Two birdsmouth or One for lean to roof

[BegBorrowSteal]

Hi all just looking at cutting birdmouths and noticed some people cut two, one for wall plate and one for the ledger board is two better or is one sufficient? Cheers

 

[noseall]

Hi all just looking at cutting birdmouths and noticed some people cut two, one for wall plate and one for the ledger board is two better or is one sufficient? Cheers

The top BM is so that the rafter can be fixed. You can achieve the same result without the BM instead by using truss clips etc. We have used both methods depending what mood I'm in - or the fact that some walls are not plum or are in 'twist' meaning that a pair of BM's could be prohibitive.

 

[^woody^]

Rafters need to be seated. A flat seat. Try eating your dinner with your chair at 35° and see what happens.

Even with truss clips, nosey. I am disappointed.

 

[noseall]

Even with truss clips, nosey. I am disappointed.

It's more borne from a horrible experience we had with a badly built wall that we were attaching a lean-to roof against. The rigid nature of two BM's meant that either the wall plate shoved out or the pole plate was shoved up out of level at one end.

I know you can cut individuals but.....

.....besides, there is also the plumb cut abutment that you are not considering Woody, you know the one that usually is butted against another rafter or ridge beam.

 

[ajstoneservices]

Why would you have two........................ever

 

[BegBorrowSteal]

Why would you have two........................ever

Good question surely the force at the top of the rafter is pushing against the ledger or ridge board (depending on the roof type) and the bottom part with birdsmouth is pushing down on the wall so is another birdsmouth really needed at the top.

 

[noseall]

so is another birdsmouth really needed at the top.

No. As long as the plumb cut is sat against the masonry. The truss clips will prevent uplift.

 

[noseall]

Why would you have two........................ever

Purlins etc.
Or a ridge beam...

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[^woody^]

Good question surely the force at the top of the rafter is pushing against the ledger or ridge board (depending on the roof type) and the bottom part with birdsmouth is pushing down on the wall so is another birdsmouth really needed at the top.

Gravity tends to push downwards, and seating rafters has tended to work well for the past few thousand years, but hey you could be on to something with this wild west method of roof framing.

In fact why have any birdmouths, when a couple of nails will hold it surely?

 

[BegBorrowSteal]

Gravity tends to push downwards, and seating rafters has tended to work well for the past few thousand years, but hey you could be on to something with this wild west method of roof framing.

In fact why have any birdmouths, when a couple of nails will hold it surely?

A birdsmouth at the bottom is needed that’s without question it’s the top one I can’t understand surely pushing against the stone/brick wall of the house is better than a strip of wood ie the ledger board also what size timber would you recommend for the ledger board does it need to be the same size as the rafters.

 

[noseall]

In fact why have any birdmouths, when a couple of nails will hold it surely?

You mean like a rafter that is positioned between a valley beam and a ridge beam.

….or a rafter that is secured against solid masonry with a top plumb cut. The pole plate is there to space the rafters and prevent uplift. Get over it Woods.

 

[ajstoneservices]

Nosey, whats the point in fitting a ridge board like that in photo 1.

Why the steel in photo 2. And why were you using eml in that blockwork?

Photo 3 is of those under sized steels again isn't it

photo 5, yeah we do sometimes need to use two birds mouths but not on a lean to roof.

 

[^woody^]

You mean like a rafter that is positioned between a valley beam and a ridge beam.

….or a rafter that is secured against solid masonry with a top plumb cut. The pole plate is there to space the rafters and prevent uplift. Get over it Woods.

The situation you describe with the valley, means that the Jack rafter and valley rafter are in compression at the valley rafter connection with restrained vertical loading, and the whole roof works as one.

That's not the same situation with a fixed vertical chunk of wall.

With the OP's situation, the loading is vertical so seats ste needed. If there are no seats, or one seat then the load becomes lateral.

 

[^woody^]

No. As long as the plumb cut is sat against the masonry. The truss clips will prevent uplift.

But the truss clip can bend horizontally if the rafter is pulled sideways. The clip provides zero lateral restraint.

When the clip is used with a truss, the bottom chord timber is providing a seat.

 

[noseall]

sideways

Says the man who informs us that gravity works downwards.
Woods- the bottom bm provides the seat and the top plumb cut resists rotation