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Building Matters

Getting Mudsills Right

For a great start to your framing, pay strict attention to the mudsill. Know the codes, mark and measure with accuracy, provide a capillary break, and plan for air-sealing.

I just finished installing mudsills on a nearly square house—the foundation is 34 ft. by 38 ft. And fortunately, the concrete contractor did a nice job of setting right angles at the corners, so it was pretty close to square in that sense too. His lines weren’t so straight though, the top of the foundation was hardly flat, and the anchor bolts, though placed precisely where I asked for them to be along the length of each wall, were all over the place in terms of their distance from the edge of the foundation. Their depth was inconsistent, at best.  

All-in-all, it was a pretty favorable situation. 

And yet, mudsills are where framing takes off from the foundation and the more accurate the start, the easier the framing that follows. So, I was fairly methodical with the process, despite the project’s simplicity. Plus, it was important to do a good job of air-sealing the mudsills as they were installed them. The aim is a tight house and this is an area prone to leaks. What follows is a look at the approach taken, some notes on what the building codes have to say about mudsills, and some thoughts from a few experts I reached out to for comments on this all-important process. 

Preparing the foundation is worth the effort 

Mudsills, or sill plates, or sole plates, as they are referred to in Chapter 4 of the the International Residential Code (IRC), are the first piece of framing lumber installed on most projects. They are commonly bolted to the foundation to make the transition from concrete to wood. Because mudsills are installed close to grade and because they can wick water from potentially wet concrete, they are generally pressure-treated lumber. Most of what you’ll find on mudsills in the 2021…

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9 Comments

  1. Tim Janson | | #1

    I'm curious how you would change your approach if this was a slab. PT mudsill then place wall on top with another plate? Build the wall with a PT sole plate? Last time I did it, I tacked the PT plate to my stem wall with small concrete screws, then put the wall up, then drilled thru both plates and the concrete and installed concrete anchor bolts.

    1. Expert Member
      Malcolm Taylor | | #2

      Tim,

      That's what I do on garages, especially if they have stepped stem-walls. It sure makes layout and standing the walls a lot easier. The only problem is if it is a house and you are using pre-cut studs, your walls end up 1 1/2" too tall.

      I'm not sure it's necessary for the anchor bolts to hold down both bottom plates. On a typical wall the studs are only attached to the plate by nails, so it's really only the sheathing ( and in some cases some hardware) that connect the mud-sill with the framing above. As long as the sheathing runs down to the mud-sill, it's no different whether there is one plate or two.

      1. Tim Janson | | #5

        Hi Malcom,

        Only reason I put the anchor bolts through both the mudsill and wall plate was skip any layout on the wall bottom plate for clearance holes for the anchors, as I didn't want to countersink the anchors. I was also using 2x2 bearing plates under the anchor bolts (overkill I'm guessing), so that would need a big clearance in the wall plate. If using smaller washers I suppose it would be straightforward to drill big holes in the wall plate for clearance.

  2. Expert Member
    Malcolm Taylor | | #3

    Good article! A couple of things I do that I think make things easier are:

    - Use a spray can of bright paint to mark the location of the bolts on the forms. You have enough to do during pour without having to consult a layout sheet, or have to think about where they should go.

    - Set a 1"x2" in the outside of the forms as a pour-strip. This gives you some wiggle room to deal with off-square or wavy formwork. It also lets you drop the exterior sheathing a bit and leave a clean line at the foundation you can seal or tape. You do have to remember to make your forms bigger to accommodate this.

  3. ThomasKansas | | #4

    What do people think about making the air seal between the foundation and walls at the stem wall/sheathing joint instead of under the mudsill?

    This could be done at various times throughout the framing process instead of having to be done before the sill goes down. It wouldn't rely on a flatish top of foundation. It doesn't require multiple beads of sealant above and below each layer - termite sheild, sill seal, etc. It doesn't require also air sealing a connection between the sheathing and mudsill to keep air out of the wall assembly. It would seal the edge of sheathing from splashback and could be done with either a tape or fluid applied.

    A downside I see is it creates a joint low on the wall that relies on sticky stuff and could catch water if it fails.

    1. Expert Member
      Malcolm Taylor | | #6

      Thomas,

      I prefer that approach for the reasons you give. You can see a version of that in this video:https://www.youtube.com/watch?v=SqP4liutJFs

  4. Tim B | | #7

    This entire article is selling me on helical piers

    1. Expert Member
      DCContrarian | | #8

      I just built a house with helical piers. The piers support a concrete beam 18" wide that sits at grade. From there up it's framed just like a conventional house.

    2. Expert Member
      Malcolm Taylor | | #9

      Tim,

      I've done a few buildings on piers - I'm building one now. It's just exchanging one set of complexities for another.

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