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Here’s how to repair a rotten mudsill

August 4, 2018

Dear James: There were some drainage problems around my home (has siding) and the mudsill on top of the foundation is rotting. How can I repair the bad lumber so my house remains stable? -- Jodi A.

Dear Jodi: It sounds as though you have correctly identified the cause of the rotting mudsill. By far, most of these problems are created by damp conditions over many years. They typically start near where patios, porches, stairs, and decks attach to the house.

Another possible cause of the problem may be termites. Look for hard mud tunnels along the exterior walls and small white bugs which look similar to wiggling white rice. These are the worker termites. They like to enter a home where the lumber is damp so you may have both causes concurrently.

The mudsill is the piece of lumber, often a 2x6, which rests on top of the foundation. In many areas of the country, double-2x6 mudsills are common. The floor joists rest on the mudsill and support the house walls. A band joint also rests on the outer edge of the mudsill to close it off from outdoors.

If you have a deep crawl space or a basement, the repair can be made from indoors. Generally though, it is easiest to repair the mudsill from outdoors on a house with siding. This makes access easier and it is also easier to see the extent of the rot damage.

Most likely, if you did not catch the problem early, the band joist and some of the floor joists are also damaged from the rot. The floors in that area may have sagged some from the disintegration of the supporting lumber. If the grout in tile floors seems to be breaking in those areas, the floor has already started to settle.

The first step is to gain access to the rotten area by removing a section of the siding and the band joist. Drill a large hole through the band joist and then use a reciprocating saw to cut through it. A tip to simplify the job is to break off the end of the saw blade so the length of the blade is the depth of the joist. This will eliminate the possibility of sawing too deeply through pipes and wiring behind the joist.

With the band joist section removed, inspect the extent of the damage to the mudsill and how far the floor joists have settled. With this information, go to your home center and purchase the necessary replacement lumber and some wood wedges.

You will likely need a helper for the next step. Before removing the rotten section of the mudsill, you must support the floor joists. Place a small hydraulic floor jack, similar to the type used for cars, on the foundation top. Jack up the floor to the height of the undamaged areas. Drive the wedges in place under the floor joists to support them.

Remove the old rotten mudsill and cut new pieces of pressure-treated lumber to replace them. You may have to cut slots in them to fit over the bolts in the top of the foundation. Put construction adhesive on top of the foundation and drive the new mudsill piece in place. Remove the support wedges, install a new band joist section and seal the area.

Send your questions to Here’s How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit www.dulley.com.

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