Tips for repairing shed siding

Q: I have a shed with fiber cement siding that is disintegrating a few inches around the bottom. The rest is in good shape. We previously replaced half a panel, using bent metal channel and caulk, then painted it over. We also tried a PVC board along the bottom, bent metal channel and caulk. Is this the best alternative? A different contractor wanted to attach PVC on the outside of the siding. Wouldn't this collect water and rot out above?

South Arlington, Va.

A: From the close-up picture you sent, it's clear that the siding on your shed is not fiber cement, but OSB, or oriented strand board. It's made of thin flakes of wood layered and glued together, topped by a thin laminate textured to resemble T1-11 plywood, which resembles rough-cut boards aligned vertically, side by side. The T1-11 look is also available in fiber cement panels, which are made from Portland cement, sand, water and cellulose fibers (wood pulp). But the crumbling wood fragments at the base of your shed's wall leave no doubt about what material you have.

OSB siding is relatively strong. It comes pre-primed, which saves time when building. And, unlike fiber cement panels, it can be cut with the same tools you’d use for wood. These features, as well as the cost, make OSB siding a bestseller for siding on garden sheds.

At the Lowe's in Alexandria, LP SmartSide OSB siding in the T1-11 style is $40.83 for a 4-by-8-foot sheet about ⅓ of an inch thick. Plywood in a similar look costs pennies less ($39.72), and it's thicker, a little over ½ inch. But it's sold without a finish, because people can opt to stain rather than paint it. Fiber cement siding is more expensive, at $57.38 for a sheet of HardiePanel Sierra 8.

OSB and fiber cement siding come pre-primed, because they must be painted; staining is an option only with plywood siding, because it has a layer of wood to stain. All three materials swell when they become damp. Plywood generally shrinks back into shape once it dries, provided the adhesive between the layers is rated for exterior use. Manufacturers treat OSB and fiber cement products to resist moisture, but the protection isn't perfect. And if the products do absorb moisture, they will swell and become permanently distorted.

One problem in your case is the way the siding meets the base of the shed. Siding on sheds usually sticks out more than the foundation or floor, which allows rain to run down the siding and onto the ground. It's similar to what campers have learned about tents and the tarps underneath: If the tarp is not completely covered by the tent, it will collect water.

Your shed, though, has a base that extends past the wall. It appears that there is flashing at the base, with one long edge of metal extending up behind the siding, and another wrapped around the base of the shed. This keeps water out of the shed's interior. If installed correctly, it could also help protect the siding. But the bend between the wall and the base must be more than 90 degrees, sloping away so water drains at the base of the siding. This kind of flashing usually comes with a suitable bend, but installers who don't understand the nuances of the angle can force the metal into a right angle — or less than one — when they nail it on.

What can you do? If you can find matching siding, it might be possible to cut off the lower portion of the damaged siding using a circular saw — or hire someone to do it for you. Check for nails before you decide where to cut, and nail a straight board below where you will run the saw to use as a guide. Set the blade depth to the thickness of the siding or a little less; you can clean up the back of the cut edge with a utility knife. Once the old wood is out, check the angle of the flashing. If it is not draining water away from the shed, replace it, nailing it to the wall in a way that keeps an angle along the base that will shed water.

Before you cut the replacement siding, factor in allowances for the top and bottom edges. At the top, between the new piece and the old siding, you will need Z flashing, which tucks behind the upper piece and covers the top edge of the new piece. In sizing the replacement, allow for a gap of ¼ inch between the top siding and the part of the Z flashing that extends below it. Also allow for a gap at the bottom edge of the replacement. Although ¼ inch might be enough here, too, a wider gap, say ⅜, will be easier to keep clean of debris.

Siding is usually installed from the ground up, so when there is a horizontal seam, such as in eave ends of sheds, the Z flashing between sheets is set on the top edge of the bottom piece over a bead of caulk, then held in place with the wide heads of roofing nails driven into sheathing, with no nails through the flashing.

If you’re doing a patch job, you’ll need to set the flashing on the top edge of the replacement piece and wiggle the top of the metal up and under the lower edge of the original siding. Prime the cut edges, and be sure to leave that ¼-inch gap, so the lower edge of the original siding can dry.

If you can't find matching siding to use as a replacement around the bottom of your shed, PVC board could also work, assuming you can find material that's as thick as the siding. Install Z flashing between the board and the siding above, and leave a ¼-inch gap. PVC won't absorb moisture, so a gap at the bottom is probably not critical.

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