Floppy Roof

[WilliamE]

Hi folks, newbie here.
I'm well underway with my outdoor workshop/shed project and am installing the roof at the moment. Rafters are 2x4 with a 12' span anchored at each end with Simpson H1 hurricane ties 24" OC. We are in a no-snow area, btw. Sloped roof, a tad better than 1:12

Using T1-11 for roofing and will continue with felt and roll shingle.

Although I'm only about halfway done installing the T1-11 I'm noticing the roof is not very rigid. I guess I should have used 2x6 but too late now. However, I could add more 2x4 - or - double up (parallel) some of the existing rafters.

I'm think the latter might be the best but I'd like to hear from the pros if you think that is worth the effort.

Thanks!
William

 

[green.bubbly]

I am not a structural engineer but 12 feet is a long distance to span using 2x4's. Especially on a relatively flat roof.

As you are already into the build process, adding additional 2x4 rafters will help. You may also think about adding a double 2x6 header beam across the middle of the roof running perpendicular to the 2x4 rafters. Not sure if you will have enough head room.

 

[Gary S]

Are you using simple rafters or engineered trusses? Engineered trusses are the way to go on a roof. My 2x4 trusses are spanning 24', and the roof is very solid even with 2-3 feet of snow on it.

 

[Falcon67]

Sister in 2x6s, cut to fit over the plates if possible. You can use construction glue and deck screws to help them become a nice rigid one piece. 12" total free span distance is not for a 2x4, as you now know. I would not double with another 2x4 - not enough cross section IMHO.

 

[Nighttrain]

Agree with above. 2x4 are no where near what you need for that roof. Fix it now before you get too far into it. ? Why are you using T1-11? That's cost a lot more than simple plywood for roof decking. What thickness roof decking are you using?

 

[SmokeyDP]

I just built a porch with a 1:12 roof and was approved for 2x6s with a 16" span and a 12' run. The header is a 4x6 and I have 4x4 posts every 4' which is a bit overkill.

I'd double up the 2x4's if you have no other choice.

 

[csp]

Why are you using T1-11? That's cost a lot more than simple plywood for roof decking.

Glad I'm not the only one wondering this.

 

[jbs]

2x4s over a 12' span at 1:12 pitch does seem inadequate.

You could also glue/screw 2x4s on flat on the bottom edge of the existing 2x4 rafters. It won't significantly increase the load-bearing capacity since the added 2x4s won't be seated on the wall top plate, but it will help a lot with the deflection or "bounciness", and it's a little easier to do than sistering additional rafters.

Here is a thread on the John Bridge tile forum discussing it in more depth:
http://www.johnbridge.com/vbulletin/showthread.php?t=63630
(ref. post 14)

 

[bczygan]

If you don't have an engineered truss (and it sounds like you don't), then your span for the ceiling joists and rafters is too long.
Questions:
What thickness is the T-111?
Are you in an area that ever gets snow?
Are you pulling a permit?
Are you meeting all zoning rules?
What is the size and layout of your shed?
Where are the openings for doors and windows?

First, you need more slope. 4/12 at least for shingles to work. Otherwise you need a rubber roof. I like 12/12.
Second, use OSB roof sheathing with clips.
Third, use 2x6 rafters and ceiling joists.

Use the 2x4's you salvage for rafter ties and misc. framing or a lean to addition. Use the T-111 for siding.
Basic answer is you need to rebuild the roof.
If you just want it to stand up under its own weight and still leak with shingles then put 2x4 or 2x6 ceiling joists and rafters at the midpoint between each existing set of rafters/joists. This will cut the loads in half and make it feel less springy.

Sistering a member that is the same depth as an existing member (Like adding a 2x4 to a 2x4) adds some strength, but not a significant amount. I does NOT double the strength. Adding a deeper member does add significant strength. And you can bear it on the top plate and get more bearing if the existing rafter has a birdsmouth notch for an overhang. Instead of sistering, put the new member at the midpoint between existing members to get more benefit by cutting the span for the roof sheathing.
Your problem is the span the member is being asked to span, along with heavier than normal sheathing. If you had a way to transfer the loads to grade at each end of the peak of the roof, you could use a properly sized ridge beam to cut the rafter spans and loads.
But the pitch is too low in any case.
Give us photos and measurements.

 

[pattenp]

To help make the roof more rigid add more 2X4's to get them on 12 inch centers. Not the best solution but a compromise.

 

[kbs2244]

If you put the rafters on 24 inch spacing you could add more at 12 inch spacing.

But if you are in a no snow load area the roof won’t need to carry any weight except rain leaves, and squirrels once you are done.

Finish it off and count it up to experience.

 

[WilliamE]

Thanks for all the thoughts guys. Gives me something to mull over. I finished the decking today and now that everything is cinched down its noticeably stouter but I think I will add in some parallel rafters. I guess that what you mean by "sistering". Standing off at a distance I can see the sag on the end rafters. 2x4 just aren't up to the job in this case and that was a bad choice on my part.
Why T1-11? I live on a small island in the Caribbean and choices are few (and costs are high!) T1-11 is the plywood 'de jour' in these parts since its cheaper than regular treated plywood. (volume) What they sell here is 5/8 thickness.
This is a "lean to" roof against the side of the house.

Tnx folks,
William

 

[WilliamE]

Pix always help. Let me try this...

 

[Eds Garage]

Way undersized. There are free span calculators on the web that would give you a better idea what is needed. My quess would be more like 2 X 10 for a 12' span with no snow load. I wouldn't bother trying to sistor onto the existing 2 X 4, just take it down and redo.

 

[Zeke]

I've seen T1-11 flipped over like that many times and I like it. 5/8ths over 24" spacing is pushing it, but if that's not a deck above, some spring is OK.


Most mild climate roof systems are designed with recovering the roof in mind which doubles the weight. Keep it simple and light and I think it will fly.

Edit: It is undersized and over spanned, I just wanted to be clear. There are ways to make the existing roof you've built stronger.

 

[betterbillt]

I don't see why he couldn't run a beam down the center and cut the spans in half.

 

[mad57]

Cool what island are you on? i know a bit about living on an island been in the bahamas on and off since about 20 yrs ago, are you planning on cool sealing this roof? is your insurance payed up? i knew a guy who would build it just good enough and wait for the hurricane to come through and blow it all down , collect his cash and get all new a few yrs later crazy sob.

 

[henrysgarage]

Not being familiar with T1-11 I looked up and it says it is a siding material. I wonder if it does not have the load strength of other materials suited for roof sheathing?

and another site for using as roof material
http://www.eng-tips.com/viewthread.cfm?qid=310396

 

[ZRX61]

I don't see why he couldn't run a beam down the center and cut the spans in half.

He could, but those wall at either end would need some beefing up to support it, they look to be 24" OC also (or more?). He's not regulated by US zoning & all that malarky though..

 

[bczygan]

William,
I've examined your photo. I've got the cheapest and quickest fix for you. The problem is the length of the 2x4's. More 2x4's won't do much for you. Adding 2x6's between would require you to hold up the end of all those rafters while you cut the tops off the studs on that wall to the left and lower your wall top plates for the 2x6's and then shim the 2x4's up. Too much work. So here is what you do. The length of your room seems to be about 20'. Whatever it is, get a 2x6 or 2x8 that long. Set it on top of the top plate of the wall on either end. Now push it to the left in your photo until it jams between the top plate and the 2x4's forming the roof framing in the triangle formed between them. It should come to a stop 3 or 4 feet away from the wall of the house. Get it a little further over with a sledge. Now put another one just to the right of it, or better yet, nail the 2 of them together to form a beam and then lift it up and slide and sledge it in place as one unit. Next, install 2 2x4 studs right below this beam on both ends to serve as columns. Make sure you sledge the beam over at each existing roof 2x4 until it is straight along it's whole length. Now cut some wedges to fill in the triangular gap above the beam at each 2x4. Hammer these in place and toenail to beam and 2x4.
This will cut your span and provide some additional support to the 2x4 roof rafters. It still won't be designed to meet any code or to hold large weights, but it will be better than what you have now. Do you understand what I'm saying?
So what are you doing for roofing? Shingles will be heavy and that low slope won't stand up to tropical driven rain storms, much less hurricanes.

Bill

 

[bczygan]

Just a note. There is a better way to do essentially the same thing as above but get a deeper, stronger and stiffer beam that can go in the middle of the 2x4 span. You would take a couple of 2x8's as long as the ones described above. You would install them both on edge. One as described above but to the right of the center line of the room. Don't do the wedges, this is a temporary support. Install 3 or 4 2x4 braces from the floor to the underside to keep it level. Then install one to the left of the center line of the room. This one will need to go below the top plates of the end walls. It will need a few 2x4 braces under the ends and in the middle as well as blocking above it at each existing 2x4 rafter to support them. You have now constructed 2 temporary walls on either side of the center point of all the 2x4 rafters. You can now cut a 3" (For a dimensional lumber beam) or 3 1/2" (For an LVL beam) slot along this center line through every 2x4 rafter so you can put a beam in the space. Size the beam properly for the 20' span and the total roof loads and install either a dimensional lumber beam (2 2x's) or an LVL. Lift the beam into place and provide double 2x4's columns at each end wall and provide temporary supports to keep it level along its length. The beam should then have joist hangers installed along each side at every 2x4 rafter to catch and support them. Then remove the temporary 2x8's and support braces.
That's how I would do the repair.

 

[WilliamE]

I like your ideas, bczygan. The room is only 16' in length and I should be able to get 2x8x16 here, if not then I can double up 2x6x16.

Part of the mission here is going to be building floor to roof shelving around most of the perimeter of the room. I didn't really look at that as a structural aid but now that I think about it....The planned shelving will be built 'rough' out of 2x4 (this is mostly a storage shed with some workbench space) warehouse style and with 2 ft shelving on each side that will cut my span down to 8 feet.

In answer to another question - yes, I will be using roll roofing shingle.

Hurricanes - yes we are in that zone and I've been thru several of them, the worst being Hugo. I'm doing whatever I can to make this thing last that's why I started with the 12" poured columns and the little concrete wall. The wooden walls are anchored in place with 5" expanding anchors. My biggest concern is roof 'lift' and it is being tied down all the way to the foundation wall with the Simpson ties. (not all in place yet in the photo).

Thanks for all the tips, fellas. I've gained some good ideas.

 

[bczygan]

William,
Now you are starting to think outside the box! Those shelving units WILL cut the span! Secure those 2x4 posts that are part of the shelving, to both the floor and the rafters for more protection from uplift as well.
Think about covering the entire roof with ice and water shield before the roll roofing for a better seal. I would use 30# felt, then ice and water, then roll roofing. Don't forget to nail the edges as often as recommended and bed the laps in roofing sealant per the instructions. Flash and counter flash where the roof meets the wall.
Let us know how it turns out with lots of photos along the way!

Bill

 

[betterbillt]

He could, but those wall at either end would need some beefing up to support it, they look to be 24" OC also (or more?). He's not regulated by US zoning & all that malarky though..

I figured the beefing up of the side walls was a no brainer. It actually looks like he has more than enough room. I figured I'd make a suggestion and see if it sparked a little activity.

You know I rebuilt a barn a few years back that had actual 2inch x 4inch rafters on a 10/12 that were like 20 feet long. They were actually from another barn that had been taken down in the late 1800's and added to a new barn. 150+ years of new york winters. All I did was sistered in new members and added Gussets to help with the load. That barn is good for another 100 years.

 

[Zeke]

Bill's post above and post # 16 are suggesting adding a purlin, just to identify the technique. Adding 2x6's with a deep bird's mouth parallel to the existing rafters would do it w/o removing a nail. That would give the highest clearance of any add-on solution. I would add those mid bay and no more floppy roof.

 

[bczygan]

Bill's post above and post # 16 are suggesting adding a purlin, just to identify the technique. Adding 2x6's with a deep bird's mouth parallel to the existing rafters would do it w/o removing a nail. That would give the highest clearance of any add-on solution. I would add those mid bay and no more floppy roof.

Zeke,
I love your common sense solutions. They show a lot of field experience. I like looking at these problems on GJ because they cause me to really think hard about the possible causes and solutions. It's fun to exercise your brain.

Bill

 

[WilliamE]

Just checking back in. Thanks to all you guys for the comments and suggestions. I'm working on roofing at the moment and it has dawned on me that my biggest problem is one single rafter. Wasn't thinking intelligently when I installed it and I should have watched for the 'bow' and flipped it over. Newbie learning lessons! Its the one 'spongy' place on the roof that caused me the alarm.
I still haven't dealt with it but all of the great ideas here will help in deciding what's best/easiest/cheapest for the situation Yeah, I know - you can't get all three!

Anyway...here's a pic. No, I didn't have access to a hammer/tacker for the felt.