header size
 

I have a porch on the front of my house that would be about 30' feet long. nad 6 feet deep. (give or take)

It's a covered porch with a balcony on top

I'm having a guy come in and dig six 10" or 12” holes so that I can pour concrete to support this structure. Right now some of the original support are in bad shape.That’s why I’m doing this.

I will be installing a header under the existing deck for the post to distribute the weight onto. I think it’s called a header. It’s the long beam under the deck to distributes the load to footings below.

How big should the header be? What size lumber.
I’ve been looking around some deck in town and I’ve seen 2 x 8 tripled up or 2x12’s doubled up…many variations…

How do you figure out what you need? Is there an easy rule of thumb for these things?

Thanks

Pabs

You can call it a header, beam, or girder -- they each sort of do the same job (although a header is usually above a door or window). The size of this beam is determined by the amount of weight it needs to hold up, how often that beam is supported, and how much deflection you're willing to tolerate.

You said the porch is 6x30 and has a covered top. You said the top is really a balcony, so I assume it is a second floor and not just a roof (floors typically must carry more weight than roofs). Is the balcony also 6x30? Is there a roof over that balcony that rests on posts from the bottom porch?

You said you're having 6 holes dug, so if I assume correctly, you'll have the holes about 6 feet apart and each hole will be supporting the beam along its length. So, this tells me you need to calculate the strength of a beam spanning 6 feet.

The six foot deep deck isn't very large which helps things, and one end is already supported at the house. The beam you're putting in will have to carry half of the deck load and the house will carry the other half. Assuming the balcony is the same size (6x30) and is rated for 40 PSF, your combined loads (assuming 10 PSF dead load) are 50 PSF for porch + 50 PSF for balcony. If each is 180 square feet, this comes to 18000 LBS total weight shared across the two beams. One beam will have 9000 lbs along its full 30 feet. A 6 foot span of that beam will carry about 2000 pounds (9000 / (30/6)).

Now for the hard part -- you need a span or load table for the type of wood you'll buy, and I don't have one handy. Just guessing, I'd suggest a 4x10 beam (or doubled 2x10's). If that balcony is much smaller, then you can probably get by with a 4x8. Since you can't buy a 30' long beam, I'd buy two 16 foot ones and make sure the butt joints lie on one of the concrete deck supports. Or dig 7 holes and buy three 10' long beams. This will shorten the span to just under 5' and maybe let you get by with a 4x8 beam.

Per the IRC 2003 Table 502.5.1 to carry two floors, a roof and a ceiling your girder spanning 6 feet may be carried by either 3 - 2x12 or 4 - 2x10 spf lumber with a deck width of 0' to 20' all lumped into one column.

A few things to consider is if this girder is within 8" of the earth it must be treated.

You did not metion the area you are from but you should dig down along the foundation to the bottom of your house footer. Then match the depth of that footer with your posts to ensure no difference of potential heave concerning frost.

Hope this helps

Wg

The difficulty with that table is the deck can be up to 20 feet deep. A 10 foot deep deck should produce half the weight of a 20 foot deck. A 6 foot deep deck will be less than 1/3 the weight of a 20 foot deck. So this table seems like overkill. Hopefully, the IRC doesn't mandate this girder spec when the deck is much smaller than what the table supports.

The table also includes a roof load, and I don't know if this deck has a roof or not. That could be another major factor if there is significant snow to worry about.

to clarify:

I live in New Brunswick , Canada. that means I'll have to dig about 4 feet to ensure I'm below the frost line. I'll probably go deeper just to be on the safe side

second thing. the balcony does NOT span the entire deck. I would guess it to be no more than 10 feet wide by 6 feet deep

also, the base of the deck sits about 3 feet off the ground. I'll probably still use pressure treated for the girder to make sure it last a long time.


and the six holes are not in line. here's a very crude "drawing" of the deck seen from above.

house side here

**************************
**************************
P1******P2******P3*******P4

******

P5***P6

stree side

P1, P2, P3 and P4 are the main support for the deck. P5 and P6 are not holding much, that's the part stairs going down from the deck. the beams there basically only hold an extension to the porch roof (to cover the stairs)
distance from P2 to p5 is about 3 or 4 feet.

that being said, what size should my holes be ? 10" , 12"?

thanks
Pabs

Your deck girder loads are much easier to manage now that you've clarified things. Now, its the posts and post holes that are important. Posts P2 and P3 are doing the majority of the work. It looks like each post is about 10' apart.

I think you can still get by with a girder of a 4x10 or two 2x10's (each 10' long) with joints on a post. The size of the post holes depends on soil strength. Posts P1, P4, P5, and P6 can probably be about a 10" diameter hole. I'd go larger for P2 and P3 -- perhaps about 16 inches or so.

How are you attaching the posts to the holes? Are you just pouring concrete at 4' down about 6" thick, placing the posts on top of it, and backfiling with gravel or concrete? Or are you placing a pier block set on top of 4' of concrete? I also wonder how tall the posts are (expecially P2 and P3), and whether you're using 4x4 or 6x6 posts?

I'll be pouring concrete in the hole right to the top using sonatubes (I think that's what they are called).
I will dig past the frost line, which is approx 4 feet here, and pouring past ground level by about 10 inches or so. then I'll install a metal bracket to accept 4" lumber. so for the remaining length to the girder ( a couple of feet max) I'll use the 4X supported on the concrete footing.


Pabs

Nothing wrong with sonatubes except that you'll need a lot of concrete. The main design trade off I see is that the metal U brackets don't provide any rigidity to posts, but are good at supporting a girder. If the posts are just going to sit on top of the girder, then you need a way to prevent them from racking. This is especially true when stairs or second floors are involved. You'll need diagonals somewhere to keep the posts rigid (diagonals under the second floor would probably work if you don't like them on the posts).

An alternative method is to pretend you're building a pole building. In this case, the posts need to be deep and sit on a conrete footing about 6" thick and 12 to 18 inches in diameter. Then, at the appropriate height, you screw 2x10's to the front and back of the posts to make your girder. The posts continue up to hold up another girder that supports the balcony.

So with the pole approach you use less concrete, poles that are longer, and racking problems are much reduced. I'd also consider making them 4x6 or 6x6 posts if more than 10' of post is above the ground. Disadvantage is that the posts must be pressure treated and will still eventually rot. This can be a chore to replace if you backfilled with concrete! If you use the post approach, I'd backfill the holes with crushed rock.

The IRC is generic fit all charts. However if you wish to use less than stated for a 20' width of deck for say a 10' deep deck in reality then a letter from an engineer must be supplied accepting that design. If a layman builds without an engineer they must stay within the limits of the IRC charts.

Engineering design not fitting the charts must only be done by a licensed engineer. Laymen must stay within the limits of those charts. There is no column depicting spanning limits for a header carrying less than 20' width therefor the 20' must be used for width for anything up to that 20' or an engineer must accept responsibility of the alternative design for the lesser span.

Posts may be set without saun tubes by placing a concrete donut on the bottom of the post or setting the post in concrete partially filling hte hole. Normally this concrete poured into the hole will create a wad of concrete around the post that will sustain the post where it was set even though the center of the bottom of that post may rot away.

Post normally will rot in two places first one from the bottom up the center of the post [concrete or donut addresses this] and rot just under the surface of the finished grade where the post enters above ground. [treated approaches this retardant desire to a certain degree]

Hope this helps

Wg

so the only advantage to using the deep post option is less concrete?

seems to me I would be better of pouring a full concrete tube and lay the post on top of it (above ground) and if that post rots then it's ery easy to change it. wouldn't it?

unless there is a dissadvantage to using full concrete . I know it's a little more money but that way I won't have to worry about it for a very long time or not?.....

humm....options.... :confused:
Pabs