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Fireblocking vs. Firestopping

October 27th, 2011

By Frank Bayer

You’ve just failed another rough inspection due to insufficient fireblocking….or is it firestopping. Confused on what is required around those holes through the top and bottom plates. (Didn’t I squirt enough of that red caulk around it?) How do I frame around bathtubs and soffitts? Hopefully I can answer these questions and demystify firestopping.


Let me start with the simplest explanation and begin with buildings covered by the International Residential Code(IRC). These include one and two family dwellings and townhouses under 3 stories. The only place firestop is mentioned in the IRC is Section 317-Dwelling Unit Separations. Through and membrane penetrations of fire-resistance-rated walls or floor assemblies need to be protected by an “approved” firestop system…. There are some exceptions which I will discuss later.

Since one family dwellings have no rated assemblies, we can leave these out. Two-family and townhouses have rated assemblies between the dwelling units. These are the only areas in IRC buildings which need firestopping. Again, I said I would start with the simple explanation, so I’ll leave a detailed explanation of firestopping to discussion about International Building Code buildings for later. That brings us to fireblocking.

We’ll start with the Code’s definition. “Building materials installed to resist the free passage of flame to other areas of the building through concealed spaces.” Section 602.8 lists 6 places where fireblocking in wood frame construction is required:

1. In concealed stud spaces vertically at floor and ceiling levels.

2. Connections between vertical and horizontal spaces (soffitts, coves, suspended ceilings).


3. Concealed spaces between stair stringers at top and bottom of run.

4. Openings around vents, pipes, ducts, cables at ceiling and floor levels.

5. Chimneys and fireplaces.

6. Cornices of two family dwelling at line of dwelling separation.

602.8.1 goes on to explain what materials can be used for fireblocking: 2 inch nominal lumber, ¾ plywood or OSB, ½ drywall and even unfaced fiberglass insulation. All these items and more can be used for fireblocking 4 of the 6 areas required to be fireblocked. For item #5 masonry chimneys and fireplaces we must go to chapter 10 where specific clearance dimensions(2” for chimneys, front and sides of fireplace, and 4” from back of fireplaces) from combustibles is required. The gap still must be fireblocked but it must be done with non-combustible materials .

Item 4-openings around vents, ducts…. is special and the most confusing. The fireblocking material around bottom and top plate penetrations must be …”approved material to resist the free passage of flame and products of combustion.” Let’s start with the definition (in the IRC) of approved. The 2006 edition of the Codes have changed this to a simple “Acceptable to the building official”. The easiest way to find out what to use is to ask the building official. When I asked an inspector of the jurisdiction I was moving to (and renovating the house I was moving in to) he gave me the standard “firecaulk” answer. Since he was near retirement and I did not want to delay my job, I did like any contractor would do and bought a couple of $16 tubes of 3M firecaulk at Home Depot and squirted them in the holes. But I must ask, why do we (inspectors) make contractors use expensive caulk around a duct or electrical penetration that will feasibly stay intact for a 2 or 3 hours in a fire, while the surrounding wood has burnt away?

When inspecting most IRC buildings and some IBC buildings, I allow tightly packed mineral wool or insulation, unfaced fiberglass insulation filling the cavity up to 16”, structolite(or similar products) and spray foam insulation as long as the penetration is not through a fire resistant rated assembly. At this point I can hear the grumbling from my fellow inspectors….spray foam insulation? Yes, the International Code Council approved a number of Dow Chemical products (mostly the Great Stuff family of foams) as an alternative fireblocking material in Type V construction (All wood framing). The conditions of use are contained in ICC’s evaluation service, Inc. report-NER-645. This report refers to the 2000 IBC and IRC, but since there has been no changes in fireblocking material requirements, this should still be a valid method of blocking.


Now I’ll turn our attention to firestopping and specifically firestopping of penetrations of rated assemblies (walls and floor/ceiling/roof). As in the IRC, the IBC requires penetrations to be protected by an approved firestop system which is installed as it was tested… To simplify, a product which was tested under certain conditions, was shown to maintain the rating of the assembly at the point of penetration.

Let’s say you have a 1hr wall separating a 2-family dwelling, your plumber installs the vent stack in this wall and a 1 ½ “pvc penetrates the membrane (one side) of the wall. This “membrane penetration” needs to be firestopped to maintain the 1-hr rating.



I’ve decided to use a Hilti system whose UL No. is W-L-2244. This means Hilti had this tested by Underwriters Laboratories. You‘ll see that there are a number of parameters which need to be met. First, this system is good for only a 1 or 2 hr gypsum wall. The U300 or U400 represents a series of “tested” wall assemblies which can be built with wood or metal studs (nominal 2 x 4 for wood or minimum 2 ½” for metal). The penetrating item can be up to a maximum of 2” PVC or cpvc plastic pipe. The closed system is that which usually carries fluids. Items 4 and 5 show the Hilti product used (FS-One), the amount (5/8” depth, ½” bead at point of contact) and the placement. Where there is an annular space, the caulk must be in it-between the drywall and the pipe. Under notes you will see that the maximum diameter of the opening is 3”, so if your plumber only owns a 6” hole saw, you’re out of luck with this system. Also, the annular space, the space between the PVC and the drywall can be from 0” (point of contact) to 5/8”. Again, if you have a piece of 1” pvc going through the wall and your plumber’s only hole saw was 3 inch, your annular space would be 1” if the pipe was centered. This system wouldn’t work.

So by this we can see that there are five important things to consider when deciding how to firestop a penetration.

1. What is the assembly you are penetrating made of (gypsum, wood, concrete)?

2. What is the item that is penetrating it (PVC, iron pipe, emt)?

3. What is the annular space (minimum and maximum)?

4. What is the size of the opening?

5. What is the hourly rating of the assembly?

Intumescent vs. flexible

Most manufacturers have a large number of products available. You may hear the term Intumescent. This type of caulk or putty expands when it is exposed to heat. It is typically used around combustible products like PVC and insulated pipe. Flexible sealants are typically used at rated joint (a whole other story) and penetrations by metal products-cast iron, emt, copper.

Passing Inspection

Most inspectors will ask for your “cut sheet” of the firestopping system used. If you have a number of different systems, a copy of all systems should be provided. If you don’t have the cut sheets available, my first question would be “How did you know how to install it”. The definitive wrong answer here is “The way we’ve been doing it for 20 yrs.” These cut sheets are your guide to correct installation. Firestopping decisions should be made before the first hole is cut (in the case of the plumber with the 6” hole saw)

Where do I obtain the cut sheets from?

There are a number of companies which manufacture firestopping (see our LINKS page). Each manufacturer has a catalog with their tested firestop systems. Which one to use would be based on access (who is your nearest supplier), cost and technical support. Most manufacturers have an 800 number where you can call for technical assistance. On larger jobs, most will send representatives to your sight and will also provide installation training. If your doing a lot of firestopping, it is worth the time to have a rep come in and train your employees (just make sure the employees who will actually be doing the installation are involved in the training).

What if I can’t find a system that meets my needs?

FS manufacturers can provide you with an engineering judgment. The manufacture’s engineers develop a system through test data of similar systems. Most Building departments require these be accepted by the architect/engineer of record and their department.

You say you never had to provide any specifications for your inspector. I’ve been known to approve a few metal pipes through a rated assembly when they have firecaulk around them without specs. For anything more complicated than that I require 2 inspections (initially) for the firestopping. First I check the packing material for required thickness, rate of compaction, and parameters of the system used. If the system calls for 1/2 thickness of caulk flush with the floor, the packing material must be recessed at least that amount. I will return once the sealant is in place. Performing dual inspections at the beginning of a large job ultimately saves time and money for both the contractor and the inspector.

The Exception

Both the IRC and IBC allow exceptions from a firestop system when the penetrant is steel, ferrous or copper pipes. The first is in concrete or masonry walls. The pipe has to be 6 inches or less and the total opening is no more than 144 sq inches. The annular space can be filled with grout, concrete or mortar as long as the space is filled for the full thickness of the wall or floor. The second exception allows you to use a material that has been prequalified to meet certain test standards (ASTM 119) for resisting passage of flames and hot gases.

Think Ahead

Hopefully, the most important point I have made is both fireblocking and firestopping should be considered long before you are closing in on rough inspection time. Whether it’s installing some OSB on a section of wall before framing a soffit, deciding which hole saw to use for vent stack or how the drywallers need to cut around openings in rated walls, forethought and buy-in by all trades will save time and money in the end.


The Code is a complex document which is open to interpretation. It is not unlikely to be in room with 5 inspectors and hear 5 different opinions. Get your local inspectors involved in the beginning and if you disagree let them know. Just make sure you’ve done your homework and can back up your opinion. Also, I am not trying to pick on plumbers. The six inch hole saw scenario was one I lived through(and can laugh at now).

While Michicode.com does not endorse any one firestopping manufacturer

click the following link for an informative video.


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