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Build an Attic Floor

Can I Build an Attic Floor With My Ceiling Joists?

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Attic Floor
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We're all looking for more space in our homes. Basements and attics both seem like prime places for offices, additional bedrooms, kids' rooms.

But sometimes, you need to spend a significant amount of time getting the basic "box" into shape before you can even begin turning that empty space into something usable. Attics, which often don't have flooring, often need this type of building-out. That's why we were asked...

Q: How can I build flooring for my attic using the joists that are already there?

Without seeing the trusses or joists in questions, it's impossible to say for sure. The conservative answer is "No." But this can be corrected.

Attic Joists For Ceiling Below

Unless expressly built so, an attic's joists are meant for the ceiling below, not for any flooring in the attic.

It's true that some houses' attics are built without flooring, but the joists are intentionally built substantial enough for the homebuyer to later build out the attic. That is a good strategy for cutting costs for homebuyers in the short term.

But in most cases, you can bet that these joists were not intended to support living space.

Existing Attic Floor No Guarantee

In some instances, you will already find flooring in your attic. Does this mean you can build? Not necessarily.

Flooring does not ensure that you have a solid-enough joist system to support a floor for living space. The previous homeowner may have simply covered those joists with plywood to use that space for a dead load (non-living space functions) such as storing Christmas decorations, kids' old toys, and the like. While not the optimal set-up, it is fine to have ceiling joists (i.e., not flooring joists) with plywood on top to use for light storage of dead loads.

So, do not be mislead into thinking that your ceiling joists will support live load functions (using it as a living or work space or even for storing heavy dead-loads) because there is already a floor there.

In attics built purely for storage purposes, you may find ceiling joists that measure 2" x 4" or 2" x 6".

Sizing, Spacing, and Span of Attic Flooring Joists For Live Loads

Attic joists that are 2" x 8" may be acceptable for building your attic floor. Heftier joists are that much better. But because every room is variable, there are no absolutes.

It's more than just joist dimensions. You need to ensure that the joist spacing is adequate. Ceiling joists for dead loads only need to carry 10 pounds per square foot (psf), as opposed to the 40 psf or greater that live-load joists must carry.

Span length is different for every room. So, span length and width between the spans can be determined only by calculations.

Sufficient to say, though, that 2" x 6" ceiling joists spaced every 24" on-center--a typical arrangement you will find in attics--will not support live loads for a bedroom, office, or bathroom.

Span Calculation for Attic Floor

The best way to get your span calculations right is to hire a structural engineer or contractor. This will be money well-spent. Some structural engineers will work on a per-hour basis.

To get a relative idea of your spanning options for attic joists, check out the span calculators listed in the link boxes below.

While not the ultimate voice in spans, these calculators do provide a sense of reality: joists you may have thought adequate for your attic floor don't even come close. For example:

To cover fifteen feet of span, you'll 2" x 10" joists made of Douglas Fir spaced every 16" on-center.

Fixing Attic Joists For Live Loads

You have a couple of options to prepare your joists for attic flooring.
  1. Sister the Joists: Sistering is the process of pairing a structural member that is damaged or is inadequate for an intended load with another member. In the case of 2" x 6" joists, you would pair them up with other 2" x 6" joists by nailing them together. Best-case scenario is to run the "sisters" the entire length of the existing joists so that you have two additional resting points.
  2. Intersperse Joists: Keep the existing joists as they are, but intersperse them with similarly-sized joists. So, 6" ceiling joists that are 24" on-center become 12" on-center.
One thing to keep in mind--which might slow down the sistering process--is that your attic may have electrical and other types of wires throughout. Because the attic is free space, DIY electricians (and even some pros) sometimes pull wires through the attics rather than through the walls.

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