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Home > Products > Porches - START HERE! > ONLINE PORCH GUIDE > Understanding the Porch > Structural Porch Components > Beam


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The Beam is the structural framing member
that spans across the tops of the Posts.

Beam - Details

Beam should be the same thickness as your Porch Posts.

The Beam must be sized to transfer and support the weight of the roof structure. It should also be tall enough to accept Cornice Brackets. 8" of height is adequate for most of our Repeating Cornice Brackets.

The Beam's finished thickness should be identical to that of the Porch Posts supporting it. This will eliminate an awkward offset between the faces of the Posts and of the Beam. Equally important, it allows easier and more graceful application of your decorative components.

Drawings below illustrate several ways standard lumber and plywood can be used to achieve Beam thicknesses equal to standard modern Porch Post thicknesses.

Type of Materials
Untreated lumber
If your Beam will be protected by an overhanging porch eave then regular untreated lumber will provide good service. However, if your Beam will be exposed, such as with a lattice porch roof, then it is best to use treated lumber or to clad the entire Beam with PVC.

Treated or PVC Cladding
Beams clad with treated lumber or with PVC should be built with a top piece the full width of the Beam to deter moisture getting inside the Beam. To eliminate a visible seam beneath these top pieces the edges of the top and face pieces can be mitered (45° bevel) where their edges meet. While not often done because of the extra time involved, you can also miter the bottom piece to the Beam face pieces.

Face and Bottom Pieces
If you are using regular untreated lumber the exposed face and bottom pieces should be paint grade. It is easier and less expensive to find presentable lumber in 3/4" thickness, rather than in 1-1/2". Also, most Beams require double 2x's for strength. Therefore, the following Beam construction instructions utilize a layered approach.

Internal 2x's
The internal 2x's should be top construction grade lumber, as it it important that your Beam be straight and true.

Beam Construction
Please note - it is your responsibility to insure adequate strength for your Beam. This is not something we can determine at long distance.

For all but the smallest porches it is easier to build the Beam in place beneath temporary supports. These should be well braced to prevent movement during construction.

The following step-by-step instructions achieve interlocked corners for strength and assume build-in-place construction WITHOUT top cover piece (as shown above) for a porch longer than the 16'. Since lumber is readily available only up to 16' long, our instructions utilize scarf (bevel) joints to extend the face and bottom pieces of the Beam to the full length you require.

These instructions are written for a standard rectangular porch. Other porch shapes may require modified construction but should follow the basic principles set forth here.

Step-by-Step Instructions
Note - if you need Beam top pieces install them first. They should be the same width as the finished thickness of your Beam.

1. First internal 2x
Prepare the first internal 2x for the Beam on one end of your porch by cutting it to length. It should be the length of the finished Beam MINUS the thickness of the face piece (typically 3/4").

Starting at one end of the porch, install the first internal 2x in the final position it will occupy, securely attaching it to the main building.

2. Second internal 2x
Prepare the initial internal 2x for one end of the front Beam that will attach to the end Beam you just started. If your Beam will have only one 'layer' of internal 2x piece, then the length of this second 2x should allow the end furthest from the corner of your porch to fall directly above one of your Porch Posts.

However, if your Beam's finished thickness requires two 'layers' of internal 2x pieces, then it is not necessary for the far end of this second 2x to fall over one of the Posts, as you can stagger the internal 2x's to achieve the necessary strength.

Install the second internal 2x into final position by butting one end against the first internal 2x so that the face of the second 2x is flush with the end of the first. Secure this corner joint with 3 nails (2 nails if using 2x4's).

3. Additional first layer 2x's
Install any addtional 2x's required to extend this first 'layer' of internal 2x's all the way to the other corner of your porch. The length of the last front internal 2x of this first layer should be short by the thickness of the face piece and ALSO short by an additiional 1-1/2" to allow space for the first internal 2x layer of the other end Beam. Be sure to maintain a consistant distance from 2x to main building.

Prepare the first 2x layer of the second end Beam by cutting to length. Install it in the same manner as the other end.

4. Internal spacer
If your Beam requires an internal spacer, install it now to the inside face of the first layer of the internal 2x. Hint - if using plywood for this internal spacer, cut it slighter narrower than the internal 2x to avoid having it end up slightly wide in places.

5. Second layer of internal 2x's
If your Beam requires a second layer of internal 2x's, install them now. Overlay their corner joints.

6. Face and Bottom Pieces
You can delay installing the face and bottom pieces until later to avoid damaging them during construction of the roof.

NOTE - Carpenters often let Beam face pieces extend slightly below the bottom piece. This is typical of Beams inside the home but is not appropriate for porch Beams since Porch Posts should occupy the entire thickness of the Beam.

When it's time to install the face pieces, start with the inside face to gain practice. Where face or bottom pieces meet along the length of your porch use a scarf (45 degree miter) joint on their ends.

3-1/4" Thick Beam
3-1/4'' Thick Beam

(left drawing above) Using two 3/4" thick face pieces, with a solid core consisting of one 1-1/2" thick piece of lumber and one 1/4" layer of plywood to match a 3-1/4" thick Porch Post. This is the recommended configuration (due to the availability of presentable 3/4" lumber), if strength is adequate for the spans (between Posts) involved.

(right drawing above) Using two 1-1/2" thick face pieces, and one 1/4" layer of plywood is an alternative way to build a laminated Beam that matches a 3-1/4" thick Porch Post. This is the recommended configuration only if extra strength is required, due to the spans between Posts.

4-1/4" Thick Beam
4-1/4'' Thick Beam

(left drawing above) Using one 3/4" thick face piece, and one 1/2" thick face piece, with a solid core of two 1-1/2" thick piece of lumber. This will match a 4-1/4" thick Porch Post. This is the recommended configuration (due to the availability of presentable 3/4" lumber), but will require also locating presentable 1/2" thick lumber. (Any cabinet shop should be able to plane down 3/4" thick boards to your required 1/2".) Of course, 1/2" plywood could be used, but care must be taken to choose a grade of plywood that will match your other lumber after painting.

(right drawing above) Using one 3/4" thick face piece, and one 1-1/2" thick face piece, together with a core of one 1-1/2" thick piece of lumber, and one 1/2" layer of plywood is an alternative way to build a laminated Beam that matches a 4-1/4" thick Porch Post. This is the recommended configuration only if a suitable 1/2" face material cannot be located for the configuration shown in above left drawing, as it is difficult to procure 1-1/2" lumber that has a presentable face.

5-1/4" Thick Beam
5-1/4'' Thick Beam

(drawing above) Using two 3/4" thick face pieces, with a solid core of two 1-1/2" thick pieces of lumber and one 3/4" piece of lumber. This will match a 5-1/4" thick Porch Post.

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