Combination truss and cut roof Birds mouth

[aturner]

Hi All,
We've had a roof design incorporating duo trusses and infill rafters. All timbers 147 x 50. Obviously the truss ties will sit on the wall plate but can't see how to put in rafter birds mouth without loosing alignment between rafter level. Any help would be greatly appreciated.
Kind regards

 

[noseall]

Once your trusses are seated, draw a vertical pencil line up from the edge of the wall plate onto the truss and measure how much truss is showing.

mimic this onto the loose fill rafter and use a bevel to determine the angle.

The actual positioning of the b/m cut is determined by measuring once you have set out your rafter spacing.

 

[aturner]

Hi,
Thankyou for replying. Maybe I'm reading more into this than I need to.
If the truss tie sits on the wall plate with the join right at the end of the wall plate, then the truss timber would sit higher than any rafter which was cut in with a birds mouth. The only way I can see this working is not to have a birds mouth on the rafter?
Regards

 

[theoldun]

For a standard duo pitch trussed roof, loose in fills are only needed for three reasons. Either velux roof lights or chimney stack over 600mm wide or dormer window.
There are three or four ways to deal with your problem and for us to know the true situation answer the following.
1 How many loose in fill rafters have you and what is there purpose?
2. Do the openings where loose in fills are going have double truss each side?
3 You state top chord is 46x147. Is bottom chord (ceiling joist), same size?
4 Overall span of plates.?
5 Pitch of roof?
6 Have they supplied in fill bottom chords?
Regards oldun

 

[aturner]

Hi oldun,
Many thanks for your input. I'll try and explain it in the form of your reply
1 & 2. There are 6 infill rafters. two before a duo, two in between duos and two after.
The purpose is to faciltate a vaulted ceiling on a single storey kitchen extension. The two duos are to be clad in oak as a feature.
3.Not sure of the chord term but the tie beam is 147 x 47.
4. Overall span wall plate to wall plate ends is 4000mm.
5. Pitch of roof is 22.5.
6.No fill in bottom chords.
Hope this helps.
Kind regards

 

[noseall]

Hi,
Thankyou for replying. Maybe I'm reading more into this than I need to.
If the truss tie sits on the wall plate with the join right at the end of the wall plate, then the truss timber would sit higher than any rafter which was cut in with a birds mouth. The only way I can see this working is not to have a birds mouth on the rafter?
Regards

I'm not sure about the design of your roof but the last raised tie roof truss system we installed involved cutting a b/m into the main trusses.

Is this the case with your roof?

 

[aturner]

Okay,
I can see that it could work that way, but only if the distance between the joist/rafter joins was short. Thought the whole idea of a truss was that the joist sits on the wall plate. Also thought that trusses were not meant to be cut in any way, else it would void any claims for failure. Seems like combination roofs are not an exact science.
Regards

 

[noseall]

With raised tie trusses it is the sloping top chord as opposed to the horizontal bottom chord that sits on the wall plate.

This design means that the truss manufacturer allows for a birds mouth cut in the top chord so that the truss is 'seated' on the plate.
The bottom image shows 'glide shoe' tuss clips being used. Can't stand 'em!

 

[aturner]

Hi noseall,
Thanks for coming in on this. The design is not to have raised tie trusses. The ties are standard and will be clad in oak veneer as a feature. The ties will sit on the wall plate as standard. It is the infill rafters which I need to sort out. With a B/M in rafters they are going to sit lower than the trusses. Someone must have had this to sort out at sometime.
Regards

 

[noseall]

Yes we have.

Use a truss clip and no b/m.

 

[aturner]

Yes we have.

Use a truss clip and no b/m.

Thank you noseall.

 

[theoldun]

With due respect your post is confusing. For a 4m span you will be using a standard fink truss with the bearing point sitting on the plate and fixed in the standard fashion.
If you wish to throw in some loose in fill rafters to form part vaulted ceiling and the in fill rafters are the same size as the top chord of the trusses, then if your plate is 50x100 then plant another 50x100 infill plate on top of existing plate and between the trusses, then you can birds mouth the in fill rafters and roof will all be on same plane.
Bear in mind, you will have to throw a ridge board in between the trusses to collect the top plumb cut of your in fills. You will also have to wind brace between trusses and someway or another you will have to fit 5 number longitudial braces across the run of roof. Suggest you ask the clever ar*se who designed the roof.
Regards oldun

 

[aturner]

Thanks oldun,
Never thought about adding height by doubling up the wall plate on the infills. This seems like the best solution. Many thanks for your input.
The design BTW was done by a rather large truss company!!
Regards

 

[theoldun]

Thanks oldun,
The design BTW was done by a rather large truss company!!

Sounds like the company based in Ethiopia. If it is them, they deliver up the Suez canal via the Red sea.

 

[noseall]

plant another 50x100 infill plate on top of existing plate and between the trusses, then you can birds mouth the in fill rafters and roof will all be on same plane.

Fine until it comes time to fix the loose ceiling joists.