2x4 Attic Floor Joists

If you want to put down flooring in your attic and want more insulation underneath that floor then you need to increase your joist size.

2x4 attic floor joists.

The depth or height of your floor joists depends on how far they can span without sagging. But you can still double the capacity of the existing 2x6s in the attic floor so first let s make sure that s enough for your situation. This is not a load bearing wall. There is a wall under part of the attic which helps support the studs at approximately midspan.

Sistering 2x6 joists next to your current 2x4 joists will simply give you a new floor height of 5 1 2 inches as opposed to the current 3 1 2 inches. Common sense tells you that large floor joists can carry more load and spacing joists closer together also increases the load bearing capacity of a floor. But larger is not always better when builders are constructing a home or adding a room addition. Strengthening attic joists for live loads by sistering.

The extra two inches of vertical distance when a floor is framed with 2 x 10 joists rather than 2 x 12s can be quite important for example. Ceiling joists are typically 2 8 lumber floor joists are usually 2 10 or 2x 12 lumber. Is an attic with only 2x4 joists holding up the ceiling strong enough to use for storage. I can t imagine a garage layout that would have a span short enough to make with 2x4 s even sistered.

Most spans are typically 10 to 12 feet so we typically use 2x8 or 2x10. There are online span calculators available that tell you the maximum span for. The studs in question span 16. Sistering installing new larger joists next to the existing ones is one way to bring your attic floor up to code.

If one end of the span is supported by a beam up the middle of the garage that probably wasn t sized to support a floor either. My calculations tell me assuming 2 southern yellow pine joists which are common in my area that 2x6s 16 inches on center will span 9 feet 9 inches with allowable deflection being the governing factor. Often existing joist bays are filled with electrical and heating ducts that will need to be relocated. You d also have to have the attachments to the top plate of the walls adequate to support the weight of the floor.

That s a big size and strength difference between the two types of attic joists even if the joist spacing is the same. If the attic were designed as living space those ceiling joists would become floor joists and be much bigger. Positioning the new joist adjacent to the existing preserves precious ceiling but it does come with some potential complications. Can it support an attic ladder and have some plywood put down and used as a storage area.