Can I remove this roof truss? Pics

[Largechris]

I have a detached garage with quite a steep pitched roof, four trusses going lengthways spanning 5 metres. Truss design is like a piggyback with a triangle on top as shown. Proposed new bits in red, so raising up ONE of the four trusses by about 0.5m (to create more clearance in that area of the garage):



The ends of the trusses look like this, so I think they just rest on the top of the walls:
[url]

The wood bits are currently held together with these perforated metal plate things:
|https://thumbsnap.com/hkceiJ2Z[/url]

I've read that I should be able to raise the span by 1/3 of the height with no issues? The current wood all looks like 4"x2".

Ideally I'd like to just bolt in a load more 4"x2" in the positions marked in red then cut out the existing lower span.

Is this reasonable? It's a detached garage, not going to bring the house down if it ends up a bit sketchy. I don't really need a structural engineer for this do I? Cheers

[Largechris]

Grumbly said:

Altering gang nail trusses is a difficult process, as they don't often have much spare in their design and do not work in the same way a traditional cut roof does. These sort of modifications require building regulation approval and often cause issues during the house sale process.

Remedial repairs are often difficult to justify and implement.

The gang nail bits don't look very strong anyway, presumably they are used for speed, so I think that's why I'm suggesting I bolt in new trusses with big coachbolts etc. ?

[Largechris]

smokey mow said:

Largechris said:

I've read that I should be able to raise the span by 1/3 of the height with no issues? The current wood all looks like 4"x2".

That guidance is for a cut roof and not trusses.

For a 5m span on a cut roof the rafters would need to be a minimum of 50x175 so yours are woefully undersized.

As already said, you need a structural engineer

So if i put a 50x175 rafter in the new position (it would only be a 4.5m span higher up) I could build off that? Actually I could build a new custom truss in that position coachbolted together then remove the original?

I can do the structural calculations myself I'm a qualified engineer. Just looks like this is the quick way to move one rafter a tiny bit upwards?

[Largechris]

Ok well I had a go anyway, minor snafu:


To be serious, I'm pretty smart at figuring out the engineering, I'm quite intuitive with a good feel for how stuff works, I'm just asking if the concept is reasonable? Cheers

[Largechris]

walamai said:

I'm not a structural engineer nor do I have any qualifications in this are. But as a lay person this intuitively seems like a bad idea.

At the moment you have the force from the weight of the roof being born mostly by the three central vertical bearers which are 'in line' with the force (ie; in compression).



But it seems you are proposing that the entire weight of the roof will be born by the outside lower section of the roof. These go from bearing almost none of the load, to bearing all of it. And they are also having to bear the force 'in bending' as Grumbly says.

The force is not in line with the timber, it's vertically down (because of gravity)



Your design doesn't add any reinforcement to the areas which are now bearing all the load.

Thank you for that helpful reply. Yes, I think this was maybe my question, in that as I understand it with a "cut" roof you can happily move the rafter upwards by upto a third of the height, as per this guide:



So the end bits look more like this, coachbolts, gorilla glue etc.:


In other words I'm replacing a truss with a cut roof design in that area?

[Largechris]

Grumbly said:

There is very little bending in the members until the truss is cut. Once cut there are bending and shear loads in all the members, the critical point will be, as you have correctly pointed out, the short section of rafter between the wall plate and the ceiling tie.

This short section of rafter now effectively acts as a cantilever with half of the dead and live load supported by the truss acting as a point load at the tip.

Is there a way to beef up the foot or so of rafter that is now "exposed". Can I sandwich more wood around it to thicken up the bottom bit?

[Largechris]

Toltec said:

A quick search for raised tie roof truss found this picture

https://murdockrooftrusses.com/wp-content/uploads/...



Note the way the lower section of the rafter is expanded with another section of timber.

Ah great minds think alike, yes, that's what I mean, beefing up the lowest bit, should be straightforward?

[Largechris]

Beyond Rational said:

Toltec said:

A quick search for raised tie roof truss found this picture

https://murdockrooftrusses.com/wp-content/uploads/...



Note the way the lower section of the rafter is expanded with another section of timber.

Also note what it's bearing onto...

It's a point load onto a wall?

[Largechris]

Beyond Rational said:

Largechris said:

smokey mow said:

Largechris said:

I've read that I should be able to raise the span by 1/3 of the height with no issues? The current wood all looks like 4"x2".

That guidance is for a cut roof and not trusses.

For a 5m span on a cut roof the rafters would need to be a minimum of 50x175 so yours are woefully undersized.

As already said, you need a structural engineer

So if i put a 50x175 rafter in the new position (it would only be a 4.5m span higher up) I could build off that? Actually I could build a new custom truss in that position coachbolted together then remove the original?

I can do the structural calculations myself I'm a qualified engineer. Just looks like this is the quick way to move one rafter a tiny bit upwards?

Why not start with the calculations?

Because I'm trying to get a feel for whether I'm on the right track in the first place or if there are other approaches, eg replace the whole truss with RSJs, that would be better.

[Largechris]

Grumbly said:

Largechris said:

Is there a way to beef up the foot or so of rafter that is now "exposed". Can I sandwich more wood around it to thicken up the bottom bit?

Yes, look at the picture helpfully posted above, the lower rafter of the truss has extra depth added, This would be difficult in situ so will need to bolt additional timber to the side.

So I'm thinking copying that to beef that rafter up all the way to the joint with the web, about 6 feet or so. If it is 4x2 also then I should be able to clamp two more pieces of 4x2, one on each side, access looks good for that. Any particular bolt size recommended please?

[Largechris]

RichB said:

Largechris said:

I have a detached garage with quite a steep pitched roof <clip>
The ends of the trusses look like this, so I think they just rest on the top of the walls:
[url]
The wood bits are currently held together with these perforated metal plate things:
|https://thumbsnap.com/hkceiJ2Z[/url]

On a lighter note, have you photographed that through a broken car windscreen or is the lens on your phone so badly cracked that it shows up on your images!

Picture taken off my other phone, broken screen....

[Largechris]

Grumbly said:

Largechris said:

Grumbly said:

Largechris said:

Is there a way to beef up the foot or so of rafter that is now "exposed". Can I sandwich more wood around it to thicken up the bottom bit?

Yes, look at the picture helpfully posted above, the lower rafter of the truss has extra depth added, This would be difficult in situ so will need to bolt additional timber to the side.

So I'm thinking copying that to beef that rafter up all the way to the joint with the web, about 6 feet or so. If it is 4x2 also then I should be able to clamp two more pieces of 4x2, one on each side, access looks good for that. Any particular bolt size recommended please?

M12 bolts with 50mm plate washers and double sided tooth plate connectors between the members. The bolt size is large, not because of the strength of the bolt is needed, but to provide sufficient bearing area of the bolt shaft on the timber.

Great thanks

[Largechris]

RichB said:

On a more serious note; are you wanting to do this to make room for a 4 post lift? Reason I ask is that my garage roof is exactly the same; steep pitch, 5m depth and three bays long. I would love to put a 4 post lift in the centre bay but to get the headroom it would require raising some of the rafter ties/joists in the central area. This thread is of interest.

Yes and no. I know it's a popular request with the four post lifts to do the same thing with the trusses, I'm only looking to move one of four trusses to make more headroom for one of my special interest hobbies.

I think from having seen similar threads, if you want to do all the garage trusses, you'll be advised to take the roof off and start again.

[Largechris]

Grumbly said:

Please bear in mind that this work would be subject to building regulation approval, and whilst problems are unlikely doing the work, it will come to light when you come to sell if you don't get the approval required.

Cheers, does this apply to a detached garage? I'm wondering for such a small job if it wouldn't be easier to restore the original truss if I do ever sell?

[Largechris]

Cold said:

Yes, cheers, he’s an engineer like me, a professional, although scissor trusses will not give me the space at the ends that I’m looking for.

[Largechris]

RichB said:

Cold said:

RichB said:

To help me, is it worth watching or did you post it for entertainment purposes

He seems to have found a reasonable solution - but I do note that calculations were done before the reciprocating saw came out.

Ok, I'll try to watch it...

I did see that video before in fact - if I was being really honest he’s used American high school trigonometry and ended up with a safety factor of 9 which is utterly ridiculous and over the top.

Anyway cheers everyone, Btw I have some good ideas for other types of roof supports, hit me up if you need me to engineer something for you.