Fire stop functions to slow spread of flames

Dear James:

I live in a 75-year-old two-story house. When some walls were opened for repairs, the plumber said there were no fire stop blocks. What are they, does this violate code and should I add them?

— Tony R.

Dear Tony:

Fire stops are an important and effective safety construction feature. Their purpose is exactly what the name infers — to stop, or at least slow, the progress of a fire throughout a home. Fire stops may not save your home from a fire, but they hopefully will give your family time to evacuate the house unharmed.

Fire stops can simply be blocks of wood at the bottom and top of a wall to keep flames from traveling quickly up through it. Even the heat from a small fire that starts in a home without fire stops can create a strong upward draft, which may quickly spread flames throughout the home. By the time the smoke alarms go off, it may already be difficult to find a safe exit path, particularly from second-floor bedrooms.

In addition to the flames and heat, smoke and fumes can quickly travel up through the walls. When many synthetic materials burn, they create toxic gases. I recall a fire in Cincinnati years ago where the people, untouched by the flames, walked out of the building and died 30 minutes later from the toxic fumes. The fire stops should also block or slow the flow of these gases.

In most modern wood frame construction, there are continuous top and bottom wall plates. They effectively seal each wall cavity to help contain a fire and the fumes. If the floor joists intersect the walls, there often are fire stops at the joists. This will keep the fire from spreading horizontally along the joists and up into another wall cavity.

For many years, building codes for all new homes have required fire stops inside the walls. If your house is 75 years old, they may not have been required at the time it was built. Most old homes are grandfathered, and code changes are not retroactive, so you likely are not required to add the fire stops to meet current codes. It would be a huge job to add fire stops through your house, unless you are already planning some major renovations in which the walls will be opened.

Some other options are adding fire-resistant insulation batts inside the walls. This is not as effective as a solid fire stop, but it will slow the airflow and spread of flames inside the wall. Fire-resistant foam insulation is also available. Make sure you get the proper type because some foam is toxic when heated. Overall, you might find it less expensive to install a residential sprinkler system. This is one of the best fire safety methods for any home, old or new.

It would also be wise to use fireproof caulk at the top and bottom of the walls where you have access. Don’t forget to use it where plumbing and wiring penetrate the lumber and walls. These caulks will withstand the heat of a fire. Some actually expand as they get hot, so they continue to seal longer even as the lumber, wiring or plumbing begins to burn away. If you have trouble finding these caulks, contact these companies for sources: 3M (3m.com) and Hilti (hilti.com).•

Send your questions to Here’s How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit www.dulley.com. The opinions expressed in this column are those of the author.

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