The dreaded Truss Question

[UKG2024]

Hello,

Can anyone offer advice with the following?

I have a garage that measures 5.4 m x 5m. The roof is constructed using trusses at 600mm centres. The truss wood is 38mm x 75mm. It's a fink truss.

(the truss height is 1.91m and span 5.4m)

I'd like to raise the ceiling height slightly to accommodate a room within a room music room.

I've read other posts on the forum and it's been suggested that the following will work:

- Creating new rafters (that are at the recommended thickness for the span) 6 x 2s to the existing truss.
- Add new rafter ties WITHIN THE LOWEST 1/3 of the truss height (lowest 0.63m)
- Add new collar ties in the top 1/3rd

I also plan to add noggins (between rafters) to assist with longitudinal forces.

My question is - will this work?

Also is it ok to add the extra weight of this new wood to the structure? Approx. 380 - 400kg.

With thanks,
Gavin

 

[theoldwizard1]

In the US, you would need to have an engineer to approve your design. If not, your insurance company would not cover any losses do to structural failures. You might even be liable for damages AFTER you sell the property.

 

[Firebrick43]

In the US, you would need to have an engineer to approve your design. If not, your insurance company would not cover any losses do to structural failures. You might even be liable for damages AFTER you sell the property.

No you don’t need an engineer in the US unless your AHJ require all structure to be engineered. Rafter tables are in the IRC 802.4.1 code and if followed engineering is not required.

 

[theoldwizard1]

No you don’t need an engineer in the US unless your AHJ require all structure to be engineered. Rafter tables are in the IRC 802.4.1 code and if followed engineering is not required.

He is CHANGING a design that was engineered. AHJ has nothing to do with insurance.

 

[UKG2024]

Thank you.

 

[SBAG]

I’ll snap a few photos of how we did mine here in a bit. No structural engineers involved - just a carpenter with 50 years of experience and his Amish helper. But I do have closed cell spray foam which makes a hell of a difference in terms of rigidity. And I don’t live in a nanny state.

 

[Firebrick43]

He is CHANGING a design that was engineered. AHJ has nothing to do with insurance.

The truss is engineered. No one is modifying or changing the truss. They are replacing it with a rafter and removing it( most of it).

Rafters have already been engineered and the results tabulated in a table aforementioned.

Its immaterial that top chord remains in place from the original truss after the rafter is in place.

If your ford 150 with its ECOPOS turbo self destructs send debris into the engine and it also grenades.

You replace that truck with another brand of truck but you save the tow hitch and ball out of the previous ford when it’s hauled off to the scrap yard.

When you use that hitch in your new truck that doesn’t mean your still driving a ford.

 

[SBAG]

Here’s how we tackled it (over a mezzanine). Yes they are still trusses and not rafters and tie beams. There was no engineer consulted. I have no permits here and the building is a damned box. It’s not like it’s a US style McMansion with 100 roof angles.



You can’t really see it, but the first modified truss is still tied in with diagonal bracing to the gable truss.

Remainder of trusses not over the mezzanine are unmolested.

 

[SBAG]

Here’s how we tackled it (over a mezzanine). Yes they are still trusses and not rafters and tie beams. There was no engineer consulted. I have no permits here and the building is a damned box. It’s not like it’s a US style McMansion with 100 roof angles.



You can’t really see it, but the first modified truss is still tied in with diagonal bracing to the gable truss.

Remainder of trusses not over the mezzanine are unmolested.

These are on 4 ft centers which I just realized are double yours (I can “do” meters and KMs…I can’t yet think in mm), so my input might be moot as rafters and rafter ties probably make more sense for you. But leaving those photos here in case anyone else are digging through threads years from now (if we all aren’t all in our pods).

 

[UKG2024]

They likely will be digging! Thank you for the photos I can see how this worked..

I'm going to go for it.

 

[UKG2024]

Can you remember what size screws or nails you used? Did you use a nail gun? Thanks

 

[cosmokenney]

Here’s how we tackled it (over a mezzanine). Yes they are still trusses and not rafters and tie beams. There was no engineer consulted. I have no permits here and the building is a damned box. It’s not like it’s a US style McMansion with 100 roof angles.

Just curious, why do you have all that bagged insulation if you already sprayed foam?

 

[theoldwizard1]

I’ll snap a few photos of how we did mine here in a bit. No structural engineers involved - just a carpenter with 50 years of experience and his Amish helper.

Experience wins for me also. Not necessarily the INSURANCE company !

 

[nmk_61802]

Here’s how we tackled it (over a mezzanine). Yes they are still trusses and not rafters and tie beams. There was no engineer consulted. I have no permits here and the building is a damned box. It’s not like it’s a US style McMansion with 100 roof angles.



You can’t really see it, but the first modified truss is still tied in with diagonal bracing to the gable truss.

Remainder of trusses not over the mezzanine are unmolested.

Work looks nice, however your gusset plates are no where near the minimum replacement for the steel plates:

https://www.mitek-us.com/wp-content/uploads/uploadedFiles/_RedesignSite/Content/documents/engineering/details/roof-truss-repair-details/ENG-ROOF-Missing%20Plate.pdf

 

[Steve W.]

Just curious, why do you have all that bagged insulation if you already sprayed foam?

I kinda noticed that the walls were looking a little bare.

.

 

[C-S-H]

Yes they are still trusses and not rafters and tie beams. There was no engineer consulted.

They are no longer trusses. And there are dozens of failure modes that need to be checked (by an engineer).

Lets just look at an estimate of the horizontal force in the bottom chord (tie) that has to go into the top chord (rafter) at that tiny gusset connection:

(30' span)(4' bay)(15psf DL + 25psf snow)(0.5 carried in truss action)(0.5 for two vertical reactions at the wall)(12/4 rafter slope)(1/0.666 rafter placement at lower third point) = 5400 pounds

Do you think those gusset plates can take that? Homemade bottom chord/top chord gusset plates are usually colossal and rife with nails. And I am more worried about the bending stresses in the rafter between that gusset and the wall. That is the first thing to check.

 

[SBAG]

Just curious, why do you have all that bagged insulation if you already sprayed foam?

Flash and bat at 3 inches on the roof and two in the walls was already at $22k (though they did put it on a thicker than that). And fire scares the **** out of me (that’s rockwool; basically a continuous fire block in cavaities). And for some level of sound proofing as the rest of the building is a shop and that might be living quarters or active office areas.

 

[SBAG]

They are no longer trusses. And there are dozens of failure modes that need to be checked (by an engineer).

Lets just look at an estimate of the horizontal force in the bottom chord (tie) that has to go into the top chord (rafter) at that tiny gusset connection:

(30' span)(4' bay)(15psf DL + 25psf snow)(0.5 carried in truss action)(0.5 for two vertical reactions at the wall)(12/4 rafter slope)(1/0.666 rafter placement at lower third point) = 5400 pounds

Do you think those gusset plates can take that? Homemade bottom chord/top chord gusset plates are usually colossal and rife with nails. And I am more worried about the bending stresses in the rafter between that gusset and the wall. That is the first thing to check.

It’ll be all right. Half the Midwest would have fallen in by some of you all’s reckoning.

And those are structural screws.

And yes, that “tiny” gusset is significantly larger than the metal version.

The top is decked with 2x6 purlins. And then glued together with the foam.

It was originally specified to handle solar panels but I decided to go with all ground mounts.

 

[The Bean]

I'm an architect, not a structural engineer, but I've done my share of truss roofs. I wouldn't have agreed to this modification without engineer consultation. If nothing else, I would have replaced the strongbacks in the modified area.

 

[kwb]

Those are no longer trusses. True trusses are comprised of triangles and members are either in tension or compression. Moving the bottom up and creating a trapezoid completely changes how forces are transmitted through the structure.

It may end up working but I sure as hell wouldn't be advising others that this will work for someone else on the internet.

 

[The Bean]

I asked my structural engineer to comment. It wasn't an enthusiastic response. I won't bore you with the details but the phrase "high probability of collapse in a blizzard" was included. His comments were in line with other concerns stated above.

 

[cosmokenney]

Flash and bat at 3 inches on the roof and two in the walls was already at $22k (though they did put it on a thicker than that). And fire scares the **** out of me (that’s rockwool; basically a continuous fire block in cavaities). And for some level of sound proofing as the rest of the building is a shop and that might be living quarters or active office areas.

The spray insulation is flammable? Man I was thinking of putting it in my garage, but that's a no go for me!

 

[TurnipTruck]

The spray insulation is flammable? Man I was thinking of putting it in my garage, but that's a no go for me!

Some jurisdictions require intumescent paint over urethane foam, but I found that to be even more expensive than the foam itself when I had my Quonset sprayed. Usually drywall is used as a fire block over foam. I used galvanized steel over the foam in a welding area.
I don’t know how high the ignition point of the foam is, but a map gas torch couldn’t do it.

 

[SBAG]

Some jurisdictions require intumescent paint over urethane foam, but I found that to be even more expensive than the foam itself when I had my Quonset sprayed. Usually drywall is used as a fire block over foam. I used galvanized steel over the foam in a welding area.
I don’t know how high the ignition point of the foam is, but a map gas torch couldn’t do it.

Mine is getting self applied intumescent paint on the underside of the roof in the main bay (36x28). Walls there will be finished. I would finish the ceiling (keeping exposed trusses) there but I don’t want to deal with gable. Anyhow, the product was around $900 which is enough for two coats. The kind I bought suggests a finish coat with standard latex so it will have BM Command over that.

My build experienced scope creep. If I had to do it over, it would have gotten scissor trusses (and made the building 20 foot longer).

 

[SBAG]

The spray insulation is flammable? Man I was thinking of putting it in my garage, but that's a no go for me!

Straight foam is flammable. Most have fire inhibitors these days…but just a few inches of foam in an empty six inch wall cavity is another name for “chimney.”