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Foundational Excellence: Perfect Mudsills and Clear Drainage

A deep dive into mudsill placement, sealing, and leveling (smooth mudsills are easier to seal)
September 26, 2023

This video by NS Builders in Massachusetts is like a Masterclass in mudsills. It describes squaring up the mudsills, making them airtight, and making them level, without using shims. The part about managing bulk water is also essential information (Hint: don't connect your gutter drains to your footing drains).

Central reference line transfers square lines to mudsills

The first step to a perfect layout is to establish a perfect reference line. Find the longest expanse of the slab and snap a line to be parallel to the longest foundation wall. This line helps ensure that everything remains straight, parallel, and square throughout the layout process. The control line can be used to transfer measurements for snapping out parallel walls.

After pulling a parallel line from the reference line, you can plumb up from the slab to the foundation walls to establish mudsill placement. Foundation walls are never perfectly straight or square. They just aren't.

Smooth the surface of the foundation wall before adding mudsills

NS Builders sandwich the sill sealer in acoustical sealant, so a smooth foundation surface is a great place to start. The layers look like this:

  • Concrete foundation wall (smoothed)
  • A couple of beads of acoustical sealant
  • A layer of sill seal
  • More acoustical sealant
  • Mudsill, which is bolted down

It is easier to plane a mudsill down than to shim a mudsill up

The crew uses a jig to set the anchor bolts perfectly—consistent distance from the outside and consistent height. They are set to not project much above the first mudsill because the crew adds a second layer of mud sill for leveling. From here, they shoot grades, marking the high spots. Now they knock off the high spots with a power planer.

PRO TIP: Don't use your boss's favorite Festool woodworking planer; use a construction-grade workhorse.

Shimming the mud sill up creates air leaks, and an uneven surface to seal. Better the bolt it down tight into the sealant and plane the top down to level.

Downspout drains should not tie into foundation drains

Outside the foundation are green PVC pipes that extend out of the ground. They are a drainage system that will carry water from gutter downspouts to a huge dry well in the backyard. Many galleys are buried in a large pit, surrounded by crushed stone. A galley is a 4' x 4' x 4' concrete structure with holes in the walls, surrounded on all six sides by crushed stone. The assembly allows massive water storage and dissipation. This drainage system is not connected to the footing drain, which is pumped out and tied into the city stormwater system.

Any issues with the galleys can be checked and cleaned out through a manhole access hole.

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