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Re-doing attic trusses possible?

dad

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dad

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Hey guys, is anyone qualified to give an opinion on a possible modification of one side of my attic to open up some room so I can actually store some shit in there? I'm thinking I could cut the existing trusses as shown only on the right side and then install vertical ones in place. Then also add some smaller ones up top as seen in the top left. I wouldn't touch the left side and I would leave the vertical center pieces in place. Would this cause any issues? I'm not going to do it if it will make it weaker or subject to imbalanced loads that could cause premature cracking in the garage walls or worse.

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Onboard 2.0

Onboard 2.0 9083

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4 minutes ago, dad said:

Hey guys, is anyone qualified to give an opinion on a possible modification of one side of my attic to open up some room so I can actually store some shit in there? I'm thinking I could cut the existing trusses as shown only on the right side and then install vertical ones in place. Then also add some smaller ones up top as seen in the top left. I wouldn't touch the left side and I would leave the vertical center pieces in place. Would this cause any issues? I'm not going to do it if it will make it weaker or subject to imbalanced loads that could cause premature cracking in the garage walls or worse.

image.png.e448b73e53d4021a6bf457b16bafa814.png

You need an engineer. I’ve never seen anyone modify trusses. Not that it can’t be done but….

They’re designed for specific locations, and if you modify load paths you have to account for the new load path down thru your house to the foundation.

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dad

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10 minutes ago, CooterBrown said:

Is there a wall under the center where all the trusses meet? If so, definitely not. That wall would be providing the support and if you move the truss as shown it would be in the center of an unsupported supported joist.

No wall under the center where all the trusses meet. Under this is a two car garage. I would definitely leave the center 2x4's alone too if I did it the way I was thinking. 

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Gil Bang

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1 hour ago, Onboard 2.0 said:

You need an engineer. I’ve never seen anyone modify trusses. Not that it can’t be done but….

They’re designed for specific locations, and if you modify load paths you have to account for the new load path down thru your house to the foundation.

Listen to this man.  Don't fucking do it without guidance from an engineer.   

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dad

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1 hour ago, Jerry Callo said:

Seems like it is an optimal time to get rid of all the shit you want to put in the attic.

You're not kidding. I've done a whole lot of that. I had a garage fire half a year ago just moved back in. I've probably donated several truck load full of shit. Several other truck loads of burnt shit to the dump. Still, there are some things that remain that need to go in the attic b/c my little yard isn't big enough for a proper shed.

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Da Fino

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18 hours ago, dad said:

You're not kidding. I've done a whole lot of that. I had a garage fire half a year ago just moved back in. I've probably donated several truck load full of shit. Several other truck loads of burnt shit to the dump. Still, there are some things that remain that need to go in the attic b/c my little yard isn't big enough for a proper shed.

Just curious, how did the fire start? Was it a free standing garage?

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Horn of Gabriel

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2 hours ago, Bevo said:

Seems like a bad idea compared to building out or buying some ceiling storage units for the garage.

The expense and risk involved in making truss changes that will affect the structural integrity of your home will pale in comparison to just renting a storage unit.  Do that, the storage unit won't cause your house to fall down either.

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Bevo

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21 minutes ago, Horn of Gabriel said:

The expense and risk involved in making truss changes that will affect the structural integrity of your home will pale in comparison to just renting a storage unit.  Do that, the storage unit won't cause your house to fall down either.

True, but the funny thing with that is once people see that it costs $1000/yr. for stuff they never use, they quickly realize that having all that stuff isn’t worth it. I love my garage ceiling storage but my garage is still cluttered with lawnmowers, tile, table, mitre saws and garden stuff and bikes and sporting goods.

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Gil Bang

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2 hours ago, Bevo said:

True, but the funny thing with that is once people see that it costs $1000/yr. for stuff they never use, they quickly realize that having all that stuff isn’t worth it. I love my garage ceiling storage but my garage is still cluttered with lawnmowers, tile, table, mitre saws and garden stuff and bikes and sporting goods.

True story here:  my uncle paid for a storage unit for more than 5 years.  Then, he rented a trailer and hauled it all to the dump.   

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UT_OB1

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On 2/13/2022 at 10:56 AM, dad said:

Hey guys, is anyone qualified to give an opinion on a possible modification of one side of my attic to open up some room so I can actually store some shit in there? I'm thinking I could cut the existing trusses as shown only on the right side and then install vertical ones in place. Then also add some smaller ones up top as seen in the top left. I wouldn't touch the left side and I would leave the vertical center pieces in place. Would this cause any issues? I'm not going to do it if it will make it weaker or subject to imbalanced loads that could cause premature cracking in the garage walls or worse.

image.png.e448b73e53d4021a6bf457b16bafa814.png

 

On 2/13/2022 at 10:58 AM, dad said:

I have the queen web config and I'd kind of want to morph that into a hybrid attic configuration except it would have a center post unlike the design below.

image.thumb.png.706b11187c5d12aa41f81bac91584111.png

I know you’ve already gotten several answers implying this is a bad ideas, but I just wanted to say it again. Unless I’m misinterpreting your drawing, you’re not picking up and distributing the forces the way you need to be. And even if it doesn’t fall down, you’ll be fucked by insurance if anything ever happens, or you’ll be fucked if you try to sell it 

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Bevo

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23 minutes ago, Gil Bang said:

True story here:  my uncle paid for a storage unit for more than 5 years.  Then, he rented a trailer and hauled it all to the dump.   

I bet there are millions like your uncle and millions more who move and schlep that stuff to a new state and put it back in storage.

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Treefidy

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 What kind of roof do you have?  It is possible to cut the trusses but the bottom chords do not appear to be substantial enough for  loading.  Trusses generally use smaller lumber than when stick framing because it’s designed to shift loads in ways yo might not even think about.

the upper chords or rafters want to sag in the middle, so the diagonal webs push against one another where the meet. The bottom chord acts as a tie helping prevent the ridge from flattening out which is what it wants to do. This keeps the ridge standing up where it’s supposed to be and the center vertical support holds the bottom chord up from sagging under the weight of the diagonal web loads and ceiling loads from Sheetrock below.    Your bottom chords look like 2x4 or at most 2x6 but either way they would not be sufficient alone to span an open space without help, much less with your shit stacked on top.   
 

Now, all that said, most everything is engineered with redundancy involved and cutting one web most likely won’t cause the roof to collapse.   I would be more concerned about the bottom chord being too thin to support much additional weight.   

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dad

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15 hours ago, Treefidy said:

 What kind of roof do you have?  It is possible to cut the trusses but the bottom chords do not appear to be substantial enough for  loading.  Trusses generally use smaller lumber than when stick framing because it’s designed to shift loads in ways yo might not even think about.

the upper chords or rafters want to sag in the middle, so the diagonal webs push against one another where the meet. The bottom chord acts as a tie helping prevent the ridge from flattening out which is what it wants to do. This keeps the ridge standing up where it’s supposed to be and the center vertical support holds the bottom chord up from sagging under the weight of the diagonal web loads and ceiling loads from Sheetrock below.    Your bottom chords look like 2x4 or at most 2x6 but either way they would not be sufficient alone to span an open space without help, much less with your shit stacked on top.   
 

Now, all that said, most everything is engineered with redundancy involved and cutting one web most likely won’t cause the roof to collapse.   I would be more concerned about the bottom chord being too thin to support much additional weight.   

Just standard shingled roof. Yes all trusses are made with 2x4's. Most people I talk to are surprised because they all have 2x6's and a much more open design. Makes sense that you can have a more open design since the stiffness of a 6" inch wide board is going to be much higher.

Thanks for explaining the load path for the existing design that makes sense. If I was just replacing the middle trusses like this, wouldn't this combat the roof rafters from sagging in a similar manner? I understand its not geometrically symmetrical BUT that doesn't mean it can't somewhat similarly distribute the loads and not flex too much in any location.

I can solve the systems of equations to obtain the loads at each point but then what? I feel like what one would need to be looking for is max deflections at key points and them having to be smaller than some threshold per some code.

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Onboard 2.0

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54 minutes ago, dad said:

Just standard shingled roof. Yes all trusses are made with 2x4's. Most people I talk to are surprised because they all have 2x6's and a much more open design. Makes sense that you can have a more open design since the stiffness of a 6" inch wide board is going to be much higher.

Thanks for explaining the load path for the existing design that makes sense. If I was just replacing the middle trusses like this, wouldn't this combat the roof rafters from sagging in a similar manner? I understand its not geometrically symmetrical BUT that doesn't mean it can't somewhat similarly distribute the loads and not flex too much in any location.

I can solve the systems of equations to obtain the loads at each point but then what? I feel like what one would need to be looking for is max deflections at key points and them having to be smaller than some threshold per some code.

image.thumb.png.a53aea79b199dfec3ce6d328d30c16b3.png

Yeah, not sure how much capacity you're gonna get with members that close to a corner.

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TexasEd

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On 2/14/2022 at 3:40 PM, Gil Bang said:

True story here:  my uncle paid for a storage unit for more than 5 years.  Then, he rented a trailer and hauled it all to the dump.   

 

On 2/14/2022 at 4:05 PM, Bevo said:

I bet there are millions like your uncle and millions more who move and schlep that stuff to a new state and put it back in storage.

Thing is there are sites now on Facebook, Craigslist, etc. that you can just post a picture and the word "Free" and the Jawas come pick most of it up for you.

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Treefidy

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On 2/17/2022 at 2:47 PM, dad said:

Just standard shingled roof. Yes all trusses are made with 2x4's. Most people I talk to are surprised because they all have 2x6's and a much more open design. Makes sense that you can have a more open design since the stiffness of a 6" inch wide board is going to be much higher.

Thanks for explaining the load path for the existing design that makes sense. If I was just replacing the middle trusses like this, wouldn't this combat the roof rafters from sagging in a similar manner? I understand its not geometrically symmetrical BUT that doesn't mean it can't somewhat similarly distribute the loads and not flex too much in any location.

I can solve the systems of equations to obtain the loads at each point but then what? I feel like what one would need to be looking for is max deflections at key points and them having to be smaller than some threshold per some code.

image.thumb.png.a53aea79b199dfec3ce6d328d30c16b3.png

On the right side you would need a beam running parallel to the diagonal rafters where there is no web, that prevents the rafters from sagging in the middle.  The ends of the beam should be supported by purlin braces which are similar to the diagonal webs in that the angle down and bear on the bottom chord, but only at a location where their load can be directed down to the slab, like a load bearing point in a wall.   If the purlin is above, for example, a header beam below, then the additional force likely means the header beam is now undersized.   Draw a straight line below the purlin and make sure it is directly supported to the ground.

the size of the beam is determined by the span and how many rafters it’s supporting.   2-2x12 with 1/2” plywood will technically span a garage door opening with loading from above, though LvLs are preferable.  As you mentioned the garage is below I would assume the minimum span possible to hit 2 walls below is 20-22, depending on direction, possible more.  That’s going to be LVLs and probably double 14” which is going to cut o your headroom. In the space intended.  

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