Sizing a beam

[Dave Falkenstein]

I have been asked to build a slatted patio cover for a friend of a friend. He wants two 6X6 posts and a 2X6 header on the house. The support members from the header to a beam on top of the two posts will be five 4X6's and the "roof" will be 2X2's spaced about 3 or 4 inches. The total weight is relatively small, compared to a conventional roof. How can I determine the size of a beam (glue lam?) to span about 11 feet between the two posts. I'd like the beam to be either a 4X or a 6X. Thanks.

[lou sansone]

I am pretty sure that "pocket ref"
will have the answer for you

lou

[Mark Singer]

On smaller spans deflection is often critical....L over 240 is the normal criteria. For a one story structure a header is usually as deep in inches as the span. So 11 feet would require a 10" or 12" deep beam...6 or 4 inches wide for appearance

[Jeffrey Makiel]

Dave,
If you are considering using engineered beams, there are three types. But none of these can be subjected to the weather elements without protection. If you are only looking at the structural characteristic, the Microllam LVL would be the choice. Second choice would be a Parallam. The third choice is a GlueLam, but this beam is predominantly used in asthetic situations when the beam is in sight.

A Microllam has almost twice the bending strength of a comperable 2x lumber. Check out the Truss Joist site...
http://www.trusjoist.com/EngSite/literature/?SubCategoryID=72&CategoryID=2

They have downloadable files that will help you size the header based on the roof load for your area. For your situation, the lower L/over deflection is fine.

cheers, Jeff

[Rob Will]

Am I missing something here? This sounds like a fairly lightweight structure and gluelam beams or equivalent would be overkill (?) I think that the structure would simply need a beam to look in proportion to the 4x6 rafters and 6x6 posts. I suggest a hollow beam with 2x6 sides and 2x6 top and bottom thus the finished beam is 5 1/2" wide by 8 1/2" tall. If you like, you can add some galvanized sheet metal attached to the inside of the hollow beam for increased strength. A sheet metal shop can quickly shear and form 2 overlapping "C" shapes to fit inside the beam.

On heavier structures, I use structural steel hidden inside of wood and brick. A full blown asphalt shingle roof i.e. a carport supported on two posts would fall in this dept.

From a saftey point of view I would want to know more about the method of attachment to the house.

BTW if you band the ends of the 4x6 rafters, that adds to the stiffness. (assuming you have an overhang past the beam.

.02 Rob

[Andy Hoyt]

Given that the structure's design (a latticed roof) is completely open and the only load on it is the structure itself (which is minimal) your bigger worry is your location - Arizona. Your hot and dry climate will wreak havoc on whatever material you use unless it is properly selected, installed and cared for.

Pressure treated stuff will be ugly and unneccessary while glulams and the like won't hold up well for more than a few years before they delaminate. A site built box beam makes sense but seems more labor than it's worth. I'd go with a solid timber of redwood (as already dry as possible) and stain it white - every year - with oil.

The posts really only need to be 4x4s but that would look too spindly; so the 6x6s are a good choice. The outside girder running from post to post should be a something by eight. 4x would work, but unless you chamfer the post tops, it will look odd. So a 6x8 is an easier timber to work with and is large enough in section to allow you to cut notches for the 4x6 joists.

Using this scheme will allow you to timberframe the entire structure with minimal use of mechanical fasteners.

[Rob Will]

[quote=Andy Hoyt]
. I'd go with a solid timber of redwood....

I like the redwood idea too...(if the $$$ are there). Also redwood would be considerably lighter than pressure treated.

Here in KY my deck has a similar roof - made of pressure treated wood & coated with an opaque stain. (Sikken)

I also have 5 - 4x6 rafters for the roof but each rafter is on it's own 4x4 post. The posts extend upward from a lattice / railing setup. So, I have no beam at all running from post to post. Used a 1/2" radius round-over bit in the router to soften ALL the edges.

Rob

[Dave Falkenstein]

Thanks for the excellent replies. All of the wood in this latticed roof structure will be stained a dark brown color (homeowner's choice) using an oil-based semi-transparent deck stain. That dark brown color might be enough to cover up the ugliness of pressure treated lumber, but I also worry about how wet PT usually is and if it will dry out straight. Redwood is generally available here in AZ, so that sounds like a good alternative also. For the posts I could use cedar.

To Rob - I had planned to lag screw a 2X6 header onto the house. The house is stucco and frame, so it should be no problem to locate the supporting structure under the stucco. The header will attach to a small vertical wall (parapet?) that forms the outside of an existing patio roof. I plan to use 4X Simpson hangers to attach the five 4X6 rafters to the header. The other end of the 4X6 rafters will sit on top of the beam that runs between the two 6X6 posts. I will tie all of that down with Simpson strap ties. Sound safe?

You guys have convinced me that a glue lam beam is not the way to go, although I understand that glue lams are available for exterior use. I'll check to see what material is available in the area and use a solid timber beam, or will site build one from 2X6 material as suggested.

Thanks again. Any other suggestions warmly welcomed.

[Rob Will]

Dave,
Sounds good, and no, the PT wood will not all dry straight.
Rob

[Andy Hoyt]

Dave - For aesthetics I'd suggest that all timber be the same species. Besides different woods will take the stain with varying degrees of satidfaction. And for sanity I'd suggest the whitest shade of dark brown possible. The darker the color the more heat will be absorbed. The more absorbed heat the shorter the life span of timber and finish.

[Dave Falkenstein]

Dave - For aesthetics I'd suggest that all timber be the same species. Besides different woods will take the stain with varying degrees of satidfaction. And for sanity I'd suggest the whitest shade of dark brown possible. The darker the color the more heat will be absorbed. The more absorbed heat the shorter the life span of timber and finish.

Andy - Thanks for the concern about the color. The homeowner gets to choose the color - I would have used redwood and stained it natural, but it's not my house!!! Anything like this structrue in the AZ sun needs to be restained every year or two regardless of the color. I'm glad I'm not doing the restaining. Smile.

[Michael Ballent]

Dave, your projects just keep getting bigger and bigger... Next think ya know you are going to be doing room additions ;)

[Dave Falkenstein]

Dave, your projects just keep getting bigger and bigger... Next think ya know you are going to be doing room additions ;)

Michael - I have helped a friend add three rooms to a house and another friend build a log cabin from the ground up. I'm helping another guy frame a small shop building next month. I like "carpentry" work, but my 67 year old back is not so sure. Smile.

[Rick Thom]

Dave, don't know the rules in your locale but many places require building permits when you are attaching to a dwelling to ensure it's safe and meets the building code for the particular type of construction.

[Dave Falkenstein]

For those that might be interested, I got pricing on redwood, cedar and rough sawn doug fir. The 6X8X12' beam needed for this project is $66 for doug fir, $246 for redwood and $253 for cedar. The pricing on the 4X6X12' rafters is about the same ratio. I suspect the homeowner is going to choose doug fir!!! It will be sealed with deck stain and nothing touches the ground, so doug fir should be fine in our dry climate.