The birdmouth Thing again!

[Sonnypaul]

Ive put a few posts on here about bird mouth cuts in rafters but still cant get it in my head and its driving me crazy!

Please help me lol

For example, an extension is ready for the roof, 4x2 wall plate is on the new block work and the 8x2 wall plate is bolted to the existing house at 150mm down from the sill.

Now what do i do ?



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Pitch is 19 degrees, before i have put a straight bit of timber on top of the wall plates and beveled my angle at the top, finished by using joist hangers on the top and truss clips on the bottom.

but i cant figure out the birdmouth without the top dropping 2 inches if the bottom was cut.

thank you

 

[gregers]

get yer head round this,

http://samples.sainsburysebooks.co.uk/9780470697986_sample_380504.pdf

 

[theoldun]

Roof calculations are easy
Give us the height above WALL plate, to the top of TOP wall plate.
The width from existing house wall and outside of wall plate
What size rafters you are using?

Will give you calcs and bevels and will tell you a way we were taught back in the 50s to scribe rafters in situ.
Regards oldun

 

[noseall]

We don't fit the pole plate (the one that is bolted vertically to the high end) until we have mocked up a rafter.

What we tend to do is cut a 17 degree plumb cut on each end of a potential rafter, i.e. a top and bottom cut.

We then offer it into place so that is rests on the corner edge of the (low horizontal) 4" x 2" wall plate and is butting directly onto the brickwork at the high end.

You can check if the rafter is sitting in plane by popping a short level on the bottom cut. You move the rafter up or down the wall until the level reads plumb.

If you have done your pitching maths correctly, the rafter should be sitting higher up the brickwork and more towards the window sill than it should be. You then strike a vertical mark using a short level from the (low end) wall plate marking off what will be your vertical plumb cut on the bottom birds-mouth.

The depth of this birds-mouth is determined by how much the rafter needs to be lowered in order for it to be in the correct position relative to the window sill. In other words the rafter is lowered wholesale keeping the top plumb cut against the wall. The 17 degree plane of the rafter does not change - just the height of the rafter changes as you lower it into the birds mouth.

The pole plate can either sit beneath the rafter or at the rafter end. We prefer it below. This can be offered up to the rafters once you have a couple in place.

If you intend sitting it at the rafter end and not below then you just take a short piece of pole plate timber and mark it off against the rafter end, whilst the rafter is resting against the masonry, then remove this material on the saw.

 

[Sonnypaul]

We don't fit the pole plate (the one that is bolted vertically to the high end) until we have mocked up a rafter.

What we tend to do is cut a 17 degree plumb cut on each end of a potential rafter, i.e. a top and bottom cut.

We then offer it into place so that is rests on the corner edge of the (low horizontal) 4" x 2" wall plate and is butting directly onto the brickwork at the high end.

You can check if the rafter is sitting in plane by popping a short level on the bottom cut. You move the rafter up or down the wall until the level reads plumb.

If you have done your pitching maths correctly, the rafter should be sitting higher up the brickwork and more towards the window sill than it should be. You then strike a vertical mark using a short level from the (low end) wall plate marking off what will be your vertical plumb cut on the bottom birds-mouth.

The depth of this birds-mouth is determined by how much the rafter needs to be lowered in order for it to be in the correct position relative to the window sill. In other words the rafter is lowered wholesale keeping the top plumb cut against the wall. The 17 degree plane of the rafter does not change - just the height of the rafter changes as you lower it into the birds mouth.

The pole plate can either sit beneath the rafter or at the rafter end. We prefer it below. This can be offered up to the rafters once you have a couple in place.

If you intend sitting it at the rafter end and not below then you just take a short piece of pole plate timber and mark it off against the rafter end, whilst the rafter is resting against the masonry, then remove this material on the saw.

Lovely noseall, really helpful, just one thing mate, why the 17degree cut? Was that just an example, the tiles are Redland 49ers min pitch 17.5.
If I do a cut 17 degree and plumb it at the low end won't the top be a different pitch.
Cheers governor

 

[Sonnypaul]

We don't fit the pole plate (the one that is bolted vertically to the high end) until we have mocked up a rafter.

What we tend to do is cut a 17 degree plumb cut on each end of a potential rafter, i.e. a top and bottom cut.

We then offer it into place so that is rests on the corner edge of the (low horizontal) 4" x 2" wall plate and is butting directly onto the brickwork at the high end.

You can check if the rafter is sitting in plane by popping a short level on the bottom cut. You move the rafter up or down the wall until the level reads plumb.

If you have done your pitching maths correctly, the rafter should be sitting higher up the brickwork and more towards the window sill than it should be. You then strike a vertical mark using a short level from the (low end) wall plate marking off what will be your vertical plumb cut on the bottom birds-mouth.

The depth of this birds-mouth is determined by how much the rafter needs to be lowered in order for it to be in the correct position relative to the window sill. In other words the rafter is lowered wholesale keeping the top plumb cut against the wall. The 17 degree plane of the rafter does not change - just the height of the rafter changes as you lower it into the birds mouth.

The pole plate can either sit beneath the rafter or at the rafter end. We prefer it below. This can be offered up to the rafters once you have a couple in place.

If you intend sitting it at the rafter end and not below then you just take a short piece of pole plate timber and mark it off against the rafter end, whilst the rafter is resting against the masonry, then remove this material on the saw.

Lovely noseall, really helpful, just one thing mate, why the 17degree cut? Was that just an example, the tiles are Redland 49ers min pitch 17.5.
If I do a cut 17 degree and plumb it at the low end won't the top be a different pitch.
Cheers governor

 

[noseall]

Sorry bud, don't know where 17 deg's came from!

The maths is fairly easy to work out once you know the distance the wall plate is from the wall and the maximum allowable rise to the rafter top, in order to be able to get your lead and tile etc beneath the window sill.

Once you know the rise and the going then an angle can be worked out using trigonometry and a tangent button.


As an example......
If the wall plate was say 3000mm distance from the wall and it was say 1300mm rise up to 150mm below the sill then a 23.43 degree pitch will achieve this.

 

[theoldun]

As you have not posted the information requested we can be of little help to you, except tp give you some very basic information on roof framing.

Fpr a 17 degree pitched roof the top plumb and heel cut is 73 degrees and the seat cut to the birds mouth is 17 degrees.

73 plus 17 equals 90 degrees which is angle C for the opposite and adjacent sides. Angle A is 17 between adjacent and hypotenuse, angle B is 73 between opposite and hypotenuse.

Nose wrote.
an example......
If the wall plate was say 3000mm distance from the wall and it was say 1300mm rise up to 150mm below the sill then a 23.43 degree pitch will achieve this.

Sorry old friend, but do not agree with you. Agree that the pitch is correct for the hypotenuse (PITCH LINE) But the top of the rafter will be plus the HAP above the pitch line and as the OP has not posted the information required, can not tell you how much above the top wall plate the apex of rafter will be.

OP . One other thing, we always allow 225 below sill, but on a 17 degree pitch you will just about get your cover flashing in
Regards oldun

 

[Sonnypaul]

As you have not posted the information requested we can be of little help to you, except tp give you some very basic information on roof framing.

Fpr a 17 degree pitched roof the top plumb and heel cut is 73 degrees and the seat cut to the birds mouth is 17 degrees.

73 plus 17 equals 90 degrees which is angle C for the opposite and adjacent sides. Angle A is 17 between adjacent and hypotenuse, angle B is 73 between opposite and hypotenuse.

Nose wrote.
an example......
If the wall plate was say 3000mm distance from the wall and it was say 1300mm rise up to 150mm below the sill then a 23.43 degree pitch will achieve this.

Sorry old friend, but do not agree with you. Agree that the pitch is correct for the hypotenuse (PITCH LINE) But the top of the rafter will be plus the HAP above the pitch line and as the OP has not posted the information required, can not tell you how much above the top wall plate the apex of rafter will be.

OP . One other thing, we always allow 225 below sill, but on a 17 degree pitch you will just about get your cover flashing in
Regards oldun

Thanks for that oldun, have read this a few times over.

Really appreciate the help guys.

Old un, is this the best way to do a roof with the birdmouth...

Block work finished at 2400mm to wall plate height. Measure down 2 inch upstand of lead and tile - battern, fix wall plate on house.

Then get a Straight bit of timber and put it on top of the wall plates.

Then bevel my top angle

Cut plumb cut on the top

Finshed with joist hangers on top and truss clips on the bottom.

That's the way I do it at the moment bearing in mind my angle is correct for the tile and velux.

If it was birdmouthed how will that differ from the way I do it ? Will the whole roof be 2 inches lower ( if I done a 2 inch cut )

If you lived in Kent or near kent I would pay you for information lol

Easier in person to explain!

 

[catlad]

There are a few clips on you tube about cutting rafters that may help.

 

[noseall]

Nose wrote.
an example......
If the wall plate was say 3000mm distance from the wall and it was say 1300mm rise up to 150mm below the sill then a 23.43 degree pitch will achieve this.

Sorry old friend, but do not agree with you. Agree that the pitch is correct for the hypotenuse (PITCH LINE) But the top of the rafter will be plus the HAP above the pitch line and as the OP has not posted the information required, can not tell you how much above the top wall plate the apex of rafter will be.

Yes bud, I know. I was just demonstrating that if he gives us the plate measurement and the rise, and rafter depth etc, that we have the maths capable of providing the plumb cut etc.
In my example, the plumb cut would need to be shallower to allow for rafter depth and soffit overhang etc.

 

[theoldun]

Thank you for expressing your appreciation.
With respect and by no means taking the p*ss, your last post was like reading a comic.
Firstly you can not work out your pitch, angle call it what you like by throwing a straight edge from arris of top plate to arris of wall plate. Depending on size of common rafter you are using your pitch can be 3 or 4 degrees out
Will give you all the help you require providing you do your bit by answering the original following questions.

Give us the height above WALL plate, to the top of TOP wall plate.
The width from existing house wall and outside of wall plate
What size rafters you are using?

Once we receive the information, will tell you all you need to know to cut pattern rafter.
You do not use hangers on your top plumb cut, or clips on your birdsmouth.
Regards oldun