How to Build a House 4: Install Aluminum or Vinyl Soffits

Our construction dictionary on our website defines a soffit: The underside of elements of a building, such as overhangs, staircases and beams. In this How to Build a House article we will concentrate on the underside of the roof overhang, the eave of the house and the overhang at the slope of the roof, the rake of the house. Generally, there are three ways to frame a soffit, where the rake meets the eave, as shown here:

Drawings of three types of soffits showing soffit-at-rake and soffit-at-eave for each type.

Before a house siding job can start, the soffits at the eaves and the rakes of the house should be already installed. The soffit of choice for a house in my area for new construction is the perforated aluminum soffit material. Other choices for a house are perforated vinyl and ventilated wood or equivalent. It is important that the soffits are ventilated sufficiently to work with the roof or gable vents to keep air movement in the attic to prevent condensation in the house. In this article I'll discuss the installation of the aluminum and vinyl soffits on a house.

With most house installations a bit of preparation is required. With soffits we need to check that the house framer installed backing. Sometimes they do, most of the time they don't. Backing consists of lumber on each side of the overhang to support the soffit material and keep it in place. On the rain gutter side this is usually built in. The rafters or trusses of the house are trimmed with a 2x4, 2x6 or piece of dimension lumber their same size. This is called the rafter or truss fascia. A 1x6, 1x8 or a finish piece one size larger than the fascia is applied to it, called the fascia board, which hangs down below the fascia forming a drip edge for rain water. The fascia also hides the edge grain of the roof sheathing, as well as that of the soffit material. A fascia gutter acts as a fascia board, eliminating the need to use a fascia board on the house. The rafter or truss fascia automatically creates the backing required for the one side of the soffit. The other side of the soffit is added after the wall sheathing is in place on the house. For light soffits such as aluminum and vinyl, simply nail a 1x2 along the wall of the house with its bottom level with the bottom of the rafter or truss fascia. For wooden soffits a 2x2 or 2x4 is usually installed. Here is a drawing to make this clear.

Diagram of soffit at the eaves showing rafter, seat cut, rafter fascia, fascia board, J-molding, siding and wall line.

Along the rake of the house, a 1x2 is also nailed to the wall, parallel and level to the rafter or truss trimmer forming the overhang.

Aluminum and vinyl materials usually come in corrugated sheets 16" wide by 12' long. They are cut to length to match the width of the eave or rake. That is the corrugations go across the overhang of the roof, not lengthwise with it. A circular saw or table saw is generally used with a fine tooth blade that is put on backwards. This prevents the teeth from catching the thin material and ripping it instead of cutting it clean. Use ear muffs, because it is noisy. Don't forget the safety glasses, as well. The vinyl can also be cut to length with a large tin snip, but snips will crimp the aluminum corrugations together. The circular saw or table saw is the best way. Allow 1/4" clearance for expansion and contraction when cutting the lengths of the soffit material.

Start the installation at a corner of the eaves. Choose the method to frame the soffit at the rake if there is one. Fasten the 1x2 to the wall, as described. Install the J-molding on the bottom of the rafter or truss fascia, as shown in the drawing. Notice that the soffit material has a tongue on one side and a groove on the other. Place the groove of the soffit material into the J-mold against the rake framing and fasten it to the 1x2, against the wall of the house with galvanized shingle nails or staples, where the flat part of the corrugation comes up against the 1x2. On the other side, nail the tongue into the fascia with one nail or staple. This will keep the material square with the wall of the house. When you have a 12' section of soffit finished, fasten a J-mold under the soffit, below the 1x2, as shown in the drawing. This will hold it in place and be ready for siding installation. The manufacturers suggest sliding the soffit material into J-mold on each side, or an F-mold on the wall side. The way I have described is how the soffit pros do their installations and is much quicker.

If the house has a cottage or hip roof, usually the soffits are all level, that is no rake. The soffit should be mitered at the corner. Cut 45 degree cuts on a 2x4 and toe-nail it in place, on the flat, from the inside corner of the two fascias to the outside corner of the wall of the house, flush with the bottom of the fascia. Install two proper length pieces of J-mold, back to back, to cover the edges of the mitered soffit material. Start the installation working out from the corner, as described above. Some pros will eliminate the 2x4 in the corner and just support the mitered J-mold in the J-mold on the fascias and fasten the wall end to the 1x2. It depends on the width of the overhang. For a 2 foot overhang and larger, I prefer to add the 2x4.

I try to start my soffits at the front of the house where they get the most exposure to view. Go both ways from the corner at the front of the house and finish at the back corner of the house. Trim the final piece with tin snips.

Checkout these pictures of the finished job.

Photo of both soffit-at-rake and soffit-at-eave on a house.

This picture shows the rake and the eave with the finished transition.

Photo of soffits from another angle.

This picture shows an alternate way of installing soffits in a corner rather than usual method of mitering. Just be consistent with either method you choose.

The next article in the How to Build a House series explains how to install vinyl siding on your house.


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