Gluelam sizing?

[Charles Freeborn]

Greetings all.
My 18' x 18' garage has developed quite a sag in the roof over the (many) years of its existence. The rafters are 2 x 4 on 2' centers, joists are every other rafter with diagonal braces to the rafters. It's all clear span below, which I must keep (no posts allowed) I'd like to put a gluelam parallel to the ridge to boost everything back up into place. I'll add necessary additional diagonals as needed. There is no live load above - it's a single story garage which is currently open through to the inside of the rafters. When this is done I'll probably lay on top of the rafters, or ledger and fit between the joists rigid foam board to stop all the heat from escaping up to the peak whilst I freeze below.
My question is what size beam should I use? My SOTP (seat of the pants) calc is at least a 4 x 8 but I suppose a 4 x 10 wouldn't be overkill either. It doesn't need to be necessarily pretty so I'm not opposed to making my own out of 3 layers of 2 x 8's with staggered scarf joints and glue / nails. Again, doesn't need to be pretty and currently there is nothing so anything will be an improvement.
TIA!

 

[loganb]

Pictures would help....I'm pretty sure I can visualize what you're planning but some pictures would help clear up any confusion

How are you intending to support each end of the proposed beam?

If gluing up you're own isn't out of the question, look up the concept of a "flitch beam" where you'd laminate a steel plate between wood beams to add additional resistance to bending. In this scenario where you can easily make the beam taller, a flitch plate is likely overkill and adding significant complexity....but it's an option

Do you have the easy ability to snake this proposed beam up and into the rafters or will you have to cut an access hole in an end wall to slide it in?

 

[Charles Freeborn]

Very sorry - should have been more clear. Here's a quick rendering of framing and what I'm asking. The beam in red is the one in question. It would be supported by additional studs set into end walls. One of the studs will fall on top of the roll up door header, which will need reinforcement. Many thanks.

 

[Firstram]

Pull a string at the top of the side walls, and pull them back in to straight while you jack the ridge up.

 

[Rusted Nut]

4x8’s/4x10’s are not glulams, they are dimensional lumber; and neither will span 18’. A glulam supplier will probably be able size one for you. I’m guessing you’ll need something like a 5 1/8” x 18 glulam.

 

[billconner]

I don't know what is sagging. Are the individual rafters bowed down? The rafter ties sagging? Or the walls bowing out? This proposed beam seems only useful to take sag out of rafter ties.

 

[carlaisle]

Are you attempting to correct a sag in the roof that is visible from the exterior or the sag in the 2x4 18' long joists that essentially constitute the "ceiling" structure of your garage?

 

[Hank11]

You could put the beam up above the ceiling joists. Plenty of room that way - if you can get it in there. Probably easier to jack up the joists and sister new ones to each old one.

 

[Charles Freeborn]

Are you attempting to correct a sag in the roof that is visible from the exterior or the sag in the 2x4 18' long joists that essentially constitute the "ceiling" structure of your garage?

Primarily sagging rafters/ roof. The ceiling joists are also sagging on account of the bracing to the rafters. By eyeball I'd say the ridge is sagging 6-8" mid point of the ridge.
My thinking is that if I put the beam under the ceiling joists and raise it up into place (I have a walkie stacker that can lift 2500 lbs), tuck the new support studs under the ends of the beam and lift the joists and roof / rafters all in one go.

 

[PCustoms]

@Charles Freeborn do you have pictures of the outside of the building, showing the roof sag as well as the side walls?

How about some pics of the "rafters" as well?

 

[mike93lx]

I don't think a 2500lb lift stacker is going to work. Straightening significant sagging beams and joists is not something you do all at once as the wood doesn't flex easily. It takes time

 

[TurnipTruck]

You will also need an equally significant beam above the rollup door, especially if it is a wide door.

 

[CraigStu]

If you can get the required beam engineered, this is something that you would want adjustable supports under. That way you could crank in maybe 1/2 turn on the adjuster every 2-3 weeks. But I am not convinced this will actually straighten the roof ridge. Also you would probably need to reinforce where the supports would sit. The slab was designed to support weight all around it's perimeter and wasn't expected to support a lot of weight at 2 points.

 

[ToolsRCool]

I am currently doing some rough estimating of beam sizing and costs as well for a complete tear down and rebuild. This website may be useful to you, has been to me. Some easy online calculation software. It does not account for everything, but may get you close. https://www.weyerhaeuser.com/woodproducts/engineered-lumber/resources/sizing-table-lookup/

You can possibly use a steel I beam as well, which will typically be of much less thickness, if that matters to you.

 

[carlaisle]

If it has survived long enough to sag that much and is not demonstrating any signs of failure/rot/decay, it should easily survive the next 70 years. Don't underestimate the strength of that old growth timber. If you dislike the appearance and just want to straighten it, getting decades of bow out of any lumber, especially old growth, will be a challenge. It will not respond quickly or willingly. If you want everything straight, plumb, and true, the fastest path to success and best value/$/outcome is to replace the entire roof structure. If you want to invest a large amount of time and effort and minimal funds into an attempt that will likely prove unsatisfactory, some variation of the plan you have proposed may suffice.

 

[Charles Freeborn]

Thanks all. Much to consider. Re-framing / replacing the whole roof structure not going to happen on my watch. The next owner of this property will probably convert to an ADU. They can do as they please then. I'm here for 10 more years, 15 max. I'm not after a perfect fix - mostly want to arrest further sag.
Cheers.

 

[PCustoms]

Thanks all. Much to consider. Re-framing / replacing the whole roof structure not going to happen on my watch. The next owner of this property will probably convert to an ADU. They can do as they please then. I'm here for 10 more years, 15 max. I'm not after a perfect fix - mostly want to arrest further sag.
Cheers.

Your sag might not be a "roof" issue...

 

[Charles Freeborn]

The offending structure..

 

[Charles Freeborn]

It's not terrible, and yes, the rest of the building has various "issues" my thinking is to add a beam parallel to the ridge & below the ceiling joists and also double up some of the joists where the verticals go up to the ridge.

 

[PCustoms]

It's not terrible, and yes, the rest of the building has various "issues" my thinking is to add a beam parallel to the ridge & below the ceiling joists and also double up some of the joists where the verticals go up to the ridge.

I think you walls are spreading.

Any shots of the inside?

What about taking one straight on, looking down the left hand wall?

Looks like you're bowed out at the top:



Overall that's not in bad shape.

 

[mike93lx]

I'd get a buddy and stretch a string line in a bunch of places. Don't touch the roof until the walls at straight and plumb.

 

[Charles Freeborn]

I think you walls are spreading.

Any shots of the inside?

What about taking one straight on, looking down the left hand wall?

Looks like you're bowed out at the top:



Overall that's not in bad shape.

It's had siding put over siding with wonky siding corners so I wouldn't trust the outside appearance too much. Overall not too terrible. The opposite wall (on the right from this perspective) needs foundation work first, then I'll shore / straighten the walls. After that I'll do the joist/rafter/beam reinforcements. Not trying to make a Taj Mahal here - just don't want a "leaner" in my back yard.