cut and pitch conundrum

[chirpychippy]

I spoke to Building Control/Regs today, to see if they would help, and what a splendid bunch they are.

They said to triple up the fifth joist and use it to hang joist from at 90 degress built into Gable Wall to fill the void created by the cranked wall.

I was also concerned with Purlins, Binders, Hangers etc until they explained the meaning of clear span, which translated into not needing any of them, yippee, although I will put some collars in high up cos it makes me feel better.

The Rafter I can't pitch at ninety degrees to the plate will have to go in at some other angle dictated by the amount of room I have left to play with. Apart from that I think I have it sussed, of course with a lot of help from the people on this site and building control. Thank you Noseall.

 

[theoldun]

Chirpy typed this last night, but noticed on your post today, that it would appear that you have the answers that you require. Did not know whether to post this or not, but have decided to post as it may give you some information for your future.

Chirpy will do my very best to help you out. Do not know how good your knowledge bank is, so will give you a rudimentary background first.
Unfortunately neither your drawings or posts make a lot of sense, so start at the beginning and try to sort out what you are trying to achieve. You state 4.4 span of ceiling joists. Now your span should be between the plates, so to span 4.4 you will require C16 /47x195 at 400 centres to support a dead and imposed load not exceeding 0.5KNm2 excluding the dead weight of joist.
You will also need two binders C16/47x150 at one third span and two third span built into walls at both ends.
To clear up your confusion, floor joists have an imposed load of 1.5KNm2, ceiling joists have an imposed load of either 0.25 or 0.50KNm2, hence the difference in depths. Forget all about rule of thumbs, you will not find them listed in BRs.
You state your pitch is 37.5 and your width is 3.2. Surely you mean your length, as your span is your width and plate is your length. Your span determines your run for length of rafter and length and size of rafter determines whether purlins are required. Your length of plate determines how many pairs of commons you require and in conjunction with rafter size determines size and length of purlins if required and purlin size determines length of purlins and centres of supports for purlins.
The above relates to a straight forward coupled roof. Read it several times and you should get the general principle together with the rest that I will write.
There are numerous ways to set out a roof. 1.With a protractor, square and folding square draw her up eighth size, quarter size or half size on a sheet of ply. If you have enough room you can even set her out full size on the ground. 2. With a few simple calculations and list of natural secants. 3. A roofing ready reckoner or calculator. 4. Roofing square. 5. A plumb bob line, passing cloud or nearby tree and a moderate wind. This last one is more complicated. These days we use Ralph Goss, Roofing Ready Reckoner and Trada roofing tables. If you intend to do a few roofs buy Ralph Goss roofing tables. No need to buy Trada you will find the tables in the Approved Document part A archived 1992 edition. They are near enough for present day purposes.
Back to the subject. You state span is 4.4, therefore overall plate is 4.6. You always work from overall plate in roofing. Half your span gives you your run 2.3.
In the tables 2.3 run at 37.5 pitch gives you a slope (hypotenuse) of 2.899 and a rise of 1.765, However these are not the figures that you will work to. These figures are the hypotenuse, opposite and adjacent sides of a triangle, and this triangle is from the top outer edge of the plate to the top of the opposing rafter, there fore you need to deduct half the thickness of the ridge from the slope. If you are using a ridge of say 50mm deduct 25mm from slope which will give you 2.874, therefore length of rafter from top of ridge plumb cut to heel cut of birds mouth is 2874 plus overhang as required. Your rise will also increase by the H.A.P. (height above plate). H.A.P. is the plumb measure from the top of rafter and corner of your 32.5 seat cut and your 62.5 heel cut to birds mouth.
Okay so now you have your length of rafter to determine rafter size, but it now starts to become a little complicated, as various roof coverings all have various dead load weights in KN/m2. It also depends on where you live with regard to imposed snow load, however snow load and man maintenance are included in different tables so as it would appear you live in Kent, providing the job in question is not above 200m in altitude you will come under zone B. As we do not know your roof covering, we will take the worst scenario of concrete plain tiles size 267x168 at 100 gauge including felt and battens will be a dead load of 0.77KN/m2. With all that in mind your rafter size without having to use purlins will need to be C16/`50 x150 at 400centres to support a dead load of 1.00 KN/m2, however if you are using concrete interlocking tile you could drop down to C16/ 50x125 at 400 centres to support a load of 75KN/m2.
Now that we have some background covered which you may or may not have known, lets look at your problem. All my comments will be with regard to the coloured picture post card, and nothing to do with the first two.
Firstly, Kings, queens, and common rafters should only be pitched at 90 degrees to the plate, never at an oblique angle, as they are not doing the job they are meant for. Queens and commons should meet their opposite partner in a parallel line at 90degree to both plate and ridge, which now leaves you in the sh8t. Before getting you out of the sh8t, going to make some assumptions from the coloured picture post card that you have drawn.
It would appear that the plate on left hand side is on the front skin of cavity wall and the plate on right hand side is on the internal skin wall which is also the party wall and this plate will be shared with both yours and neighbours roofs. You will have a lot of work at this point forming a tapered valley gutter. Hope assumption is right
What you have is an oblique verge and gable, and the way to do these are with 50mm noggins by one size up from the depth of rafters. Pitch your roof from rafter 1 to 5 making sure that your ridge will project into inner skin of gable brickwork.
Left hand side, pitch no 6 and you may even get no 7 in. If not double up no 6, hang noggin as stated low enough down from ridge to collect no 7 rafter. Noggin to be long enough to project and be built into gable wall Easy way to do noggins, fix to rafter, tack 50x50 slat on top and fix slat across rafters to hold noggin level to collect rafter. Brick in when gable goes up
Right hand side, more or less the same as other side but noggins will be low enough down to collect bottom of rafters. Joists will be same as rafters, with last joist bolted to wall.
Couple of other pointers . Plates always go in level and the same overall measure each end. The two other roof dead loads that you may wish to know are slates including felt and batten 0.41KN/m2 and plain clay roofing tiles ditto 0.66KN/m2. Have already given you concrete plain tiles and concrete interlocking tiles.
As Nose has already said you do have a rain water run off problem with oblique verges but there are a couple of ways to cure this that any decent roof tiler should know.
Hope some of this information will be of help to you in the future..
Regards oldun

 

[gregers]

WOW what a reply,wish i could understand it

 

[chirpychippy]

http://i253.photobucket.com/albums/hh64/chirpychippy/09042012107.jpg

Thank you very much theoldun, I have another piece of art work to try and expalin further with more clarity. But to clarify a few points first, the clear span for the ceiling joists is 4250mm not 4450mm as originally said "not that I knew this until someone explained it to me. And the angle of the roof is in and around 34 but not 37.5. The clear span of the rafter is 2800mm.

The roof covering is Concrete Interlocking Tiles Marley Romans to be exact. According to the span tables I am within the clear spans for the sizes and grade timbers being used and I have been through this in detail with building control and they agree that I don't need to add additional support such as purlins, binders etc.

You are quite correct in saying that I have to form a tapered velley gutter between the new roof and the exisitng roof at the party wall as shown in the colourful diagram.

However, there are a few elements which I haven't explained that well relating to the rafters, whilst I fully agree that rafters should always go in at 90 degress to the ridge and the plates and form a pair, I am not sure of the use of noggins to collect the 7th and 8th rafters to achieve this for I have to build or form a gable ladder to collect the external rafter pair that the barge board will be nailed to.

Lastly, if you can understand my recent art work, you will see that the roof of the neighbours extension is lower than the roof of the existing building, thus meaning the wall plate will be lower, Hyperthetically, I would have to set the new wall plate to this height on the front of the new extension to ensure the joists go in level, however, should I do this then I know that the rafters at the front will miss the new wall plate, for this I can see some solutions.

Solution 1,

The new internal wall plate at the front of the new extension has to be set so that the HAP on the new rafters tallies with the top of the existing truss roof, thus making the roof line equal at the front. The external wall of the existing extension would then have to be built up so as to give the correct height to allow a new plate to be attached thus the new joists go in level.

Solution 2,

Set new wall plate at front of new extension to the height of wall plate in exisitng extension and introduce a pole plate fixed to the top of new joists parrallel to the wall plate but set back to achieve equal rafter/ truss height and HAP on new rafters, however this will put additional loading on the 8x2 joists and render them under sized "I think"

Solution 3,

Any takers for solution three welcome.

BTW, I do appreciate the advice that Theoldun has given and whilst I do understand most of it, such as ready reckoners etc, KN/M2 I don't, but I will keep on reading and re reading.

 

[theoldun]

Chirpy, no problems son. Have a clearer picture now and think solution 3 may be best bet. Will explain solution 3 to you plus answer all your queries and explain KN loading to you,
However need a little more information. Assume clients house is a semi? Draw a site plan and post showing clients and neighbours house and extension, also show road frontage, side and rear boundaries of plot.
If you do not get a reply back straight away, do not worry as sometimes loose my vision for up to three or four days.
Regards oldun