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psychomti 04-04-2010 09:22 PM

New Addition roof
All, I am new to this forum, so please forgive me if this question has already been asked and answered in another thread. Also, I am not a professional contractor, but a DIY a heart.

I am adding a 16x42 foot addition to the back of my house and will be using a shed type roof that will extend from the house 18 feet. Here is my question...

What is the best wood to use (2x6, 2x8, 2x10) if I choose to use 16 on center for the joists. I usually get most of my building products from Lowes, and they only offer 16ft in length max, so I will more than likely have to go to a sawmill or somewhere and have them made. Unless I can marry the boards together and make them longer.

I am going to use 5/8 for the roof, felt, and shingles. then use either Sheetrock or bead board for the ceiling. I would rather have the open "Cathedral" look, but how would I insulate this? If I choose to just go with the sheet rock or bead board as previously mentioned, i will be insulting between the beams as normal.

I would like to mention that I am on a tight budget, so if I can get away with doing it in a way that I have no described, please let me know.


San Antonio, TX

CrpntrFrk 04-04-2010 09:51 PM

Your framing members need to span from bearing point to bearing point. As far as what size lumber to use that depends on the pitch of the roof. So when you you say shed roof I picture in my head something near flat. On that note you should not use shingles on a roof with less then a 12/3 pitch.

oh'mike 04-04-2010 09:53 PM

I know you are just trying to figure out if this is a practical project--So here are a few things to think about--

You will need plans drawn to get a permit---

You need to talk to a real lumber yard-Not a Big Box store. A lumber yard will check the span tables for you--and tell you weather a 2x8 -2x10 or 2x12 is required.

You need to go to your local building dept.--often 2x12 will be required in order to get the insulation R-value needed to meet today's codes.

Longer lengths of 2x10 and 2x12 are available easily from any decent lumber yard.

Often hemlock/fur is required (these are stronger woods) Lowes does not carry this wood as far as I know.

Get some plans together--even home drawn,simple ones and post them here--Someone will be glad to help.

---Good Luck--Have fun--Mike--

Scuba_Dave 04-04-2010 10:03 PM

You do need one piece rafters, unless you have an engineer design a connection method your building Dept will accept
---usually not worth the hassle & cost more then buying 20's
I ordered my 20's from a Lumber yard...not a big box store
2x12's for Max insulation

I needed 3 sets of plans for my building Dept for a permit
If you go with an open Cathedral look all the way to the peak then usually you need an engineered ridge beam
All depends upon your layout/design

psychomti 04-04-2010 10:06 PM

CrpntrFrk, There used to be a covered deck there before that was only 12' out from the house. I am wanting to extend that out to 16 feet and have already gotten my permit, and going to pour a slab here in the next month. I already have a 2x6 header board that is attached to the house that I will be connecting to, but from the sounds of it, I may have to go with something a little larger. As far as the pitch is concerned, it is really only going to be dropping about 3 feet in the entire 18foot span. So, if i don't use shingles, should I just go with a metal roof?

oh'mike 04-04-2010 10:14 PM

If my math is correct you have a 1.6 pitch. I think that will be treated as a flat roof and require a
rubber roof.

I am NOT a roofer---someone who knows more than me will have to help--Mike--

psychomti 04-04-2010 10:20 PM

Pics of the mess!!
3 Attachment(s)
Ok, here is a few pics of what I am trying to accomplish. You will see the original roof, that was strictly a covered porch. This is what is coming down and a 16x42 foot slab is going to be poured. From there you will see the header board on the wall, which was used to attach the 2x6 headers. Now, if I go with 2x10s, will I need to attach a 2x10 header on the house, or could I just leave the same header on there since it is already attached? Also, the addition will be the entire width of the house from one side to the other.

Thanks a bunch!!!

oh'mike 04-04-2010 10:27 PM

Could you add a new (wider ) ledger up a little higher?

Right below that window ? that might give you a pitch steep enough for regular 3 tab shingles--

I'd sure think about that--stronger and cheaper,(Better looking,too)

psychomti 04-04-2010 10:28 PM

By the way, I may be asking a bunch of questions, since I am learning as I go. HAHA

psychomti 04-04-2010 10:30 PM

I could probably add one a little higher. I didn't think about that and it would look better. Just curious, how would it be stronger and cheaper?

oh'mike 04-04-2010 10:31 PM

You need to get some venting up near the top to cool that roof--There are some roof guys here --hope one is still up.

Scuba_Dave 04-04-2010 10:35 PM

You can't put a 2x12 rafter on a 2x6 ledger
...or are the rafters going above 2xt - ie resting on it ?
But you still need the face of the 2x to be up against solid 2x for the entire cut face
You may be able to add another ledger below/above it
For that distance & low slope you will most likely need 2x12's
For a 2x12 rafter cut at an angle I ended up with (2) 2x8's to fully land the rafters

oh'mike 04-04-2010 10:35 PM

The longer the pitch the less stress on the rafters,(Thinner 2x's)---
Cheaper?---You can put on a 3 tab shingle roof.

A flat roof will have to be installed by a Pro.

psychomti 04-04-2010 10:38 PM

Can I just insulate and let it vent out through the sofit at the end or is that not going to be enough? I'm wondering if they make some sort of vent like a ridge vent that I can install up at the top where the flashing is? By the ledger board is where I am referring to. I know what you mean, and I am sure it will trap alot of heat, especially here in San Antonio, but I thought that as long as I had at least a 2 inch gap between the insulation and the plywood for the roof, then it would have a little air flow from the sofit vents. Am I on the right path or do I need more than just sofits?

psychomti 04-04-2010 10:40 PM


Originally Posted by oh'mike (Post 424116)
The longer the pitch the less stress on the rafters,(Thinner 2x's)---
Cheaper?---You can put on a 3 tab shingle roof.

A flat roof will have to be installed by a Pro.

Oh, I see what you are talking about now. Makes alot of sense.

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