I-Joist Rafter birdmouth detail

[WannabeBob]

Hi all,

Our architect have specified a couple of roofs using I-joist rafters, and our SE have the following details on how this connects to the wall plate.
I've a bit of concern about the bird mouth, as the diagrams seem to indicate the bottom flange terminating before the plate, and so isn't bearing any load.

At least a couple of the I-joist MIs I've seen specifically say that the bottom flage/cord should not be cut short, for example:
https://www.jamesjones.co.uk/assets/downloads/subdir/Technical manual 5th edition.pdf


Before I send off an email to the SE, I thought I'd check with the BH experts if my concerns are justified, or the SE details are actually OK and I'm just getting my panties in a twist for no reason.


Our SE's details below:

[JohnMo]

Your drawings state by specialist contractor. So are only indicative.

 

My roof worked like this

 

Approach company to build roof, they used specialist roofing fabricators, specialist company design and provide drawings

Contractor approved or modified as appropriate. Full drawing and calculation package sent to structural engineer for approval. Again any modification made to enable structural approval. This aspect becomes part of the overall structural certification.

 

Same will be true for you pozi joists.

[WannabeBob]

Thanks @JohnMo

So building regs wont have an issue with this?

 

And once we get building regs approval, we’d approach a carpenter and joist manufacturer to liaise with our SE to get this all to work?

[JohnMo]

I just approached the joiner (carpenters make furniture), they did all the other leg work, I just made sure it was done and reviewed everything so I was also happy.

[nod]

There won’t be. An issue if it’s in your roof design I spent a full day doing the birds mouth on nearly all our trusses 

[Russell griffiths]

What’s the steel joist for in d35 if it’s just to support those rafter ends and it’s going to be visable and need boxing in, then that’s a terrible detail. 
 

my I joists fitted to a ledger board fitted to my icf blocks. 

[saveasteading]

8 hours ago, WannabeBob said:

building regs wont have an issue with this?

They should ask for the connection details.

 You need it anyway.  It doesn't work as drawn, and saying on the drawing that it doesn't work as shown is a cop-out. 

[WannabeBob]

38 minutes ago, nod said:

There won’t be. An issue if it’s in your roof design I spent a full day doing the birds mouth on nearly all our trusses 

I’m might end up doing much of it myself too. I reckon there’s going to be almost 100 birdsmouth to cut. 

 

9 minutes ago, Russell griffiths said:

What’s the steel joist for in d35 if it’s just to support those rafter ends and it’s going to be visable and need boxing in, then that’s a terrible detail. 
 

my I joists fitted to a ledger board fitted to my icf blocks. 

Yes it’s pretty much just to support the rafters. The alternative was to rip the ICF off to fix to concrete and we felt like that had more chances of creating a thermal bridge if not done correctly.

 

It’ll be hidden above a false ceiling, so wont be visible once completed. 

[WannabeBob]

11 minutes ago, saveasteading said:

They should ask for the connection details.

 You need it anyway.  It doesn't work as drawn, and saying on the drawing that it doesn't work as shown is a cop-out. 

yes i think i’ll get them to fix that detail even if it does need to eventually go through the contractor. 

[JohnMo]

I did posi joists, similar detail to I joists, but no bell mouths, way simpler

 

[WannabeBob]

That could be a good way to do it too. What connector did you use to fix the rafter to the plate @JohnMo?

[Russell griffiths]

Not a single birds mouth in my roof. 
all the wall plates were cut to the pitch angle of the roof. 
 

if you have a false ceiling, is it at actual ceiling height, or higher up. 
if at ceiling height then can you not use a standard truss. 
 

do you have a cross section drawing of the whole thing. 

[Russell griffiths]

With this photo of yours, why is there an independent I joists as a rafter, then a steel web posi joist as a ceiling/ floor joist when both jobs could be done with a fabricated truss. 

[Chanmenie]

I didn’t have bird mouths, I had the trusses cut to sit on top of the wall plate, which fixed with bolts imbedded into the concrete, the wall plates were flush with the top of the ICF block, I then fitted the rafter overhangs after.

[WannabeBob]

49 minutes ago, Russell griffiths said:

Not a single birds mouth in my roof. 
all the wall plates were cut to the pitch angle of the roof. 
 

if you have a false ceiling, is it at actual ceiling height, or higher up. 
if at ceiling height then can you not use a standard truss. 
 

do you have a cross section drawing of the whole thing. 

It’s actual height. D35 is for the lean to roof on the right. The insulation is at the rafter as there will be mvhr and pipes run above the false ceiling. Wouldn’t a truss still need to be supported where it meets the wall?

 

18 minutes ago, Russell griffiths said:

With this photo of yours, why is there an independent I joists as a rafter, then a steel web posi joist as a ceiling/ floor joist when both jobs could be done with a fabricated truss.

The main roof spans a rectangle about 11 by 12m, and has rooms in it. I didn’t think a trussed roof would span that distance especially with maximising the room above?

[WannabeBob]

13 minutes ago, Chanmenie said:

I didn’t have bird mouths, I had the trusses cut to sit on top of the wall plate, which fixed with bolts imbedded into the concrete, the wall plates were flush with the top of the ICF block, I then fitted the rafter overhangs after.

You mean something like this? (Sorry about my terrible drawing)

[Chanmenie]

Yes exactly like that 

Before the concrete was poured I had the wall plates sitting on the scaffold with m12 rods pre drilled through the timber and made a float 47mm tall so I could set the height of the wet concrete, then just pushed the m12 rods with nuts into the concrete.

After the trusses were fitted and nailed to the wall plate I cut away a little of the inner eps to fit the truss clips

 

[WannabeBob]

That looks like a good way to halve the number of cuts @Chanmenie; I think we might use this thanks!

[IanR]

On 01/02/2025 at 21:30, WannabeBob said:

You mean something like this? (Sorry about my terrible drawing)

 

 

Mine was like this, but with packers between the flanges where the load is taken.

 

Design:

 

Cuts:

 

Installed with packers:

 

[WannabeBob]

Thanks @IanR, that web stiffeners do seem consistent with the MI.

I don't see any truss clips in your images - how have you fixed them to the top plate? Just nails?

[IanR]

1 hour ago, WannabeBob said:

I don't see any truss clips in your images - how have you fixed them to the top plate? Just nails?

 

That's right, no clips or straps, just nailed.

[Chanmenie]

3 hours ago, WannabeBob said:

Thanks @IanR, that web stiffeners do seem consistent with the MI.

I don't see any truss clips in your images - how have you fixed them to the top plate? Just nails?

I put truss clips on as building control wanted them