How to Support Floor Joists & Girder in a Crawl Space

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Does the part of your floor that’s directly above your crawl space give off a bouncy feel when you walk on it? If so, it’s highly likely that your crawl space has lost its structural integrity, leading to sagging and sinking in the middle. In this blog, we discuss how to structurally reinforce your crawl space. We’ve also included a brief section on the benefits and drawbacks of a crawl space.

How do you support a floor joist in a crawl space?

If the floor joists and girders above your crawl space are sinking, it could be due to rot damage, or that the structural supports are spaced too far apart. To fix this problem, you’ll need to install additional supports in the form of lally columns. Alternatively, you can use structural support jacks, which work to restore floor rigidity and structural integrity. To properly install joist support jacks in your crawl space, follow the procedure below:

  1. Establish the installation points- depending on how large your crawl space is; and the span of the floor joists above it, you can measure out the points at which you’ll need to install additional support.
  2. Make your concrete footings- after determining the points where you’ll install your support jacks, make footings at each installation point by digging out two-square foot holes that are two-feet deep. Then, fill in the holes with compacted, crushed stone and level them out with pre-cast concrete footing. The compacted, crushed stones underneath each footing ensure rigid support that’s not vulnerable to shifting and settling.
  3. Next, it’s time to measure out the distance between the footings and the joists above, while cutting out the structural support jacks as per these measurements.
  4. After cutting out your jack posts, install each of them at the appropriate spots, with the top end contacting the girders and the bottom end fixed into the concrete footing.

How do you reinforce a crawl space foundation?

To provide additional structural support for your crawl space foundation, you can use the fixes detailed below:

Install additional beams

As discussed in the previous section, adding steel or wooden beam/jacks helps to facilitate proper weight redistribution all over the floor frame, thus taking the pressure off the floor joists above your crawl space, consequently reversing the collapsing/sinking effect.

Install sister joists

If the floor joists in your crawl space lack sufficient support or are too small, you’re likely to experience a bouncy and sinking floor. You can solve this problem by sistering up each joist with an extra joist of equal dimensions. This method is also called ‘double-up reinforcement’. Sistering joists is a great way to improve the structural integrity of the floor above your crawl space.

Overlay the subfloor

Sometimes, the floor above your crawl space could be sagging due to a problem with the subfloor- not the floor joists. If your subfloor is made of a thin, wooden boards that can’t support the weight of the house, we’d recommend you consider overlaying them with additional structural support material. These may include plywood boards or Oriented Strand Boards (OSB), as both are known for their structural strength.

Block the floor joists

This method involves the installation of rows of wooden blocks between the joists to facilitate uniform weight distribution among the floor joists. Blocking works best when the supporting lumber pieces are cut out to the same size as the joists they’re supporting, and when the blocks are installed from wall-to-wall.

How do you fix a squeaky crawl space?

Floor squeaking usually accompanies floor sagging, as both are as a result of joist movement. Therefore, to fix the squeaky floor above your crawl space, you can use the same methods discussed in the preceding section.

Is crawl space good or bad?

There’s no definite answer to this question, as crawl spaces have their upsides and detriments. For instance, in comparison to a slab-on-grade foundation, a crawl space foundation allows for easier access to your plumbing, electrical, and HVAC systems. For concrete foundations, you may have to tear up the flooring to access these systems, which can be quite inconveniencing.

On the other hand, crawl spaces can trigger mold and mildew infestations, which can consequently cause significant health problems to the house occupants. Being that crawlspaces are barely exposed to sunlight, they hold moisture for longer, thus presenting a suitable habitat for mold and mildew fungi. These fungi are known to trigger attacks and allergic reactions for people with respiratory problems like asthma. What’s more, the moisture tends to trigger rot damage in the floor joists above your crawl space.

However, this shouldn’t deter you from installing a crawl space foundation and enjoying the aforementioned advantage that come with it. You can solve this moisture problem by simply installing a moisture barrier or filling in the crawl space with a concrete slab, as discussed in the next section.

Can a crawl space be filled in?

Yes, you can fill in the surface of your crawl space with concrete to prevent moisture problems in future. The process is typically preceded by the installation of a vapor barrier to act as a moisture seal for the porous concrete. Then, you can pour crushed stones and level it off with a four-inch layer of concrete. Remember, however, that filling in your crawl space floor with a concrete slab will only minimize your moisture problem- but won’t completely solve it. For instance, you’ll still have some moisture coming from your foundation walls.

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Joe McGuinty
I’ve been working with floors for over 12 years. I started as a flooring contractor, primarily in materials selection. Then, I switched careers into accounting, so my wife and I began buying, renovating, and re-selling homes on the side. You’d be surprised how much value you can add to a home simply by adding new floors.

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