The New Normal, Light Weight Wood Construction
Al Mullins, Traditions Training, LLC
This article will talk about some of the construction components that we see going into new buildings, specifically single-family dwellings. Many of these items have been around for a while but it seems that we are really starting to see some significant fires in “newer” lightweight building construction and that should concern every firefighter in the country.
We in the fire service are at a cross roads in our history, many of us have grown up with and have fought many fires in structures that were constructed out of “sawn joists” or “dimensional lumber”. These products were cut at the sawmill and were the dimensions that they were said to have been, that is if you bought a 2×4, that piece of wood measured 2 inches by 4 inches. Many mill owners and lumber companies looked at cutting costs and found that they could generate another couple of board feet out of a tree by cutting down on the size of the finished product. Now when you walk into your friendly Lowe’s or Home Depot and look at the lumber aisle you will not find “true” 2”x4” pieces of lumber. Instead, you may find a 1 ½” x 3” piece of wood, certainly not, what is advertised on the bin.
This change in size is due to economics and to the fact that we are taxing our natural resources to the limit. Today people are looking to economically and structurally do more with less, and when you go out into your first due areas you will see examples of this everywhere. Just look at the work sites and you will see trusses, plywood I beams and OSB (opposite strand board) everywhere in new home construction. The plywood I beams, trusses and OSB are all developments that are known as engineered wood. With engineered wood, the architects and the engineers out there are looking to make more from less and as you can see, they are.
Engineered wood, as Frank Brannigan called it is great and the above-mentioned construction items are easy to use and allow for a lot of flexibility on the part of the architects when it comes to designing a home. The load carrying capacity of the plywood I is and the trusses are there, talk to any construction supervisor and they are impressed with the capacity of engineered components. While this is true there are a couple of things that should worry us, we do not normally come across these building components when they are in their best shape. Usually when firefighters encounter lightweight building components is when they are under distress.
Another thing that Mr. Brannigan brought up in his books on Building Construction for the Fire Service” was the concept of “fat” in especially in regards to sawn wooden joists. Fat is the extra wood in a beam or a column that is not part of the load carrying capacity of that member. The 2 × 12 beam that is used to support the floor has a lot of fat, and this fat actually equates into time. While the joist is, burning the fat is being burned and since this fat does not lend itself to the load carrying capacity of the beam then that’s “OK” at least from the perspective that the beam can still carry its load.
Once the fat or excess wood is burned away then the fire starts taking away from the ability of that beam to carry its designated load.
With that being said my question is “Where’s the fat?” With engineered wood, there is no “fat” or excess wood. Look at plywood I-beams and then look at the same thing in a sawn joist, notice all of the fat. That extra wood is what is giving us time to work when the building is on fire, without the excess wood there is no time. In the past Incident Commanders (IC) looked at the 20 minute mark as a bench mark on how things were going, with light weight construction there needs to be a rethinking of the bench mark and we need to seriously look at adjusting the timer fro those personnel operating in a structure.
Now, how about those trusses? They are everywhere and while Plywood I beams are bad in floors trusses are worse. The reason I am saying that is at least with a plywood I you have some fire separation in the floor void. While they do not hold up well in fire conditions, they may limit the spread of fire and give us time to access the void and put the fire out before it compromises the ability of that assembly to carry its load. Looking at the parallel wood chord truss, do you see any fire separations? None, at least at this time, so a fire that gains access to the floor void of “truss loft” has a plethora of wood to burn. Remember the old adage from Fire 101 the more surface mass exposed the great the potential spread and burn rate.
I would like to say that these two components the Plywood I beams and the wooden trusses are the end of the story but we really need to look at what homes are being sheathed in and that typically is Vinyl Siding. Vinyl siding is great; it looks good, it maintains its look, and is easy to work with. These facts are seen by the many new homebuilders using Vinyl Siding to enclose their homes. Like I said it looks great, but from our perspective as a fire service, it reminds me of the reason that wood shake shingles were banned for many of years and that is because it burns and helps fires spread. In many instances where you have a fire in a home that is enclosed with vinyl siding you will see a rapid spread of fire on the exterior of the structure. A fire that comes out a window will now easily gain access to the attic. Fires that are starting on the exterior are going from being small and a nuisance are now quickly involving the siding and gaining access to the attic.
You can definitely tell these fires involving vinyl siding from a distance they have that heavy black smoke output that is normally seen in fires where hydrocarbon based shingles become involved with fire. An important aspect of these fires is the fact that there is a lot of unburned carbon material that gets into the attic, and sets us up for a big problem.
We get hose lines in to the buildings quickly and get up the stairs so we can get water into the attic, what we usually do is pop open the attic scuttle and put lines in service from that point. This is where we need to be careful; with all that unburned material gathering in the attic, we have a very dangerous situation. On more than one occasion, this build up of un-burned gasses can light off quickly and explosively. On more than one occasion, this explosive ignition of unburned gasses causes the ceiling to come down on personnel operating on the second floor. Once this occurs, the fire burns unchecked and the engine company that was operating is knocked senseless or is blown out of the house.
We are now in a new and different world, we are not working in the or on the same types of buildings that those who came before us are. We are working on larger structures made with lighter components. Therefore, we as a service need to make a concerted effort to be aware of these facts and to act accordingly. As a friend of mine, Battalion Chief Chuck Ryan recently said we need to replace “aggressive interior attack with educated interior attack”.
Al Mullins, Battalion Chief, Fairfax County, VA Fire Department
Al has been in the fire service for over 30 years, and with Fairfax County for more than 27 years as both a volunteer and career firefighter. Al has been a Battalion Chief since 2003 and is currently working in the 2nd Battalion on B shift in Fairfax. Chief Mullins has worked in Operations and at the Fire Academy in Fairfax with both the Recruit and Field Training Sections. He has also been a presenter at the Firehouse Expo 2006 and Firehouse Central in 2007.
Al has completed several Associates Degrees in Fire Science and is finishing his Bachelors Degree at the University of Maryland University College. Additionally he has attended several classes at the National Fire Academy and is currently enrolled in the Executive Fire Officer Program at the NFA.