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I am not a carpenter, so please forgive me if I use some terminology wrong. I will do my best to describe my situation, and then my question.

I have an existing "lean-to" style porch roof that itself seems in good shape, but the structures that support it don't make much sense to me, so I am starting a project to upgrade\repair\reinforce the supports for the roof. On one side, the rafters are toe-nailed to a ledger board at the house. On the other, they are set atop a single 2x6 beam (the roof beam length is about 15'). The beam is supported by 4 columns\posts (this is where we get to the odd part). The posts consist of 4x4s that run from the footings (currently a cinder block buried in the ground) up to along side the beam and connected to the beam with lag bolts and nails. There is a 2x4 attached to the side of the post with lag bolts that sits below the beam but does not extend all the way down to the footings. All 4 columns are of this type of construction. I tried to make a simple sketch of what I am describing:

enter image description here

My plan to upgrade this entails digging new, proper footings ( I am in the north east US), replacing the columns with 6x6 posts, and adding an additional 2x6 to double up (laminate?) the existing beam. Like this:

enter image description here

So my question is about attaching the new 2x6 beam to the existing one. All the information I can find online says that the proper way to do this requires a fair amount of nails driven through the boards in both directions. My concern is that all that hammering against the beam in the direction away from the house may cause stress or start to pull the rafters away from the ledger (the roof is currently finished with asphalt shingles on top). Is there any type of screw that might be acceptable to use in this scenario? It seems like driving screws in might be a lot less stress on the structure. Or would a nail gun be better or worse in this case than a hammer?

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    If you set 6x6 columns correctly, they will not move from driving nails. It should take a jack or chain fall or other heavy equipment to move them. If you set them incorrectly enough that there is potential give, the first tack screw or nail to tie it to the rafter will lock it in. Using the right screws or bolts will be fine as well. Super fancy screws can get quite pricy. Last year I saw ones chosen to hold a retaining wall made of 4x8 and they were 2.5 currency for each 6 inch torx screw even in large quantity.
    – K H
    Apr 16, 2021 at 2:07
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    We can assume that the rafters are (tenuously) connected to the existing beam. A bunch of hammering would jeopardize those connections.
    – isherwood
    Apr 16, 2021 at 2:27
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    You'll also have to take time to enlarge the "bird's mouth" in each rafter. That's the cutout that your single 2x6 beam is sitting in. When you double that beam, the bird's mouth will need to be larger so the joists can sit on the wider beam. That's very doable, but will take some time with a jig saw to cut each one.
    – FreeMan
    Apr 16, 2021 at 12:39
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    The elephant in the room: "The posts consist of 4x4s that run from the footings (currently a cinder block buried in the ground)" Is that not on your "make it right and structurally sound" list?
    – Alaska Man
    Apr 16, 2021 at 17:27
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    @AloysiusDefenestrate Yes, I plan to add Simpson LRUZ hangars to the rafters at the ledger. The current birdsmouths on the rafters are sloppy enough that it would not take much effort for me to clean them up and expand them for the additional board. This is the route I will take.
    – habelson
    Apr 17, 2021 at 19:20

3 Answers 3

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How about drilling holes and using bolts, washers and nuts? This will be structurally stronger than a million nails driven in from both sides.

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  • While this will work, it's massive overkill and a bit unsightly.
    – isherwood
    Apr 16, 2021 at 12:13
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borrow or rent a nail gun. they shoot so fast that noting will move.

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  • This is a good solution. I'm not sure why it's getting downvotes. The question asks specifically about preventing movement in the existing beam member, and here's a good strategy. Downvoters need to explain themselves.
    – isherwood
    Apr 16, 2021 at 14:57
  • @isherwood This depends on exactly what movement is not desired. If OP wants to simply minimize the magnitude of the displacement of the existing member then this is fine. However it's important to realize, a nail gun still dumps a very large amount of energy into the nail, and thus the beam. Without getting too deep in the physics weeds here, the end result is that beam is going to vibrate considerably, the magnitude of the vibrations will be small but the frequency will be very high which would produce significant fatigue.
    – PGmath
    Apr 16, 2021 at 20:02
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    Well, you can have your physics (about which I have both education and respect), and I'll take my decades of actual experience. A nail gun is far and away better than a hammer here.
    – isherwood
    Apr 16, 2021 at 20:35
  • @PGmath If OP is worried that hammering that beam will make something fall apart, then it's definitely not attached correctly. The ledger at the house should be lagged in with heavy fasteners, the rafters would be secured in hangers, and the beam should be lagged or bolted to the posts. Whether a nail gun or a hammer, it shouldn't be moving in any case.
    – J...
    Apr 16, 2021 at 21:09
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    @habelson My only reservation is that structural decisions should not be made based on gut feeling or random worries - if you calculate that you need a built-up beam to support the required load then it needs to be done properly. If you don't need the extra beam, then it really doesn't matter. Yes, more is stronger but that tells you nothing about precisely how strong it will be and whether or not that will support the load. For every person whose instincts are on the right side of safe, there are people the other way around. If you calculate, there is no ambiguity.
    – J...
    Apr 17, 2021 at 12:08
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When talking about structural issues it's always important to work to the rules. Like electricity, this is something that can kill when it's done wrong. If you're worried about a beam, the first thing to do is to figure out how big it should be.

You've told us the rafters are 10 feet between the ledger and beam, 16"OC, and your beams span about 4 feet between support posts. We don't know the snow load requirement, so you must find this information for your area. Once that is known you can find the beam size required by consulting a beam table.

From such a table we find that for No1 grade lumber either a single 2x6 or a single 2x8 should suffice for this, depending on the snow load requirement for your area. You noted 1.5' of snow as a requirement in comments - typically this is given in "PSF" (pounds per square foot) as a standardized measure, however. A single 2x6 is OK for your scenario if code for snow is 30psf or lower in your area. Note that the ground load and roof design loads are different - these tables are for a typical pitched roof. For a steeper roof the snow load goes down, for a flat roof it goes up. Also note that using lower grades of lumber makes a significant difference in the required beam size.

Now, If you intend to construct a built-up beam for an application that requires multiple plies of dimensional lumber there are rules about how the members are fastened together, using specific cross-nailing patterns or structural lags, bolts, or screws.

If you decide on screws, note that most types of general-purpose construction screws are not suitable for structural applications. The required screw for beam building is a listed structural screw with a long shank.

enter image description here

These screws have several features that deck or construction screws do not. Firstly they are listed for structural applications and meet the required shear load, strength, and corrosion requirements for same. Secondly, they are threaded such that the screws only bite into one of the plies in the sandwich, causing the screw to pull the members tightly together when fastened. This clamps the boards together rather than just tacking them together.

This does two things a regular screw does not. First, it loads the screw in tension, where it is strongest, and it maximizes the force between the pieces of wood, allowing them to function as a single unit and preventing them from sliding against each other (in which case the screws would be forced to take up all of the shear load, where they are weakest). Regular screws bite the top board as much as the bottom so they do little to force the boards together and most of the loading is concentrated around the screw head.

For the best strength, ideally you would also apply a good bead of construction adhesive between the plies. When the structure screws pull the two members together this also ensures that the adhesive spreads out as much as possible to produce a stronger, uniform bond over the mating surfaces.

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  • The linked table is a big help. According to this bulletin (nj.gov/dca/divisions/codes/publications/pdf_bulletins/…) as well as this site (hazards.atcouncil.org) my area snow load is 20 psf. Am I right in saying that your recommendation would be to stay with the single 2x6 rather than go through the hassle of adding another ply?
    – habelson
    Apr 17, 2021 at 13:57
  • @habelson For 20psf it's not needed. Your clear span will actually be a bit shorter than 4ft since the total span you said was 15ft with four posts, so 2x6 seems ok. Adding another ply certainly won't hurt and will improve the stiffness. If you're prone to high winds or seismic activity, of course, those require separate consideration.
    – J...
    Apr 17, 2021 at 14:06
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    @habelson Also note that a single 2x6 is fine only if it is No1 grade lumber. For the 30psf ground load example with a 20ft (full) span (same as your 10ft half-span), a No1 2x6 is fine, but for No2 lumber it steps up to 2x8 and for No3 lumber to a 2x10. If you're not sure about the grade of the existing member or if it has not aged well then this would be another reason to consider doubling up the beam.
    – J...
    Apr 17, 2021 at 15:28

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