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Add Truss Elements to Existing Common Rafter/Ridge Board Roof?

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Forum topic by Underdog posted 04-19-2019 04:56 PM 678 views 0 times favorited 11 replies Add to Favorites Watch
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Underdog

1303 posts in 2397 days


04-19-2019 04:56 PM

I’m thinking I need to shore up my slightly sagging shop roof, and hoping you guys can help me figure out what to do.
Here’s the deal. I have a 16’ x 24’ framed shop on piers. (I didn’t place the recommended amount of piers either, but that’s another question.) The roof is laid out w/ 2×6 common rafters on 16” centers w/two 12’ 2×6 ridge boards spliced in the center.
The roof has started to sag just a bit, and perhaps the walls are bowing out. I haven’t measured either the sag or bow, but I’m guessing it’s an inch or so…
I’m thinking I can pull the walls together w/ some eyebolts in the top plate and a come-along. Then I would jack the ridge up to level, or just slightly above level before adding some kind of scissor type truss structure.
I don’t want to add traditional truss elements, because the top of the plates are only at 89” and that would cut head space a great deal.
So… 1×4 or 2×4 truss structure added to the rafters? Scissor type? Or just tear off the roof and start over?

Whaddya think?

Need pix? I wanna wait out the rain first… It’s coming down pretty hard right about now.

EDIT

I just measured the sag in the ridge board at around 1.5 to 1.75 inches- it’s hard to see at the bend in the tape when you’re standing on the floor. The ridge boards has only got the one splice in it.

Roof pitch is 5” rise in 12” run.

The rafters don’t have any splices.

Top of the plate is at 90”, the bottom of the ridge board is between 129-3/4 (center) and 131-1/2” (at the ends).

-- Jim, Georgia, USA


11 replies so far

View cjfarmer's profile

cjfarmer

80 posts in 74 days


#1 posted 04-19-2019 05:32 PM

Yes add stiffening elements . the 2×6 rafters are over span. Triangulate the truss structure its the strongest roof u can build. A come along will pull your walls back in ive done it before on a building someone had removed roof and joists. Dumb mistake. But u will need some type of a permanent tie acroos the span to prevent future sagging. How long are the splices on your joists? Are they all in line or staggered? If they sag u will need to fugure out how to stiffen them and u will add weight to the existing structure with every board and bunch of nails. And the best way i know to add piers is pour concrete in buckets as tall as your piers. Oil the inside of the buckets and the new piers slide out when cured. You can roll them under the shop if u have room. Ive dragged square piers under pier and beam houses and rolling beats dragging any time. If theres no room to roll piers im sorry u are in the pier dragging business. You got nothing without a solid foundation. Good luck

-- Who is we and where is here? - bullwinkle

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Carlos510

266 posts in 733 days


#2 posted 04-19-2019 05:44 PM

Scissor additions would work unless you are very tall or require the clearance for the type of work you do. Definitely use 2 X 4 to much stress for 1 X 4 and I like to overbuild. Heres a rough sketch The dashed cross piece will greatly strengthen the assembly and makes the construction simple all flat surface joints, gusset the dashed center piece to the rafters. Your walls are 7’5” so depending on the slope you should be well over 8’ down the center.

-- "If time is money, then I need a loan" , http://www.hobbyworkshopprojects.blogspot.com/

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Carlos510

266 posts in 733 days


#3 posted 04-19-2019 09:35 PM

Just read your edit, nice to have actual numbers, if you decide to convert to that scissor truss there’s no reason you can’t maintain at least 9 1/2’ center height.

-- "If time is money, then I need a loan" , http://www.hobbyworkshopprojects.blogspot.com/

View Underdog's profile

Underdog

1303 posts in 2397 days


#4 posted 04-19-2019 10:23 PM


How long are the splices on your joists? Are they all in line or staggered? If they sag u will need to fugure out how to stiffen them and u will add weight to the existing structure with every board and bunch of nails.

The floor joists, like the rafters, have no splices either. They’re the full 16’ long on 16” centers. Problem is, that I’ve only put 4 rows of 6 piers, instead of 5 rows of 7 piers. Now the 4×4s that the floor framing sits on are sagging between piers. Shoulda, Coulda, Woulda. Now I’m faced with putting some more in…. which makes your advice below so very welcome!

And the best way i know to add piers is pour concrete in buckets as tall as your piers. Oil the inside of the buckets and the new piers slide out when cured. You can roll them under the shop if u have room. Ive dragged square piers under pier and beam houses and rolling beats dragging any time. If theres no room to roll piers im sorry u are in the pier dragging business. You got nothing without a solid foundation. Good luck

- cjfarmer

That advice is great. I’ll definitely be calculating how many bags it will take to fill up a 5 gallon bucket!

Thanks!

-- Jim, Georgia, USA

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Underdog

1303 posts in 2397 days


#5 posted 04-19-2019 10:28 PM



Scissor additions would work unless you are very tall or require the clearance for the type of work you do. Definitely use 2 X 4 to much stress for 1 X 4 and I like to overbuild. Heres a rough sketch The dashed cross piece will greatly strengthen the assembly and makes the construction simple all flat surface joints, gusset the dashed center piece to the rafters. Your walls are 7 5” so depending on the slope you should be well over 8 down the center.

- Carlos510

So how do the 2×4 scissor parts join in the middle? Or are they on either side of the rafter with the dashed line member in between?

-- Jim, Georgia, USA

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Underdog

1303 posts in 2397 days


#6 posted 04-19-2019 11:15 PM

So I saw this video, and this is another sort of scissor truss for a small building:

I’ve drawn approximately it to scale here:

What do you think?
I think it looks harder to do than your sketch…

-- Jim, Georgia, USA

View Carlos510's profile (online now)

Carlos510

266 posts in 733 days


#7 posted 04-19-2019 11:25 PM

That is a faulty scissor design and completely defeats the intent of the scissor concept. The scissor arms must attach from the wall plate to the opposite side rafter thats where the resistance to crushing comes from. Any significant weight on your design and the scissors will open up where they meet at the peak completely defeating there purpose.

As for the scissors they join in the middle with the cross piece between them. The easy way to go is a bolt with fender washers through all three members. Check out the small detail in the center of the drawing with the arrows pointing to where they go. Who ever gave you that design does not know what the purpose of a scissor truss is.

-- "If time is money, then I need a loan" , http://www.hobbyworkshopprojects.blogspot.com/

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Carlos510

266 posts in 733 days


#8 posted 04-20-2019 12:43 AM

You really don’t think I was going to spent an hour drawing a detailed scaled design, that was going to be shot down by a design that completely defeats the whole purpose of the exercise do you? lol. No need to answer I’m joking now, lol.

-- "If time is money, then I need a loan" , http://www.hobbyworkshopprojects.blogspot.com/

View therealSteveN's profile

therealSteveN

2602 posts in 935 days


#9 posted 04-20-2019 03:23 AM

Do the piers at ground level first, they are why your roof looks like it is sagging, unless you are storing great weight from/on them. Now if you have snow, get the top shored up too. But the first fix is floor, it supports the entire works. Kinda like spend money on good shoes…..

Brings a lot of truth to “build it right the first time”

-- Think safe, be safe

View Underdog's profile

Underdog

1303 posts in 2397 days


#10 posted 04-20-2019 01:06 PM

Yep. I’m thinkin’ I probably have the cart before the horse too. I’ll have to sketch out the pier arrangement and get some advice on that too. The good thing is that the floor joists are 16’ foot long so there’s no splices. I’ll just have to figure out how to get more support under there.

On the roof project, it looks like I’ll need 12’ 2×4s. And looks like I’ll need at least 17 pairs (instead of 19 because I probably don’t need scissor truss on the end rafters since they’re framed in?).
So thirty four 2×4 x 12’ @~ $6 each is about $204. Lowes has these about $5.68 and Home Depot has some at $5.93 (spruce/pine/fir).

Sound about right? Or should I plan on putting more bracing on the end rafters also?

More on the piers later. That’ll certainly have a bearing on the rest of the project.

-- Jim, Georgia, USA

View Pjonesy's profile

Pjonesy

297 posts in 1187 days


#11 posted 04-21-2019 04:09 AM

I am thinking that if your walls have bowed out and the ridge board has sagged it can be rectified quite easily with a barrup bar. If however the external walls have sagged you need to fix that first then proceed to take the sag out of the roof.
If the ends of your ridge board are supported with good solid posts you can use the barrup bar method. This bar is needs to be at least 16mm steel rod threaded at each end and securely attached to substantial brackets at each end of the ridge board. In the middle of the span you will need to attach 1 or 2 50×50mm steel posts attached to the ridge board maybe 1 to 2 metres apart. These posts need to be long enough to bow the 16mm steel rod down at lest 70mm more than your sag. The 16mm rod should be installed with nuts and large washers, then by tightening the nuts up it will pull the sag out of your roof and if the rafters are attached correctly to the top of the side walls it should pull them in straight as well.
I am not sure if barrup is the correct terminology for this method but it was shown to me many years ago and I have used it successfully in building career.

-- Don't tell me it can't be done.

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