rafter size and spacing , lean to.

[robgil]

any one know what the regs for rafter size and spacing and wall plate size on the house side would be on a lean to with cedar shingles?
or is it more complicated than that?

 

[Flyfisher]

I don't know, but I suspect the answer will lie with a structural engineer rather than the building regs themselves.

 

[Jon Maine]

It is dependent on:-

1/ Roof pitch
2/ Length of rafter between supports.
3/ Dead load of roof covering

Yes an engineer could easily work this out (mainly based on item 3 ) but also it would be covered under the building regs and easy to establish your self with the above info.

 

[Gareth Hughes]

The relevant approved document is here, should be pretty straightforward (it's about page 11 I think)
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf

(Edit) actually I see they've taken the useful bits out and refer you to another publication now

But they have left in the bit about house longhorn beetle which always used to make me wonder what was so special about Woking?

 

[Nigel Watts]

You need to think about the roof pitch as well in the context of the choice of roofing material. This may be of interest:
http://homerenovations.about.com/od/houseexteriorframework/a/shinglespitch.htm

 

[Flyfisher]

The building regs give lots of guidelines and principles but seem to give little hard data.

I suspect Robgil wants to know (or calculate) the sizes of timber needed to support a roof of x Kg/m2 across a span of y metres at a pitch of z degrees, plus the required rafter pitch. The answer is probably traceable by wading through various references and supporting standards but what's really needed is a "builder's ready reckoner" with simple formulae and worked examples.

I'm basing these comments on my experience of the IEE wiring regulations. Worthy though they undoubtedly are, each new edition was always accompanied by various independent "Guidelines to the nth Edition" books that explained simply, but concisely, how to design a wiring system that complied with the relevant regulations.

Is there nothing similar in the building game, sorry, profession?

 

[christianb]

For that, 6x2" C16 or C24 rafters at 400 centres would do the job and be, for all intents, complete overkill.

C.

 

[Flyfisher]

I agree that construction would seem more than adequate, but it would be interesting to know the actual figures. Perhaps, for example, 4x2 timbers would be perfectly adequate?

Just a thought; when determining the roof loading, are there any guidelines or requirements for supporting people walking around up there?
I think it's a safe bet that I'm heavier than a few cedar shingles!

 

[christianb]

Oh, that would take you, your family, your car, and the local rugby team.

As one building inspector said to me: "if it looks like it will do the job, it usually will".

As for de facto calculations, there's a good book by a chap called Roger Greeno called the Building and Construction Manual.

C.

 

[Flyfisher]

Oh, that would take you, your family, your car, and the local rugby team.

Probably just about OK then. :wink:

The book reference was very interesting. It's on Google Books and the roofing section contains a wealth of information with timber sizes clearly shown.
Thanks for that.

 

[christianb]

It's a great reference manual. Suits everyone from self-build to major construction projects - and there's plenty of info on roof construction, including all the calcs.

C.

 

[robgil]

i found this on tinternet , it says its for more common timber sizes used in traditional roofing construction.
could i take this literally do you think?

http://www.salford.gov.uk/d/purlin-and-rafter-roofs.pdf

 

[robgil]

The relevant approved document is here, should be pretty straightforward (it's about page 11 I think)
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf

(Edit) actually I see they've taken the useful bits out and refer you to another publication now

But they have left in the bit about house longhorn beetle which always used to make me wonder what was so special about Woking?

cheers Gareth , i may have to take a day off to read through it all though.

 

[chuckey]

I reckon the rafter size is a bit trivial. The real problem is to fix the top end of the rafter to the wall without it having a tendency to slide down the slope. The victorian way would be to birds beak the rafters and have a purlin, so the purlin resists any sliding down the slope. The lower end of the rafters would have their bottom surface bird beaked or planed down so it sits on the wall plate. So any sliding down the slope would have to be resisted by the wall and wall plate. Unfortunatley bird beaking the top end also destroys its strength so should not be indulged in.
Frank

 

[robgil]

i thought that was the norm anyway Frank , although i have seen brackets used to secure to the lower wall plate. this roof does have a purlin though and i was wondering if it was entirely necessary.

 

[chuckey]

I replaced a roof not too disimilar to yours, the purlin (17' X 10" X 4") terminated in a wall that I was removing. The "new" length would have been 23', so to get any stiffness in it it would have had to be at least C16 9" X 3"(£££). I decided to fix a triangle of wood to the underneath of each rafter (cut up rafter off cuts), these were fixed with a huge 8" X 8mm twist nail (galvanised) into predriled holes and a couple of 6mm X180mm screws.
The basic idea is to make the two mounting surfaces for the rafter at the right angle (for the roof slope) and parallel. There will be only downwards thrust on the wall.
Frank

 

[robgil]

that sounds interesting. i think i need to get back up there and measure up. the longest span for the purlins is only around 4 meters or so so its not too bad. the rafters are around 3 meters.
ive never done a roof before so this will be a sharp learning curve for me , i just hope i get it right.

 

[Louis Henry]

The old builders would have used 4" x 3" and it would have stayed up for ever. Nowadays you would probably need at least 6" x 2" to leave room for some insulation (probably plus some over the rafters). Shingles will need a much steeper slope than the one in the picture.However you do it the building inspectors will tell you you've got it wrong, as will the conservation officer, but of course their advice will be conflicting! Good luck.

 

[MdB]

A 6 x 2 will be approx 50% stronger than a 4 x 3 even though they both have the same amount of wood in them.

 

[christianb]

Don't know if I'm missing something here, but why are you worrying about purlins on a lean-to roof? If reducing any "play" is an issue, once you put the battens for the slates up there you won't get any whatsoever.

Or, just make the whole thing a monocoque cosntruction and stick some wbp ply on top before nailing in the battens.

C.