2 hour fire wall between townhouses

[Jmeng1026]

I have been asked to design a townhouse unit that will be constructed on the rear side of an existing townhouse building.

I have designed townhouses before with a 2-hour firewall between units but that was all new construction.

Is there a way to get a 2-hour firewall without having to redo the existing rear wall of the existing unit?

Basically a one-sided firewall.

 

[dik]

Not likely, if it's a code requirement. This is a life safety issue. The firewall has to stand independent of either part of the two buildings.

 

[Greenalleycat]

Can you do it in masonry block? A 20 series block would easily provide that and could be built from one side while providing protection to both sides

 

[Thomas_T]

To meet fire separation requirements for townhomes, you'll need to match one of the approved fire-rated wall assemblies, such as two separate 1-hour fire-rated walls, where each wall assembly is rated for fire exposure from both sides.

Code allows for adding layers to approved UL assemblies. For example, if the existing wall has 5/8" Type X gypsum board on the interior side and structural sheathing on the exterior, you can add a second layer of 5/8" Type X gypsum board to the exterior side. This modification would create a 1-hour fire-resistance-rated wall with protection from both sides. You can then build a matching 1-hour, both-sides-rated wall as part of the new unit.

Watch out for the rimboard area of the existing structure. It might be difficult to achieve a rating in this area.

As an alternative, you can construct the new unit with a wall assembly rated for 2 hours from both sides, like:

• UL U301: A common 2-hour-rated gypsum wall assembly
  UL U301 Assembly
• UL V337 with LP FlameBlock: Offers enhanced shear capacity and can be rated for either 1 or 2 hours
  UL V337 Assembly

Roof protection is also required. You’ll need either:

• A parapet that extends at least 30 inches above the roof, or
• A fire-resistance-rated roof, achieved using either fire-rated sheathing or 5/8" Type X gypsum board installed below the existing sheathing.

And... this is architect's business! Better to stay in your lane and avoid liabilities; just nudge your arch with your preferred solution until they approve it (and that might take time.)
Good luck!

Thomas

 

[dik]

Can you do it in masonry block?

It requires lateral support in the event either half of the building burns down.

 

[Greenalleycat]

It requires lateral support in the event either half of the building burns down.

Just design it to cantilever? Super easy for single story. Two story would be a bit niggly as you'd need midfloor support on the existing building side - may be tricky to attach. The post fire load here is only 0.5kPa though so they're pretty small loads

 

[phamENG]

It requires lateral support in the event either half of the building burns down.

I don't think you would in this case, dik. The existing unit already has wall. The intent is to keep the adjacent unit from burning and prevent progressive collapse across units. The free standing masonry wall that has to stay up is a thing when it's the only wall between units. But if it comes down during the fire in this case, it's not necessarily going to endanger the existing unit. Not a great thing, having a block wall fall down, but if the building has burned to the ground anyway...

So I'd say building the back wall from masonry and tying it into the new unit for stability should be a winner. It's an atypical situation, though, so make sure you have buy-in from the architect and the building official before going too far down that road.

 

[dik]

I don't think you would in this case, dik.

The firewall has to remain intact to prevent the passage of fire. Either half of the building may have the fire.

 

[phamENG]

Agreed. If the existing side burns, it falls down and the masonry remains standing - while also preventing the passage of fire to the new.

If the new side burns, and the masonry falls, the existing remains standing and didn't catch fire because the masonry prevented the passage of the fire. Once it falls, there's nothing there to burn the rest of it anyway. It would be essentially the same as a dual wood wall with a gap.

 

[RWW0002]

The free standing masonry wall that has to stay up is a thing when it's the only wall between units.

I am not sure this is accurate. I think a "firewall" needs to be one of three things. 1. A Freestanding wall that will remain with collapse of either structure. 2 A double wall where each wall has a rating that meets code intent (there is some code wiggle room here with combining assembles) where each wall stays in case of collapse of either wall. 3. A rated wall tied to one or both structures that is designed for the force of collapse of the adjacent structure.

If the existing building exterior can be justified as a rated wall then you may have a case 2, or similar, but otherwise I am seeing that this likely would need to be free-standing.

 

[RWW0002]

Agreed. If the existing side burns, it falls down and the masonry remains standing - while also preventing the passage of fire to the new.

If the new side burns, and the masonry falls, the existing remains standing and didn't catch fire because the masonry prevented the passage of the fire. Once it falls, there's nothing there to burn the rest of it anyway. It would be essentially the same as a dual wood wall with a gap.

The code is pretty specific on these, and unless the existing wall can be considered to have a rating (using existing brick finishes or similar), it is likely a no-go. This topic is often misunderstood and misinterpreted by contractors (and sometime code officials)..

Here is some good content summarizing firewalls. A little dated but still good.

https://www.pdhonline.com/courses/g125/g125content.pdf

 

[phamENG]

On second thought, it's probably easier to convert to a double light frame fire wall. Strip the exterior sheathing off of the original, install 5/8" type X gyp, h-clips with insulation, and then build the wall of the new unit. Then you have structurally independent walls with the necessary ratings.

 

[phamENG]

using existing brick finishes or similar

Why would you need brick? Most double wall scenarios use a pair of structurally independent, one hour rated walls with insulation and burn-away clips between them.

 

[phamENG]

Reference:

 

[RWW0002]

Rated walls (Fire barriers, fire partitions) and firewalls are two related but separate things.

I agree that a 2 hour wall may be able to consist of 2 - 1 hour walls (see the "wiggle room" I referenced above with combined assemblies), and there are other materials besides brick that can make up the "existing" portion of the double wall. I suggested it since it has been useful in the past for justifying existing construction that may not have been built to meet a specific assembly meeting requirements of a rated wall.

OP may need to clarify whether they are after a rated partition between units or a firewall between buildings.. Two significantly different things.

 

[dik]

If the new side burns, and the masonry falls,

The fire moves to the existing building?