Structural Truss Noggins at Ceiling Level

[MortarThePoint]

Like @pocster I'm messing about with Noggins, but mine are structural rather than for the plasterboard so I thought I'd start a different thread.

 

We've got attic trusses and the roof designer said to put noggins in at the base of each side wall and then two noggins spaced in between that. I've circled what this ends up as below and the designer has shown some noggins 172mm aligned with the bottom of the bottom chord (222mm).

 

Is that alignment the best choice (compared to flush with top of bottom chord or in the middle)?

One run of noggins is basically along the support wall near where there look to be clips. Does it make sense to have them so close to anchors?

 

[tonyshouse]

Is that a 100mm load bearing wall ?     
 

I would align with bottom of chord 
 

I can see zero point in having structural noggins above that.  I can see a good reason for them in the mid span point as they will help spread point loads on the floor and spread it a bit to the truss each side. 
 

I would expect diagonal bracing to the underside of the rafters of the trusses, I use to swap these foe X runs of steel band. 

 

is there a water tank on the flat roof? 
 

I would add 6x2 purlins in the triangular void, either top or bottom of the vertical member 

 

 

trusses are designed to minimise everything, we like strong and robust 

 

tonys danger warning — Never drill a truss 

[MortarThePoint]

36 minutes ago, tonyshouse said:

Is that a 100mm load bearing wall ?     
 

I would align with bottom of chord 
 

I can see zero point in having structural noggins above that.  I can see a good reason for them in the mid span point as they will help spread point loads on the floor and spread it a bit to the truss each side. 
 

I would expect diagonal bracing to the underside of the rafters of the trusses, I use to swap these foe X runs of steel band. 

 

is there a water tank on the flat roof? 
 

I would add 6x2 purlins in the triangular void, either top or bottom of the vertical member 

 

 

trusses are designed to minimise everything, we like strong and robust 

 

tonys danger warning — Never drill a truss 

 

Yes the wall is a 100mm load bearing and that helps a lot with the span. Like you I was confused as to why the noggins were above that and not shifted off to split the two spans.

 

There are 4x1 bracing timbers in currently, but they will get in the way of the plasterboard so I expect to replace them with the noggins that are shown circled green here. the lowest ones are where you are suggesting purlins aren't they?

 

The truss designer dotted water tanks all over the place for good measure.

 

The manufacturers have given me a cheat sheet as to where I can drill, but I hope to avoid it as much as possible.

 

Edited February 21, 2021 by MortarThePoint

[Oz07]

Align with bottom. 

[Oz07]

If your not beam filling the blockwork or fixing trusses on that spine wall surely the noggins are doing a job. I'd bring them maybe an inch or so away from the wall can double up as plasterboard perimeter nogs then. 

[MortarThePoint]

The truss designer has incorporated noggins at each of the joining plates (circled green in above post), 150mm which is less than 75% of the top chord 222mm. Is that OK? I asked for the top chords to be increased from 197mm to 222mm and he may have forgotten to revise the noggin timber.

 

In terms of the higher up noggins, all but a couple of gaps have 220mm vertical noggins right at the top (shown here added in blue). I might install a 150mm noggin at the lower position shown here as a plasterboard edge noggin.

 

Edited February 21, 2021 by MortarThePoint

[Hobbiniho]

2 hours ago, MortarThePoint said:

The truss designer dotted water tanks all over the place for good measure.

 

What are these water tanks for? and why the flat roof? i have never seen a partially flat roof attic truss before, you will have difficulty fitting noggins at all the points shown due to the fact that there will be metal plates on all the joins it will be difficult to nail through these and will be difficult to fit the noggings tight as the metal plates sit a few mm proud of the surface, it might be easier to ask if the design can be changed slightly, rather than having noggings it would be easier to fit thick sarking and have 3x2 or 4x2 "brandering" on the inside, this could go at 400 centres and would give a better fixing for plasterboard. 

[MortarThePoint]

8 hours ago, Hobbiniho said:

 

What are these water tanks for? and why the flat roof? i have never seen a partially flat roof attic truss before, you will have difficulty fitting noggins at all the points shown due to the fact that there will be metal plates on all the joins it will be difficult to nail through these and will be difficult to fit the noggings tight as the metal plates sit a few mm proud of the surface, it might be easier to ask if the design can be changed slightly, rather than having noggings it would be easier to fit thick sarking and have 3x2 or 4x2 "brandering" on the inside, this could go at 400 centres and would give a better fixing for plasterboard. 

 

They aren't actually water tanks as we won't have any. They are just a form of additional loading capacity.

 

It's a flat roof at the top because the planners wanted the ridge height reduced. An advantage of a flat roof (aka crown top) is that it divorces the depth of the house from the ridge height. 

 

The 222mm noggins at the top (shown blue in previous post) should meet the requirements of the top two 150mm noggins shouldn't they? That only leaves the noggins at the top of the vertical members (side walls). I'll ask, but I was expecting it would be OK to shift those noggins down off the nail plates which is good for the reason you mention and would provide plasterboard support.

[MortarThePoint]

Just spoke to the truss designer and he said that the noggins at nodes 2, 3, 4, 7, 8, and 9 aren't needed. That's good as the large blue noggin will therefore be more than what's required.

 

I'll still fit  noggins at nodes 2 and 9 as it will provide support for the plasterboard.

[MortarThePoint]

What size of screw do people tend to use for this? I am running in two 5.0x90 and one 5.0x100 per noggin end. Should I be using some 6.0x100 screws?

 

The joists are 222x47 @ 600mm centres and the noggins are 222x47 or 172x47. I'm mostly able to screw though the joist into the noggin (staggered configuration) but there are some double trusses that need skew screws.

[markc]

A noggin is classed as a compression member from a design point of view, its purpose is to prevent the trusses or cords from deflection towards each other. Therefore the fixings are merely to hold it in place.

[MortarThePoint]

21 minutes ago, markc said:

A noggin is classed as a compression member from a design point of view, its purpose is to prevent the trusses or cords from deflection towards each other. Therefore the fixings are merely to hold it in place.

 

That's kind of what I was thinking too as things have gone very wrong when the screws would pull out of shear damage.

 

What about floor stiffening to reduce bounce, that won't just be compressive will it?

[markc]

@MortarThePoint, Again, most floor stiffeners/noggins are there to prevent the joists/beams from rolling over when loaded.

If you look at a pozi type joist (its easier to explain than a solid member) then floor bounce occurs when the top cord compresses and/or the bottom cord stretches allowing the joist to deflect downwards. Solid noggins or Diagonal (top/bottom/top/bottom etc) mid or part span braces have very little effect on deflection.

If you want to reduce floor bounce you have to increase beam depth or increase section on the top and bottom cords

[MortarThePoint]

I have a supporting wall close to midspan (62%) and all the trusses look to be resting on it well except for one. This one is about 1mm above and deflects down onto the wallplate if I hang my weight from it. In the photo I've slipped a piece of galvanised steel between the two. Is this a good idea or should I do something else?

 

[Russell griffiths]

Have you put a straight edge on the trusses, you might find it is bowed up high when you need to pull it down, do that first before you pack it up too high. 

[MortarThePoint]

50 minutes ago, Russell griffiths said:

Have you put a straight edge on the trusses, you might find it is bowed up high when you need to pull it down, do that first before you pack it up too high. 

 

I've only got a 2m straight edge, but I'll give it a try. 

 

I'm not clear how the remedy would change though as it's the same net effect isn't it. 

[Russell griffiths]

Have you taken the packer out and seen if you can pull the truss down. 

TBH your only talking 2mm it won’t matter if it’s up or down. 

[MortarThePoint]

27 minutes ago, Russell griffiths said:

Have you taken the packer out and seen if you can pull the truss down. 

TBH your only talking 2mm it won’t matter if it’s up or down. 

Without the packer I can pull it down if I hang on it