Seat cut too long?

johney · 22-07-2019 12:02am #1

hello Folks,

1 question out of curiosity please.
he has made the birdsmouth rafter seat cut for the purlin 4 inches, but the purlin thickness is only 3 inches. This in effect means the bottom inch of rafter has no bearing. I always thought the heel needed to be fully grounded as this is where the maximum strength is.
Rafter is 13 foot from purlin to ridge. Also as his saw was a small bladed one, he had to overcut the rafter in order to knock out the complete birdsmouth piece.
All in, Im wondering will this have any effect on the structural integrity of the rafters or will it really make no difference?

Thanks a lot,

John.

Gumbo · 22-07-2019 9:41am

johney wrote: »

hello Folks,

1 question out of curiosity please.
he has made the birdsmouth rafter seat cut for the purlin 4 inches, but the purlin thickness is only 3 inches. This in effect means the bottom inch of rafter has no bearing. I always thought the heel needed to be fully grounded as this is where the maximum strength is.
Rafter is 13 foot from purlin to ridge. Also as his saw was a small bladed one, he had to overcut the rafter in order to knock out the complete birdsmouth piece.
All in, Im wondering will this have any effect on the structural integrity of the rafters or will it really make no difference?

Thanks a lot,

John.

Your Engineer should inspect and confirm.

Hootanany · 22-07-2019 9:45am

Do you mean wallplate?

sydthebeat · 22-07-2019 10:00am

johney wrote: »

hello Folks,

1 question out of curiosity please.
he has made the birdsmouth rafter seat cut for the purlin 4 inches, but the purlin thickness is only 3 inches. This in effect means the bottom inch of rafter has no bearing. I always thought the heel needed to be fully grounded as this is where the maximum strength is.
Rafter is 13 foot from purlin to ridge. Also as his saw was a small bladed one, he had to overcut the rafter in order to knock out the complete birdsmouth piece.
All in, Im wondering will this have any effect on the structural integrity of the rafters or will it really make no difference?

Thanks a lot,

John.

pics or sketches as this is all very hard to understand???

its not unusual for the end of the rafter to be effectively cantilevered to form an overhang

Bonzo Delaney · 22-07-2019 8:09pm

Depending on the size of the rafter but generally the rule is thumb is that the birds mouth doesn't eat in to more than 1/3rd the depth of the rafter so if it's s 6x2 no more than 2 inches of a birds mouth depth

johney · 22-07-2019 9:13pm

thanks lads,

its not on the wall plate but at the purling supporting the rafters. Rafter length 21 foot. Purlin 9 foot in from wall plate, 13 foot down along rafter from ridge.
Rafter seat cut for purlin 4 inches. Purlin 9x3.

Bonzo Delaney · 22-07-2019 9:25pm

What size rafter 5x2. 6x2 .7x2. 9x2

sydthebeat · 22-07-2019 9:25pm

johney wrote: »

thanks lads,

its not on the wall plate but at the purling supporting the rafters. Rafter length 21 foot. Purlin 9 foot in from wall plate, 13 foot down along rafter from ridge.
Rafter seat cut for purlin 4 inches. Purlin 9x3.

what size rafters?

the rafter should not be cut more than 1/3 of the depth.

Bonzo Delaney · 22-07-2019 9:33pm

And what pitch roof
It'll be easy enough to figure out what size seat cut is allowed then one rafter size is determined
Hips will generally have more cut out due to keeping top edge on the same plane . Don't any one go quoting dihederal angle planes here to allow depth😉

TheBoyConor · 24-07-2019 5:29pm

It is hard to say for sure without seeing the exact situation but if I was a resident engineer on a job and I saw that i would be saying "no. substandard workmanship. Take it out and replace" . You basically will have an inch gap between the side of the purlin and the lower part of the rafter! Anyone seeing that would be like WTF?

Golden rule - measure twice, cut once. Insist that it is changed at his expense. He will be more careful the next time.

And another piece of advice, be hard here. If the builder sees that you will turn a blind eye to or accept poor workmanship then you can expect the quality of construction to deteriorate even further.

Hootanany · 24-07-2019 5:40pm

Bonzo Delaney wrote: »

And what pitch roof
It'll be easy enough to figure out what size seat cut is allowed then one rafter size is determined
Hips will generally have more cut out due to keeping top edge on the same plane . Don't any one go quoting dihederal angle planes here to allow depthðŸ˜‰

The oul Dihedral angle I have never seen anyone every do this to a hip rafter and mean every.

kevcos · 30-07-2019 2:14pm

TheBoyConor wrote: »

It is hard to say for sure without seeing the exact situation but if I was a resident engineer on a job and I saw that i would be saying "no. substandard workmanship. Take it out and replace" . You basically will have an inch gap between the side of the purlin and the lower part of the rafter! Anyone seeing that would be like WTF?

Golden rule - measure twice, cut once. Insist that it is changed at his expense. He will be more careful the next time.

And another piece of advice, be hard here. If the builder sees that you will turn a blind eye to or accept poor workmanship then you can expect the quality of construction to deteriorate even further.

You were correct, it is hard to say for sure without seeing and perhaps you should have left it at that.

TheBoyConor · 30-07-2019 2:19pm

Well alright then. Did you come here just to say that to me?

kevcos · 30-07-2019 2:35pm

TheBoyConor wrote: »

Well alright then. Did you come here just to say that to me?

Without seeing the install you are condemning the roof and advising the OP to fire a shot at the builder.
And regardless of what actually has been installed your technical appraisal is flawed as well.

OP, what degree is the roof pitch and what dimension are the rafters?

Calahonda52 · 30-07-2019 10:02pm

kevcos wrote: »

Without seeing the install you are condemning the roof and advising the OP to fire a shot at the builder.
And regardless of what actually has been installed your technical appraisal is flawed as well.

OP, what degree is the roof pitch and what dimension are the rafters?

Pls clarify

Seat cut too long?

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