cut and pitch conundrum

[chirpychippy]

May I ask for some help. I am trying fathom a cut and pitch gable roof. The span is 4.4mtrs plate to plate and the width is 3.2mtrs. On the spec it states use 47x195 C24 grade timber for the ceiling joists but I have always followed the rule of thumb of span x 2 plus 1 for joist depth so really they should be 47x225. Or is it because they are ceiling joists that the load won't be the same as floor joists therefore the depth can be less?

Which leads onto my second question, the roof pitch is 37.5 degrees the width is 3.2 mtrs, under certain circumstances purlins and struts would be used however because there is no central wall, or any wall, would a King Post be sufficient on each pair of rafters or should further bracing be incorporated.

I know there are people on this site that have profound knowledge of cut and pitch roofing and may wonder why I am asking these questions, it is only because I take my vocation as a Carpenter very seriously, whilst I am only six years in there are still things I have to learn, my nemesis of Cut and pitch being one of them.

Thank you in advance, Chirpy

 

[tim00]

With respect, i understand that you apparently dont have much cut roofing experience so i'll start at the beginning,so to speak.

Roof cutting refers to pitched roofs and is the traditional term, ie a cut roof. Perhaps cut and pitch is a local term?

Your references to span and width and king posts and walls confuses me. For rafters, the span is the only dimension you require. The span in roof cutting refers to the dimension : building line to building line, or outside of the plate to outside of the plate.
For joists the span is inside of the plate to inside of the plate. Google "joist spans" for sections. Note that spacing and span determine the section.

Perhaps you can post a diagram - plan and section, or explain more clearly - google one of the many excellent usa roof cutting tutorials esp. for terminology.

 

[gregers]

I know there are people on this site that have profound knowledge of cut and pitch roofing and may wonder why I am asking these questions, it is only because I take my vocation as a Carpenter very seriously, whilst I am only six years in there are still things I have to learn, my nemesis of Cut and pitch being one of them.

only 6 years eh,well ive been in the game over 20 and LACK vast knowledge of roofs,dont worry about it.ive allways found better to ask then fook it up.
do you not own any roofing books,i:e goss's ready reckoner?
i kindly got given a copy by an old chippy years back.
still dont understand the bloody thing,

which to be honest is a shame because i would like to improve my limited knowledge of roofs,but to be quite honest where i am based its nearly all 2nd fix work.

 

[noseall]

Which leads onto my second question, the roof pitch is 37.5 degrees the width is 3.2 mtrs, under certain circumstances purlins and struts would be used however because there is no central wall, or any wall, would a King Post be sufficient on each pair of rafters or should further bracing be incorporated.

I know there are people on this site that have profound knowledge of cut and pitch roofing and may wonder why I am asking these questions, it is only because I take my vocation as a Carpenter very seriously, whilst I am only six years in there are still things I have to learn, my nemesis of Cut and pitch being one of them.

Thank you in advance, Chirpy

I build a roof according to design.

Give me a drawing and i build the roof unless the roof is a simple or typical jobby.

Your question about purlins and posts makes no sense. Either the roof has been designed with clear spans or it hasn't. As Tim has stated, if the rafter is man enough for the span then there is no need for additional support. There may be the need for collars high up in a roof but these stiffen rather than support.

Do you have a drawing?

 

[chirpychippy]

OK, where to start, forget the joists this matter has been cleared up. There are no drawings for the roof, just a lot of blurb in the spec which could relate as much to this roof as it could to any other.

The span is 4.4mtrs building line to building line or outside of plate to outside of plate. The pitch is 35 degrees taken from the drawing, but can be adjusted to suit on sight. The rafters are to be cut from 5 x 2 C24 grade timber spaced at 400c/c. "this is all the information I was given"

So whilst I can Cut and Pitch quite a few roofs and in theory this roof, I was unsure of the structual requirements as I had nothing to work from.

In the end I registered with the Trada website, from which I have found the relevent information I require, thus I opted for the close couple roof design as it corrolates to the span and the neccasery structual needs.

Thank you to those of you whom did reply and I apologise for my original posting being confusing, cheers jason

 

[gregers]

sounds like you got a result,

 

[chirpychippy]

I am going to post two drawings of my own, the probability of you making sense of them is questionable.

Diagram 1 is the way I intended to pitch the roof. The rafters are green the plates are in pink, the yellow is irrelevant. There are three pairs of rafters on diagram one which give me concern but only from the LBC point of view and as they are reluctant to assist with whether its an acceptable form for pitching a roof which leaves me a bit stuck. Rafters A and B i intend to spider out towards the crooked wall thus meaning I would have to spider the joists to go along with a close couple roof, in my head it works but is it acceptable to building regs?

Diagram 2 Is more aesthetically pleasing but I can't work out how to pitch rafter A1, do I bring it down to the plate crooked or could I put a trimmer between adjacent joists and pitch to the trimmer? Which in this diagram leads me onto the problems I have with the joists. Up to the 5th rafter the joists will sit happily plate to plate, but past the fifth rafter the only way I feel they can be located is by way of trimmers and hangers.

I am relatively comfortable with diagram one and can't really see any reason why it won't be accepted, but I would appreciate your opinions before I advance.

Thank you once again[/img]

 

[chirpychippy]

 

[chirpychippy]

 

[^woody^]

Is that a ridge line (black line) down the middle, or is this a mono-pitched roof?

 

[chirpychippy]

hello woody, yes that black line is the ridge.

 

[noseall]

So, instead of having a square end some kind soul has given you a cant end, no doubt to use up the whole of their boundary!

Diagram 2 makes most sense.

You also realise that unless the roof tiler can fabricate something (like we had to a couple of years ago), some of the water run-off on the 'A' side will drip off the verge end and not into the gutter.

 

[chirpychippy]

yep spot on noseall, the cant end is to use up all the boundary, marvellous!

We have had conversations in the past about my lack of experience in Cut and Pitch and it is this lack of experience that drives me to learn about Cut and Pitch.

Going back to my original question, The rafters at the rear are striaght forward as are most of the front one's apart from Rafter A1 would it be simpler to just pitch it to the plate at an angle rather than put a trimmer between its adjacent rafters.

Which leads me onto the joists, I can get 5 full joists in plate to plate but the 6th 7th and poss 8th I am not sure what to do with. Would it be feesible to double up the 5th joist and then fill the remaining void with shorter joists hung from the doubled up joist and built into the gable wall at 400 centres.

And finally my last question, looking at diagram 1 the plate with a length of 3000mm is their neighbours exisitng extension and their neighbours plate. Now there is a Party Wall agreement in place stating that this plate can be used for the new extensions joists and rafters. So to my mind the new plate measuring 1900mm at the front has to be set at the same height as the existing plate to enable the new joists to be level. However, whilst this would work for the joists and the rafters at the rear, I don't think the rafters pitched to front would land on the new plate and still carry the roof line of the exisiting building.

The only solution I can think of should this be the case is to put a pole plate on top of the new joists to retain the roof line, however, in pitching the front this way would i not be putting extra load on the joists and therefore would have to beef the joists up. They are currently 8x2 which is the minimum permitable size allowed for the distance being spanned.

I know I have said this before and I will continue to say it, I really do appreciate the help I have been given thus far, thank you

 

[chirpychippy]

In answer to your question about the tiles and the water run off, I spoke to the tiler about this some weeks back and his answer was simply he would cut the tiles to the angle of the cant and that would be that, pretty poor really, saying he has many years experience of tiling.

 

[noseall]

I shouldn't worry about the lack of the last couple of joists, just make sure that you build the ridge board in. You can also fix the last (splayed) joist to the wall and then manufacture some cradling/framing to carry the ends of the plasterboards.

Keep the plates level. I would need to see the existing roof to get an idea of what you want to achieve. We have fixed a plate to the top side of the joists in the past although this was directly over the wall.