Fire Blocking Basics

Fire Blocking Diagram | Construction Pro Tips
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How to prevent fires

Holes and gaps in wall and ceiling cavities allow a fire to spread rapidly. They also allow airflow, which feeds a fire. Sealing these gaps slows, or may even stop, the spread of flames, smoke and gases.

The purpose of fire blocking is to prevent or at least slow the vertical movement of flames, smoke and gases by sealing off concealed spaces like stud cavities and soffits. Smoke and gases travel horizontally as well, so preventing horizontal air movement, such as within a dropped ceiling, has its own name: "draft stopping." We'll cover draft stopping in a different story.

Code requirements for adding fire blocking and draft stopping don't apply to existing houses that are not under construction. But if you’re finishing a basement, putting on an addition, remodeling a room, or just running pipes or wires through a plate, you’ll need to include fire blocking. And of course all new houses have to be built to code.

What are approved fire blocking materials?

Drywall, certain sealants and expanding foams, framing lumber, fiber cement board, and densely packed fiberglass or mineral wool insulation are among the acceptable materials to use for fire blocking. But to be on the safe side, ask your local building official which will work best for your project.

In this story we’ll show you the most common fire blocking applications so that you can know and apply the principles in your work.

Fire blocking at the top of basement walls | Construction Pro Tips
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Basement Walls

The gap behind a 2x4 wall has to be sealed off from the joist bays above the wall.

Fire blocking behind bathtub | Construction Pro Tips
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Drop-in Tubs

If the wall behind a drop-in bathtub is finished, no blocking is required. If the wall isn’t finished, install 2-by material between the studs, flush with the top of the tub. Insulation batting in the wall extending at least 16 in. above the tub is also sufficient.

Concealed Spaces Fire Blocking | Construction Pro Tips
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Concealed spaces

A dropped ceiling needs fire blocking where the joists meet the wall.

Fire blocking at stair landings | Construction Pro Tips
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Landing

Install blocking in stud cavities above a landing over an unfinished space.

Fire blocking at plumbing and electrical penetrations | Construction Pro Tips
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Plumbing and electrical

Where plumbing pipes or electric cables pass through wall plates, fill any resulting gaps with an approved fire-blocking material.

Fire blocking at soffits | Construction Pro Tips
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Soffit with an unfinished back

Soffits provide an easy path to floor and ceiling joist bays if they’re not fire blocked. Without a soffit, the top plate provides the fire blocking. If a soffit has an unfinished back, block off the stud bays at the base of the soffit with 2-by material. Or install 2-by blocking (or approved sheathing) along the back of the soffit.

Fire blocking behind soffits | Construction Pro Tips
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Soffit with a finished back

If the wall and ceiling were drywalled before the soffit was put in, no further fire blocking is needed.

Fire blocking under stairs | Construction Pro Tips
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Stairs above unfinished space

Angle-cut 2-by fire blocking to fit between the studs on the outside of the stair stringers.

Fire blocking at the top of and between stair stringers | Construction Pro Tips
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Stringers

Install 2-by blocking between stair stringers to prevent fire from entering the joist bays. Cover the underside of stair stringers with drywall.