Modifying roof trusses to increase height for a two-post lift

[Guineafowl]

Here is the area. If you start counting at the back truss, above the doorway, I need to remove all members of trusses 2 and 3 (ideally 4 as well, but not essential), apart from the top rafters.

The roof is box profile with ply underneath. Approximate position of the trusses arrowed:


Looking longways on the scale drawing, there’s plenty of potential room to lift my Landy. It doesn’t need to go this high (one benefit of being a short*rse). The 1800mm dimension is to the bottom of the chassis rails. 1600mm would be plenty. Angled lines show the bottom edge of the rafters. Dotted box is the outline of the car lift:

Side on, you can see that truss 4 might be a problem if the bonnet is open, although you can remove the Landy one. Line of required lift shown:

So, advice, please, on gutting trusses 2 and 3, at least, and maybe modifying no. 4 less drastically. Some points:

- The workshop is well sheltered by trees, roof pitch is decent and snow loading light here in the Moray microclimate.
- I’m half thinking the posts of the lift itself could be used structurally.
- Otherwise, stout plywood gussets at the apex, as big as I can make them, maybe some brackets to the walls?

 

[Guineafowl]

One more detail. Here is the Landy lifted up into the truss, showing not all of it has to be removed. Orange highlighter shows minimum theoretical removal, but more clearance would be needed for wing mirrors, opening doors, etc.:

 

[Agroshield]

Starter for one point:

The bottom chord of the truss is a tie, in tension. Whatever restraining ability you remove by cutting it, you need to replace in an alternative manner

 

[frank horton]

before get that far are u shore the floor is strong enough....
wouldn't be happy chopping the roof up to fit the ramp....I'd redo the roof.....sorry....

 

[Pigeon_Droppings2]

Personally I'd get a structural engineer to do some calcs...might be cheaper than diy over engineering.

I would have thought you could insert a steel or wood frame roughly the (floor) profile of the lift and then tie all the existing stuff to it
...basically redirect the existing forces around the area you need to open.

I'm not a structural engineer so this is purely brain storming the problem!

 

[Guineafowl]

Starter for one point:

The bottom chord of the truss is a tie, in tension. Whatever restraining ability you remove by cutting it, you need to replace in an alternative manner

Ok, the left-hand wall (top pic) is against the room that comes out at 90deg, which is lined on the inside with OSB, including ceiling, so I can’t see that spreading. Right-hand wall is to the outside, so might need bracing somehow. Big apex gusset should help, too?

before get that far are u shore the floor is strong enough....

Yes, I’ll drill a test hole. If not, I can chop out a big section of floor and pour a decent thick beam for it to sit on.

 

[tom2207]

lower the floor

 

[Guineafowl]

lower the floor

By 1.5m?

 

[roofman]

Those horizontal beams are stopping spread load on the walls.

 

[tom2207]

By 1.5m?

install a pit and a set of jacks ...

 

[BrokenBiker]

Just asked a roofer on site about this...he asked how much you like getting crushed by falling walls and debris?

 

[BrokenBiker]

install a pit and a set of jacks ...

I'd take a pit over cutting trusses out any day

 

[chunkytfg]

Considering the outer walls are just wooden stud walls I would guess the only way to do it would be to rebuild it completely.

You could probably do some clever trickery with it where you use extra trusses either side of the gap for the Lift. Then run stronger roof joists from a ridge beam down to the wall plate all while beefing up the walls to take the spreading load n them.

I all honesty though I cants see it being much cheaper to do than to rip it all off and start again.

Speak to a structural engineer though as quite frankly you're asking for trouble trying to figure it out yourself.

 

[MoreWellie]

not an informed comment, just another idea for the pot
lift the roof and increase the wall height by 1.5m
I expect that would have planning implications though

 

[dan treg]

is the roof covering just metal sheets . if so you could build it like a portal frame ie 2 roof trusses tied together then purlins every 1m spaning the gap you need so the purlins are horizontal like on ann industrial building if that makes sense

 

[Agroshield]

...so I can’t see that spreading. Right-hand wall is to the outside, so might need bracing somehow. Big apex gusset should help, too?

It depends on how firmly the roof is fixed to the walls. The walls might not spread, but the roof could do so.

The truss is like a lever - a pair of nutcrackers is a good analogy. A small force with your hand (the current tie) translates into a huge force on the nut (the new gusset).

Fabricated trusses like those, with the pressed in perforated metal joints are carefully engineered to have the minimum amount of material in them to do the job. They are not like a traditional carpenter-built roof which has a bit more redundancy built in.

The two diagonals which are to be cut support the midpoint of the rafter. With them gone, the span doubles, which means the deflection increases by a factor of 16 (see https://uploads-cdn.omnicalculator....simply_supported_beam_deflection_formulas.png, third from top).

I am sure what you want can be done, but it does need some professional input. You could help by indicating to the engineer whether you prefer a structural steel solution (Z-purlins as suggested above are a good idea) or a timber solution (glu-lam purlins might also work).

To me, you need new super duty trusses at positions 1 1/2 and 4 3/4 with everything in the ramp area bearing on them. And that might need the wall beefing up locally at the truss supports.

 

[gaz1]

it is not possible with the framing that you have unless you remove the framing itself and rebuilt brand new framing to make the roof higher

closest your going to get is a scissor frame and you wont get full height in doing so

 

[Guineafowl]

95% of me is taking all the advice and warnings on board.
5% of me is thinking: jeez, lads, it’s only a little tin-roof shed.

Can I be forgiven for that ^?

Two problems I see from what’s been said:
1. Wall spreading. Left-hand wall is ok. Right-hand one can be braced from outside and with diagonals inside. Could even involve the lift posts.

2. Rafter bowing: purlins from the intact trusses. Again, could even use the lift posts as extra support.

Still thinking. Thanks for the replies.

 

[BrokenBiker]

95% of me is taking all the advice and warnings on board.
5% of me is thinking: jeez, lads, it’s only a little tin-roof shed.

Can I be forgiven for that ^?

begining of the year i did a steel structure on a building that had caught fire, a lot of the steel had been ripped out and was being replaced with new and upgraded...part of the building was still standing and deemed safe structurally but they wanted a new roof to match new part of the building...they took out 3/4 of the roof sheets and purlins then had to very quickly get me to slew round, extend the jib to full stick and hold the steel in place after it started moving in the wind...and it stil had a fair bit of structure left...it is surprising how quicly a roof becomes a hazard when vital parts of the structure are removed...to look at it, you would think it was still plenty strong enough but you could push the rafters about by hand and 3 or 4 would move where they were still connected by a few purlins here and there...

 

[roblane65]

95% of me is taking all the advice and warnings on board.
5% of me is thinking: jeez, lads, it’s only a little tin-roof shed

listen to your 95% light gauge trusses should be installed at no more than 600mm centers , far be from me to p**s on your parade but I'd be more concerned that there isn't enough trusses in there to start with never mind entertaining cutting anything out ! every part of a truss forms part of its structural integrity even the gang nails and should not be cut . Just my opinion mind you , did that build ever see a building inspector