Storage shed strength

Reply

  #1  
Old 04-02-13, 06:20 AM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Storage shed strength

We just built the frame for a storage shed
12 by 24
Walls = 2x4 studs 16 on center
Roof joists 2 x4 24 on center
Ridge beam 2x6. Put 3 8 ft together to get span

Everything up with plywood put down but building just does not seem overly sturdy
Is there a way to strengthen roof at this point?
 
  #2  
Old 04-02-13, 06:41 AM
M
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,845
Received 361 Votes on 318 Posts
Is the plywood or siding on the walls yet?
 
  #3  
Old 04-02-13, 06:43 AM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Yes plywood on walls and roof.
 
  #4  
Old 04-02-13, 06:55 AM
M
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,845
Received 361 Votes on 318 Posts
Are there collar ties on the roof rafters? Is it only the roof that feels weak/wobbly or the whole building?
 
  #5  
Old 04-02-13, 06:55 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
Ceiling joists run from wall to wall. Rafters run from each wall up to the ridge beam forming a triangle. From your list it sounds like you do not have the ceiling joist from wall to wall, just rafters.

How high are your walls?
Did you use a double top plate on top of those walls, with the rafters and ceiling joist resting on top?

Bud
 
  #6  
Old 04-02-13, 07:29 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
How thick is your roof sheeting?

If you do have ceiling joists going horizontaly from wall to wall what dimension is that lumber? I hope it's a 2x6 and not 2x4 with your 24" spacing.
 
  #7  
Old 04-02-13, 08:27 AM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Ok - we are using rafters that go up to a ridge beam. - rafters are 2 by 4 (spaced 24 inches on center)

walls are 8 foot high with a double top plate at the walls.

Roof sheeting is 5/8 inch
walls are solid enough - the weakness is appears to be in the roof.

I was hoping to get some advice before we went any further so any suggestions would will be appreciated. we are in the south so no worries about snow weight however we are in a hurricane zone.
 
  #8  
Old 04-02-13, 10:40 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
Hi, again you did not mention a board (ceiling joist) going directly from the top of one wall to the top of the other wall. Without that you have no stability. Step #27 at this link is what I'm referring to, the triangle:
How To Build A Shed DIY Backyard Construction Step by Step - YouTube

Bud
 
  #9  
Old 04-02-13, 10:51 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
Without ceiling joist to directly hold the walls together the weight of the roof will push the walls outward. So, unless something else is done to handle the outward push of the roof rafters there will be trouble.

 
  #10  
Old 04-02-13, 11:15 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,113
Received 3 Votes on 3 Posts
Just my opinion...but 2 x 4 for the rafters on 24" centers seems very small to me. Engineered trusses can use them, but regular rafters? I would have prob gone 2 x 6 and poss even 16" centers. Min 16" with 2 x 4.....

But thats just me.
 
  #11  
Old 04-02-13, 11:30 AM
BridgeMan45's Avatar
Member
Join Date: Oct 2011
Location: USA
Posts: 3,194
Received 1 Vote on 1 Post
Rafters (2 x 4s at 24" centers) are not stiff enough to provide a rigid roof structure. Ridge beam splices are also probably questionable (based on the "put three 8' together to make the span" comment), making things worse.

An easy fix would be to first strengthen the ridge splices in a manner consistent with what their configuration is. Then add a bottom flange to each existing rafter, using the glue-and-screw method of attaching flat 2 x 4s centered on the bottom of each rafter, making inverted "T"s.

The difference in stiffness will surprise you.
 
  #12  
Old 04-03-13, 08:17 AM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Bridgeman45,

you will have to excuse me but could you explain the bottom flange that you suggested .

Are you talking about attaching the full length of the rafter or at the sill plate?

thanks
 
  #13  
Old 04-03-13, 10:04 AM
BridgeMan45's Avatar
Member
Join Date: Oct 2011
Location: USA
Posts: 3,194
Received 1 Vote on 1 Post
The ability of any member to resist applied loads is dependent on both what it is composed of, and its geometric cross-section. By adding a flange to the bottom of each rafter as I described, you will be greatly increasing the stiffness of each. In technical terms, you will be increasing the moment of inertia (I) of each rafter, resulting in lower applied member stress and deflection, since "I" is found in the denominator of the equations for computing each. Meaning the entire roof support system will firm up and be less likely to deflect under load. Doing the same to the over-spanned ridge beam could also help, as will making sure the splices don't allow any movement between individual 2 x 6s.

Adding the flange pieces as close to full length as possible along the rafter (and beam) bottoms will be most effective, as that's where the applied bending stresses are greatest.
 
  #14  
Old 04-03-13, 10:11 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
I guess I'm still missing a confirmation of the triangle as shown by Pilot. You state rafters going up to the ridge, but you have not stated you have the ceiling joist going from wall to wall??

Bud
 
  #15  
Old 04-03-13, 11:46 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
Woachubby look at the picture I posted. Yes or No, do you have the horizontal piece labeled "ceiling joist"?

This question has been asked before. It is a very important piece and whether or not it is there makes a big difference. If you don't know, posting a picture of your shed from the inside looking up at the roof should clear things up.
 
  #16  
Old 04-03-13, 03:19 PM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
No I do not have a ceiling joist going from wall to wall.

Span is 12 foot

Will try and post picture this evening or tomorrow .
 
  #17  
Old 04-03-13, 05:09 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
You will never achieve any stability without something holding the walls together. Was there a reason, like vertical clearance, that you did not install them?

Bud
 
  #18  
Old 04-03-13, 06:24 PM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
In the drawing I posted the load from the roof rafters gets transferred downward and outward (shown by the arrows). The shallower the roof pitch the greater it tries to push the walls outward. Ceiling joists solidly tie the two walls together and prevents them from being pushed outward. It solves the problem and conveniently provides for a flat ceiling.

Long ago cathedral builders wanted tall open spaces. To go without something tying the two walls together like a ceiling joist they had to use huge flying buttresses on the outside of the building to prevent the walls from being pushed outward. The important thing is that the outward push of your roof rafters must be countered somehow. If you're not building a cathedral I'd say go for the ceiling joists. It's a much simpler & cheaper solution to the problem.

 
  #19  
Old 04-04-13, 04:14 AM
M
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,845
Received 361 Votes on 318 Posts
Would collar ties halfway up the rafters be enough to do the same thing? That would still give him extra ceiling height if needed.
 
  #20  
Old 04-04-13, 06:31 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
Has Woachubby said that the roof was done that way for ceiling height?

Collar/rafter ties are an option but I think the roof is marginal with 2x4 on 24" centers and ceiling joists. I would not feel comfortable just adding ties unless at the bottom of the rafter where it meets the walls.

I have heard the terms collar and rafter ties used interchangeably over the years. Generally I think a collar tie lives in the upper 1/3 of the roof structure tying the top half of the rafters together and does NOT prevent the walls from spreading. I think (but not certain) collar ties may be as light as 1 x 4" lumber. Rafter ties are in the lower 1/3 of the rafter and do prevent the walls from spreading and must be at least 2x4 sized lumber. This also means that the ceiling joist can also be called or considered a rafter tie.

The upper limit for a rafter tie is 1/3 the way up the rafters. I think you can go above that as is often done with vaulted ceilings with the proper engineering. But it's not something you just throw together in the back yard.


 
  #21  
Old 04-04-13, 06:34 AM
M
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,845
Received 361 Votes on 318 Posts
I didn't know there was a difference between collar ties and rafter ties
Guess I better stick to painting
 
  #22  
Old 04-04-13, 09:30 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
The only reason I know is I built my house 12 years ago. I learned a lot about some things and not enough about others.
 
  #23  
Old 04-04-13, 06:08 PM
BridgeMan45's Avatar
Member
Join Date: Oct 2011
Location: USA
Posts: 3,194
Received 1 Vote on 1 Post
I take back what I said earlier about beefing up the flimsy rafters. I think the OP's shed needs some flying buttresses to stiffen it up!
 
  #24  
Old 04-05-13, 09:46 AM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Great discussion.
I appreciate the feedback.
the walls are 8 ft high so putting in a joist is not a problem.

to recap from all the conversations I guess attaching a joist to each rafter seems to be the best option.
 
  #25  
Old 04-05-13, 10:09 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
Yes. Because of your span a 2x4 may be too lite though it is often done with engineered trusses. I think 2x4 will be too small especially if you plan to hang sheet rock or use the attic area for storage and because of the 2' spacing. Most tables for ceiling joist span distance start with 2x6 lumber and go up from there and call for 2x8 in most cases with 2' centers but it also depends on what wood you are using.

Here is one table to give you an idea;

 
  #26  
Old 04-05-13, 01:27 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
The ceiling joists are probably the most important step you can take, but think about what is holding the 8' x 12' x 24' rectangular box below the ceiling together. Example, if you removed the plywood from both ends, a small breeze could push it over. Now, if you are putting a door in one of those ends, there goes some of the plywood that is currently providing most of the side to side strength.

During construction, builders will attach diagonal braces, corner to corner on those gable ends. But the usually get removed. A similar approach would be corner braces on each end, 45 degrees about 2 or 3 feet long.

On the interior, smaller corner braces or even angle braces like shelf brackets can stiffen each 90 degree angle. Does this shed rest on a wood base, concrete, or dirt?

Bud
 
  #27  
Old 04-05-13, 08:35 PM
W
Member
Thread Starter
Join Date: Nov 2007
Location: Louisiana
Posts: 57
Received 0 Votes on 0 Posts
Shed rests on a concrete base and is bolted down

Do I need a joist on every rafter?
 
  #28  
Old 04-06-13, 06:09 AM
P
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 24,479
Received 755 Votes on 694 Posts
Yes, install a ceiling joist at every rafter.

Since the walls and roof are on and have possibly moved I would not cut the ceiling joist to fit the space. I would cut them the length they should be and pull the walls in if needed. A come along, large ratchet strap, rope and block & tackle can be used to pull the walls together and hold them in position while you nail in the joists
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: