Learn how to prepare walls for vinyl aluminum siding in this expert how-to, with diagrams and steps for preparing, sheathing, and furring walls for siding.

This article is a continuation of the discussion that began with Step 1: How to Install Vinyl and Aluminum Siding. Once you’ve gathered your tools and materials, you’re ready to prepare the walls for vinyl or aluminum siding and trim.

Vinyl Siding Diagram
Vinyl Siding Diagram

As shown in the diagram above, vinyl or aluminum siding are part of a system designed to keep the weather out. Because these materials—especially vinyl—are very flexible, they need a sturdy, flat backing. Otherwise, the resulting walls will look bumpy or wavy.

As discussed in the article How to Install Vinyl and Aluminum Siding, sheathing or backerboard normally provide the necessary rigidity and surface.

In some cases, notably when the “board” width of the new siding matches the home’s existing wood lap siding, vinyl or aluminum can be installed directly over the existing material. Below, you’ll see how to prepare existing siding. Then, we’ll discuss sheathing and other methods.

Before you begin, clear the area around the house and tie back any shrubbery to allow for plenty of working room.


Preparing Existing Siding

Provide a sound, flat surface for installation of vinyl or aluminum siding.
Provide a sound, flat surface for installation of vinyl or aluminum siding.

If you’ll be siding over the top of older siding, here are a few important steps to take:

1) Nail down loose boards, and replace any rotten ones.

2) Scrape off loose caulk and re-caulk around windows, doors, and other areas to protect them from moisture penetration.

3) Remove all protrusions, such as gutters, downspouts, and light fixtures.

4) Seal all cracks to make the house airtight.

5) Check all walls for evenness.


osb vinyl siding sheathing
OSB sheathing provides a flat, solid base for vinyl siding. © Wendy Kaveney | Dreamstime.com

Sheathing for Vinyl Siding

Applying weather-resistant sheathing over old siding is the fastest, easiest way to provide a flat, nailable surface for vinyl or aluminum siding installation. The tricky part can be dealing with the added thickness where siding meets windows and doors. Be sure door and window frames can be built out relatively easily before taking this approach. Otherwise, you’re better off stripping off the old siding and starting fresh with sheathing.

To obtain the best finished appearance for solid vinyl siding, use quality sheathing with no buckling or warping. The most p0pular wall sheathing material is oriented strand board (OSB)—it’s both serviceable and relatively inexpensive. Properly conditioned and fastened, exterior quality sheathing will eliminate the possibility of swelling and buckling that can occur with unstable wood.

If you’re dealing with new construction, the wall framing studs should be plumb and positioned uniformly to provide a flat surface for the sheathing. Be sure your builder uses quality, kiln-dried studs.



preparing base for vinyl siding
Over existing siding, 1-by-2 furring provides a flat base. This vertical application of furring is for horizontal siding.

Furring Out a Wall for Siding

Another way to provide a flat, nailable base for siding is to install spaced 1-by-2 strips (called “furring”) across the walls and around their perimeters. This works well on uneven or masonry surfaces. Slip small pieces of shingles (called “shims”) behind the furring strips to even out the high and low spots so the overall surface will be completely flat.

For horizontal siding, you apply the furring strips vertically. For vertical siding, you apply them horizontally. The strips also must be placed alongside all door and window frames and at building corners.

Keep the overall profile as low as possible because the alignment of the siding at doors and windows may be difficult. When you use furring, window and door frames generally have to be extended.

Next: Step 3

How to Install Vinyl Siding Starter & Trim Strips

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About Don Vandervort
Don Vandervort has developed his expertise for more than 30 years as a remodeler and builder, Building Editor for Sunset Books, Senior Editor at Home Magazine, author of more than 30 home improvement books, and writer of countless magazine articles. He appeared for 3 seasons on HGTV’s “The Fix,” served as MSN’s home expert for several years, and is featured as Yelp's home improvement expert. Don founded HomeTips in 1996. Read more about Don Vandervort