How to Identify a Firewall
LESLIE: Now we’ve got Cynthia from South Dakota on the line who’s got a question about a firewall. Tell us what you’re working on.
CYNTHIA: I have an old house and I’ve been ripping out the plaster walls. And I found, along this one wall – see, the whole entire house is this pretty durable and tough plaster-board stuff. And I was wondering if that is a firewall, because that seems to be where all the cold-air returns and stuff are and if I should or should not rip it out. And if I do rip it out, is there a certain kind of drywall that I should use there?
TOM: Where is this wall located exactly?
CYNTHIA: It could have been on the outside of the house at one point but it’s (audio gap) under the furnace.
TOM: Well, first of all, the only place that you typically would have a firewall – in other words, a fire-rated wall with a certain rating – is between the garage and the house. All the other walls and ceilings inside the homes are – usually have traditional, ½-inch drywall. If it’s an exterior – an interior/exterior wall – an inside surface of an exterior wall, like a garage wall, then you would use a 5/8-inch-thick, fire-rated drywall. But all of the other places in the house, you’d have regular plaster board – I’m sorry, regular drywall.
CYNTHIA: OK. Have you ever seen this plaster board before?
TOM: Well, sure. Now, how old is the house?
CYNTHIA: I believe it was built in 1896?
TOM: See, there’s different stages of wall construction. In 1896, you would have had something called “wood lath,” so there would be wood strips on the wall and then plaster put on top of that.
CYNTHIA: Yep. That’s on most of the walls. But this one particular wall – which could have been an outside wall at one point; I’m not sure exactly – it’s like in 2-foot strips.
TOM: Yeah, OK. So that’s a later addition. And what they did with that is when they stopped using wood lath, they started using rock lath or – you would think of sheetrock in those 2-foot-wide strips? They put that on and then covered that with wet plaster. So that’s just a more modern version of the way walls were constructed. So it went from wood lath to rock lath to sheetrock. That’s, essentially, the progression of wall construction over, roughly, the last hundred years.
CYNTHIA: OK. Well, thank you.
TOM: A little lesson on building history. Hope that clears it up for you.
CYNTHIA: Yeah. Alright. Thank you.
TOM: Alright. Good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.