Joist and beam span questions

I'm in the process of building a roof over my 8'x36' front porch. It's just a simple cover with a metal roof. The current plan is 8' -2x6 rafters 24" OC on a 2x6 girder with 4x4 posts 8' OC . I've looked at span calculators and such , but I don't claim to be a carpenter or an engineer so any advice from someone more knowledgeable in the craft ? -Thanks

I’m not an engineer either but if I’m understanding correctly you are spanning a 2x6 beam 8’ from from column to column. That beam will see much more load than the joists of the same span therefore why would both be the same size? I’d up the size of the beam. Depth is your friend when it comes to deflection and eventual sagging.

Would using double 2x6's do the same thing ? I don't know, just asking . Or would 6' OC posts work ? If not then move up to a 2x8 ? -Thanks

I’d use a 2x8 or even a 2x10 if depth isn’t an issue.

I think I'll go with the 2x8 and also go 6' oc on the post just to be safe.-Thanks

If concerned about head room, double girt 2x6, one on each side of 4x4.

The 2x6 joists will be fine but the beam is carrying roof load and should be a doubled 2x10 minimum with posts 8'oc. The joists should be on load bearing at the wall or ledgered onto a board lagged into a load bearing. Attach the joists at the beam and to the ledger board, if used, using joist hangers or a ledger strip and notch. Run a hog back aka rat board on top of the joists (1 2x4 nailed perpendicular to the run of the joists with 1 2x4 or 2x6 attached vertical) to stiffen up and unify the joists.
When building the beam lap your joints so the breaks are on the posts. Additional strength can be gained by using construction adhesive when marrying the boards.

Edit: misread rafter as joist. 2x6 is fine but need to know more, local code which is usually based on snow load, before I could say 24oc would be good. It would be down here, with mid span bracing.

This. Are you cutting notches in the post for the 2x6s to rest on?

you should be good on rafters as long as they are No. 2 graded or better - I'd recommend 16" centers, but 24 should work. Im not sure about your snow loads there in Indiana.

what is your roof pitch and are you hanging the rafters on the face of the beam or are they sitting on top of the beam?
those may change it, but based on a 4/12 pitch and sitting on top of the beam, you want 2-2x10 beam

A lot depends on if it’s a flat roof or what your snow load will be, typically rated at 25-50 lbs per square foot, i.e.; 50lbs in a northern area. The second factor is what type of lumber you plan to use, each species has a different modulus of elasticity. None of that is helpful yet- the math needed calculates the load a roof rafter can carry (different from live load calculations for floor joists). Here’s your example:

Calculate the rafter load capacity:
8’ x 36’ = 288 sq ft.
288/ 50 lbs per sq ft = 14,400
Calculate the maximum bending moment of the rafter:
Now divide the 14,400 lbs by the number of rafters in a 36’ span spaced 24” OC or 14,400/18 = 800 ft lbs

800 ft lbs x 12 = 9,600 inch lbs
9,600 / 1,150 (fiber stress of a wooden beam) = 8.3 section modulus required


Calculate the section modulus for the rafter.
The formula for the section modulus is rafter width times rafter depth squared divided by 6. A two 2-by-6 standard rafter has actual dimensions of 1.5-by-5.5 inches which would give a section modulus of 1.5 x 5.5 x 5.5 / 6 = 7.6
Since you need a section modulus of 8.3 and a 2 x 6 yields of modulus of 7.6.
If you have a low snow load you would be fine with a 2 x6 but I would beef it up to a 2 x 8 or go to 16" on center. Below is a rafter guide that utilizes a #40 lb load rate.

Did you figure on the snow load? What's the pitch of the roof? The flatter the roof the heavier the joist' need to be.

Just wondering if you’ve looked at engineered lumber and buying rafters here in Iowa some insurance companies make you have an engineered roof.

I would go with a double 2 x 10 setting on top of the 4 x 4's. Don't screw them into the side of the 4 x 4. Simpson makes a connector plate from the 4 x 4 to the 2 x 10.

I study structural failure for a living and this is what can tell you. If you slope that roof 12/12 you can frame it with anything stronger than 1x2's and you will be fine. If you are a 3/12 you want to go beefy because of the snow loading & I am not talking about that fluffy stuff that blows across the ground that skiers love I am talking about the wet sticky stuff that weight 5 times that.

Your porch framing will work so long as you do 3 things:
1) Slope it properly, you want that load getting OFF of your porch not piling up.
2) Tie into the exterior framing of your structure (this is KEY- your centers need to match- if it is 16 in OC then your porch roof should be the same)
Screwing a 2x10 onto the existing siding is quick and will almost always lead to failure in the future. I actually have had 2 collapse under me and then I quit accessing them to get on the 2nd story. Its just not worth it.
3) Properly flash the wall / roof intersection. If not water will trickle in and no matter how strong you build it they framing will decay and eventual fail.

Ppl think I am a "doomesdayer" on this type of subject but it is what I do for a living.

Here is another pro tip- never screw new metal roofing down over shingles unless you are selling the place with in the next 7 or 8 yrs. Why? Because the screws will rot off in 10 years and the roof will be prime to get pealed off by the wind in the 10-15 yr range

You're welcome

Both Eagleye and Leftlane make a couple good points but they might not apply to your situation entirely.

You haven't said what the pitch is going to be on this roof nor how much snow you have on the level at one time. From what I can find, Indiana rarely has more than 10 inches remaining at any one time. I assume that is pretty wet.

But a cubic foot of solid ice weighs 57 pounds so I think you'd be safe calculating half of that or less for your load.

The US legal span for rafters in snow regions with a dead load of 20 PSF allows a horizontal span of 9'9" for 2x6 on 24" centers using #2 material. Greater than that, depending on the slope. Example: if your slope is 4/12 , you multiply by a conversion factor of 1.054 and you come up with a span of 123.318"

Personally, I wouldn't feel comfortable going that far. Nevertheless, the roof you describe is exactly the size of the roof over my deck. 8x36. Mine has a 4/12 pitch and 2x6 on 24's for rafters. Nailers with metal on top. No plywood. All that does is add weight. Currently I have 60 inches of snow on that roof. Without a doubt it is not as dense as the snow you likely get, but I probably have 5 times as much as you will have at one time.

I agree with Leftlane when he says you don't want to just screw a ledgerboard up there. BUT....you can do that just to hold it in place while you do things right. Once that ledgerboard is where you want it...drill through it and into the studs of the existing exterior wall. Next run a 3/8"x 5" galvanized lag into the existing studs. Do it every 4 feet. That won't fail on you unless there is a tornado.
I also agree that you must use the proper flashing where the new roof meets the existing building. You could even run some caulk on the backside of the flashing if you wanted.

Lots of ways to do things but it's always best to do it the best possible way the first time. It might cost a bit more but not as much as re-doing it !!
I am going to insist that you NOT use two 2x10's for your header that carries the rafter tails. Two 2x10's measure 3 inches thick. The 4x4 post you are setting that on measure 3.5 inches thick. That half inch difference is just enough to allow for movement. It is imperative that everything be plumb and stable if you expect to support a load. Posts and header need to be plumb in all directions.

So.........do it right. Buy two 4x10x18' beams, and the correct Simpson hardware to hold it where it is supposed to be. All the pieces are engineered to fit. Be sure to include some diagonals to eliminate sway.

One more thing PLEASE do not set your uprights on the deck surface. Run them through the decking and support them on concrete. Either adjustable piers or just a poured concrete pad about 10"x10"x10". You need to transfer that roof load to the ground. Not the deck. Be sure to use pressure treated material in contact with the ground or concrete

I'm including some pix of mine that has withstood the test of time, snow, and earthquakes. Mine has a hip roof so that may be different from yours as it wraps around three sides


This is how you need to join your headers


Use plenty of Simpson connectors.Don't scrimp on doing it right

Thanks guys, great info. I should add that the ledger board will also be supported with 4x4 post to help keep the weight off the house itself. We don't get much heavy snow anymore 6"-8" would be average 10" to 12" would be a lot. Last several years been under 6". Pitch will be around 2/12 or 3/12 and the distance from the ledger board to the support beam is 7' with a 12" overhang past the beam. No plywood, just 1x4 's and metal roof. White17, do those "Y" post help distribute the weight better than a single post ? Also the perimeter of the deck is framed with double 2x10's that sit on concrete pilings, would it be okay to mount the post on top of those vs. using concrete pilings for each post ? -Thanks to all.

kyron4 , If your going to be open underneath so that the metal shows,instead of 1x4s,step up to 2x4s. you will have less blow off trouble and the screws will stay tighter. 1x4s let the vibrations from wind unscrew the fasteners from less friction holding onto the screws.

what you are calling Y posts are knee braces, they can be decorative, but most often are used to stiffen from lateral motion.your holding a bunch of weight up in the air and it wants to tip when outside forces are applied.

As Trapr says those knee braces prevent lateral movement so your posts remain plumb. Think of them as a gusset without the plywood.

Yes, you could set the posts on top of the double 2x10's.. they will act as a squash block between the post and the concrete pilings. Be sure you use some sort of mechanical connector where the post meets the double 2x....either a Simpson connector or a plywood gusset on each side. If you end up with a post that is not directly over a piling or pier, I would add one at that location.


EDIT: Trapr's comment about the nailers reminds me that the roofing screws that are used to attach to plywood are different than the screws used for attaching to dimensional lumber. Be sure you ask for the right ones.

White 17- Id walk on your porch and I almost never say that!