than 13R.

On the other hand, you would still have to meet FHA accessibility requirements.

But if it meets the code, it meets the code and you would have two one-hour fire barriers on the property line.
 
brudgers said:
You would be in the IRC rather than the IBC - and depending on local requirements might not require a design professional - and would be allowed to use 13D rather than 13R.On the other hand, you would still have to meet FHA accessibility requirements.

But if it meets the code, it meets the code and you would have two one-hour fire barriers on the property line.
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You are going to accept the design of fire barriers/walls without a design professional?
 
They would have to be built under the IBC. The IRC is for detached one-and two-family dwellings and townhouses. These are ATTACHED two-family dwellings.
 
TABLE R302.1 seems to permit a building to be built with a zero lot line fire seperation distance however it lacks direction on protecting the roof such as a parapet or fire treated wood with no openings in the roof that you would find for town houses.

The IRC is prescriptive and was not written to address this situation and I would look at R104.11 as authority to require a design to cover roof be located adjacent to the property line
 
If they are connected, I would classify it as a 4-plex and use IBC.

If they decided to go separate structures, the each would be built per IRC.

Fire barriers are not a trigger of requiring a design professional in any jurisdiction I'm familiar with. Usually based on area and/or number of dwelling units.
 
Paul Sweet said:
They would have to be built under the IBC. The IRC is for detached one-and two-family dwellings and townhouses. These are ATTACHED two-family dwellings.
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Only to play devil's advocate: I cannot find a definition of "attached" or "detached" in the code. What is proposed is actually two separate structures on a zero lot line - completely independant of one another, including foundations. Walls are protected in accordance with R302.1.
 
Attached is attached. Do you really need a definition?

Your latest post describes two buildings, two permits on two lots designed per the IRC (if adopted).
 
Lot line defines the structure for me. 2 seperate units of 2 dwelling each. Need to rate the walls on the lot line.

See the following terms:

BUILDING LINE. The line established by law, beyond which a building shall not extend, except as specifically provided by law.

DWELLING. Any building that contains one or two dwelling units used, intended, or designed to be built, used, rented, leased, let or hired out to be occupied, or that are occupied for living purposes.

EXTERIOR WALL. An above-grade wall that defines the exterior boundaries of a building. Includes between-floor spandrels, peripheral edges of floors, roof and basement knee walls, dormer walls, gable end walls, walls enclosing a mansard roof and basement walls with an average below-grade wall area that is less than 50 percent of the total opaque and nonopaque area of that enclosing side.

FIRE SEPARATION DISTANCE. The distance measured from the building face to one of the following:

1. To the closest interior lot line; or

2. To the centerline of a street, an alley or public way; or

3. To an imaginary line between two buildings on the lot.

The distance shall be measured at a right angle from the face of the wall.

LOT LINE. A line dividing one lot from another, or from a street or any public place.

When you get through all that then go to TABLE R302.1 EXTERIOR WALLS and notice there is no requirement for any roof requirements between either building except for penetrations/openings or projections.

So the short answer is yes. The longer answer is when shown all the details (plumbing, roof vents, and location of porches and decks, skylights, etc) may change that to maybe. When the plan leaves here all those areas listed on the table are shown on the plans as to what if anything can go in the space.

It's amazing what discussed but not put on the plans changes when you go out and look at back to back plumbing stacks going thru the roof in the area that no allowed and you ask the contractor why? That's why it on all the plans.
 
I guess you could do it with a firewall(separate buildings), fire rated bearing walls(R317.1.1) and rated floor separations......No DP for fire barriers here...anyone good with a UL book can spec that...
 
If they are truly "detached" then there would be a gap, let's say minimum 1/4" wide, from the ground to the sky. If you put roofing or flashing over that gap for waterproofing purposes, you've "attached" the buildings.

So, these parallel fire walls on each separate building need to be both weatherproof, and be capable of construction from one side only. There is only one system that comes to my mind: a CMU wall. The walls probably need to be separated by at least an inch for constructability, perhaps more if you had lateral movement issues. You would also need to figure out how to remove any stormwater that falls between the walls.

Conclusion: you might get it to work in theory, but it's probably not cost-effective or practical in the real world. The FHA comments in previous posts also apply.

Yankee: if I understand your comment correctly, I think a building offical can accept a UL-listed or a Gypsum Association tested fire assembly; I'm not sure what the registered design professional otherwise adds to the credibility of the lab-tested assembly.
 
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Yankee said:
You are going to accept the design of fire barriers/walls without a design professional?
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I don't accept anything without a design professional - of course I don't accept anything with one either.

I simply are one.
 
Paul Sweet said:
They would have to be built under the IBC. The IRC is for detached one-and two-family dwellings and townhouses. These are ATTACHED two-family dwellings.
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When in their infinite wisdom, the ICC changed the IRC from a separation distance between buildings to a separation distance from lot lines, they allowed this situation.

Two buildings with their own walls are not attached.
 
steveray said:
I guess you could do it with a firewall(separate buildings), fire rated bearing walls(R317.1.1) and rated floor separations......No DP for fire barriers here...anyone good with a UL book can spec that...
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Actually, properly detailing a frame bearing wall for that type of situation is difficult because you only have access to one side.
 
I agree with yikes, in theory I don't think the IRC would prevent it. In reality, it would be easier to install the sprinklers.
 
Yikes said:
.....a CMU wall. ......Conclusion: you might get it to work in theory, but it's probably not cost-effective or practical in the real world. .........
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Actually the CMU wall works very well until someone wants to add trusses.

Had one zero lot line duplex where they wanted to stick frame one side and build the other later(read yet to be built). Ought to be fun doing that on the second one since the first one was a pain to get the contractor to make sure they sheetrocked the entire wall and roof without the subs making holes in the wall. Went great until the plumber forgot to think about the 5/8 wall and installed the tub/shower without it.
 
fatboy said:
I agree with yikes, in theory I don't think the IRC would prevent it. In reality, it would be easier to install the sprinklers.
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Not necessarily - tap fees for sprinklers can be significant particularly if you have to bore across a road - and with only four dwellings the cost per dwelling could easily be thousands of dollars before you get inside the building.
 
FredK said:
Actually the CMU wall works very well until someone wants to add trusses.
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CMU walls and trusses are no big deal.

Done all the time down in Florida.
 
I stand corrected - you are right - one wall could be built with conventional framing and stucco, the other wall would be one-sided construction (CMU). I still say that waterproofing the bottom will be a real pain, and someone, sometime, is gonna try to attach the buildings with flashing at the parapet top - -and perhaps at the wall as well, to hide all those spider webs in the gap.
 
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