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.