Approved Document Part A

[chirpychippy]

Good evening all, the last time I couldn't work a roof out I found the people on this site extremely helpful. Thank you.

Now I am back in the same boat, I have tried to use the approved documents for guidance but it only covers timbers up to 150mm in deptj. I have a salt box roof with the longest run of 4.7meters which equates to a rafter length of 6.2mtrs on a pitch of 40 degrees which this is.

If! I used a 6x2 rafter and introduced a substantial purlin at the midway point of the rafter would this effectively divide the actual run in half, thus making a 6 x 2 C24 possible.

I can upload the architects plans if it helps?

Any useful replies much appreciated, thank you

 

[tony1851]

Sorry, but what's the question?

 

[chirpychippy]

Maybe I wasn't clear, a 6x2 c24 according to the approved document isn't deep enough for the actual run on its own, is introducing a purlins at the mid point along its length acceptable and if so what size would I need. Otherwise what depth rafter would I need?
Thank you

 

[tony1851]

From back-of-envelope calcs I would think you're looking at something like 200x50s @ 400.
But perhaps more important for the structure as a whole is: how will the rafters be restrained? Is it an open (vaulted) roof or closed with a ceiling?

 

[chirpychippy]

It is a timber framed extension on top of an existing garage, it's not a vaulted ceiling but the ceiling joists sit on the rear wall/rafter plate and are bolted to the side of the rafters that are pitched on to a lower plate at the front. Thank you for your calculations,the plans are pretty useless, I just want make sure the roof structure is correct.

 

[chirpychippy]

Hello Tony1851, may I ask? You seemed to calculate the rafter size for this run with relative ease which leads me to think that there must be formulas for all roofing members. Would this be information you are willing to share with me or would you be able to guide me a source where I could learn, thank you.

 

[tony1851]

PM me if you want.

 

[theoldun]

Salt Box. Not done one now for years

It would appear that your long pitch is on the front, therefore on a 4700 half run from overall front plate to centre of ridge will be 6135 slope (hypotenuse).
50x200 C24 at 400mm centres will be way, way undersized and you will need a purlin.
If you post following information then will give you size of rafters and size of purlin.
Covering of roof. i.e interlocking concrete tiles,plain concrete tiles, clay plain tiles etc.
Span of building width.
Overall length span of front and back plate.
Regards oldun

 

[tony1851]

50x200 C24 at 400mm centres will be way, way undersized and you will need a purlin.

Sorry, Oldun, it's not undersized.

 

[theoldun]

50x200 C24 at 400mm centres will be way, way undersized and you will need a purlin.

Sorry, Oldun, it's not undersized.

Well, if we are wrong Tony, then we apologise to both you and the OP.

In our defence we admit that we are not clever enough to calculate rafter sizes, instead we have to rely on TRADA SPAN TABLES.

The OP states his rafter length is 6200, We make it 6135.
We quote Trada span table for a 40 degree pitch roof with imposed snow load of 0.75KN/m2 and a dead load of not more than 0.75KN/m2, as being a maximum length of slope for a 47x195 C24 as being 5140. Roughly metre short of span table Plus no allowance has been made for the additional loading of ceiling joists being bolted to the long pitch.
If the carpet to the roof is clay plain tiles then the dead load is going to increase to 1.00KN /m2, which will make the situation even worse.

With respect Tony, who are we supposed to believe.
Regards oldun

 

[tony1851]

Hi Oldun,

It's not a case of who's right and who's wrong - it's a difference in approach.

The TRADA tables (and other published timber tables) are necessarily conservative. This is because they are intended for use by builders, inspectors, DIYers etc, who may not have the time or experience in evaluating loads, or in applying the various factors. The tables also have to allow for slightly sub-standard workmanship in cutting, notching and fixing etc.

It is well-known when dealing with timber that calculating member sizes from first principles will almost always give more accurate results, enabling the designer to either reduce the section-size, or increase the span, when compared with the tables.

This applies whether we use the old BS 5268 ('permissible stress' design)
or the Eurocode 5 ('limit-state' design) for calculations.

thanks,tony.

 

[chirpychippy]

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Hello Tony1851 and the Oldun nice to hear from you again. I have attached the plan to give you a better insight into the proposed extension.

It is difficult to measure the longest span accurately from the drawings as 1 plan scales at 7900mm and the other at 8100mm , I will work with 8100mm.

The garage is 3275mm wide by 5600mm long, at the rear of the garage and adjoining the house there is a small utility extension measuring 2300mmx2300mm hence the two pitched roofs.

So for the larger part of the extension roof the span is 8100mm with two runs of 4600mm and 3500mm. So I would imagine that all the rafter widths will be dictated by the longest run 4600mm. There are ceiling joists/collars sitting on the rear wall which will be bolted to the side of the rafters at the front of the extension.

To my mind despite the size of the rafters 50 x 200mm I think I will still need a purlin at the front "the size of which I have no idea" it can be housed into the Gable wall and picked up by a 100mm x 100mm C24 post in the timber framed wall to the right of the patio doors with the load transferred to the brick garage below. I also feel that I will need a binder for the joists which will run from the very rear of the building to the front rafters again housed into the gable wall and sitting on a 100mm x 100mm corner post C24 at the rear of the smaller section, again the size of which I have no idea.

Where the dormer is to be formed do I need to triple or quadruple these rafters for the sides of the dormer. I ask this because whenever I have done loft conversions 9 times out of 10 we have only had to put doubles in.

I know I keep saying this but I do really appreciate the guidance you are giving me, I am very comfortable with cut and pitch and setting out but have absolutely no idea of how to calculate loadings etc.

As a way of indirectly paying you for your assistance and guidance would you be able to tell me of any charities you support then I can make a donation to them.

Thank you Chirpy.

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[tony1851]

Hmm.... now let me see - where did I put that magnifying glass?

 

[chirpychippy]

You may borrow mine, I will try again

 

[chirpychippy]

//www.diynot.com/network/chirpychippy/albums/

I am having a go at sending the images again, these images are in my album which may be easier to view, thanks for now Jason