Pfffft
3x2's were used back then because they knew no better and besides most older roofs have purlins, reducing the spans considerably. To chuck 3x2's into the thread without knowing the span is comical. Try and accept defeat graciously!
Roof timber queries
- Thread starter garymcl
- Start date
3x2's were used back then because they knew no better and besides most older roofs have purlins, reducing the spans considerably. To chuck 3x2's into the thread without knowing the span is comical. Try and accept defeat graciously!
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Just to confirm your posted information. Flanks 4000, length 10000, lean to roof, hipped each end, pitch approx 23 degrees, centres 600mm If that is all correct then it is a very straight forward roof, although pitch bothers us a bit, but that will sort itself out once we do the calcs.
Joists First.
4000 overall, assume 300mm cavity wall therefore internal span 3700 less 50mm ledger plate = 3650, therefore you need 47x200 C16, 600 centres.
Rafters. 22.5 to 30 degree pitch
Once again, say 3650 internal span plus 100mm wall plate = 3750 run @ 23 degree pitch = 1592 rise and 4074 slope, pitch line, setting out line, hypotenuse call it what you like, but basically it is the length of the rafter from the top plumb cut to the plumb cut of the heel on birds mouth.
Now as rafter tables only go up to 47x150 C16 to span 3200 at 600mm centres, you have a problem. It is easy to resolve if and repeat if you can find a lad who knows how to throw a mid span ring purlin round, treble up 4 number joists to make 4 bressumers to support purlins. Now in situations like this we often take a flyer. Increase joists to 47x225 and rattle rafters in the same at 47x225 all at 400 centres, not 600 centres, with a hanger on every rafter and joist. Up to now we have never been knocked back by BC with this construction, although we are confident that if a SE did calcs for the above it would come in at more than 0.50 and not more than 0.75kN.m2. Must stress that you must get BCs approval before going down this road. If you do go this route and you want 450mm overhang then rafters will need to be 4500 long. You might even have to clunk a bit on the tails, depends how neat your chippy is. You can get them that length, but will have a fair bit of searching.
We mentioned concern in the beginning with the pitch. If your new extension has a standard 2400 storey height, a rise of 1592 at 23 degree pitch, rafters at 225 deep with 75mm deep heel leaving approx 175mm HAP, battens and tiles 50mm, lead apron 150mm. = 2400 + 1592 +175 + 50 + 150 = 4367 from DPC to top of lead apron.
Now if your house is a standard house with 2400 ground floor storey 200 joists and 2400 first floor storey height = 5000. Knock say 300 of for your soffit level gives you 4700m so your new roof will scratch in but means you have no windows on your first floor rooms other wise they will all be in the new roof space at that pitch.
No point in going any further until you confirm whether any windows in back wall above new roof. Let us know and will finish it of for you ref treatment, the Longhorn Beetle Myth and fixing ledger plates and wall plates. etc.
Read this through a dozen times and you will get the gist of it. Do not think our figures are wrong.
Regards oldun.
Catlad. You do write a load of ****e.
Joists First.
4000 overall, assume 300mm cavity wall therefore internal span 3700 less 50mm ledger plate = 3650, therefore you need 47x200 C16, 600 centres.
Rafters. 22.5 to 30 degree pitch
Once again, say 3650 internal span plus 100mm wall plate = 3750 run @ 23 degree pitch = 1592 rise and 4074 slope, pitch line, setting out line, hypotenuse call it what you like, but basically it is the length of the rafter from the top plumb cut to the plumb cut of the heel on birds mouth.
Now as rafter tables only go up to 47x150 C16 to span 3200 at 600mm centres, you have a problem. It is easy to resolve if and repeat if you can find a lad who knows how to throw a mid span ring purlin round, treble up 4 number joists to make 4 bressumers to support purlins. Now in situations like this we often take a flyer. Increase joists to 47x225 and rattle rafters in the same at 47x225 all at 400 centres, not 600 centres, with a hanger on every rafter and joist. Up to now we have never been knocked back by BC with this construction, although we are confident that if a SE did calcs for the above it would come in at more than 0.50 and not more than 0.75kN.m2. Must stress that you must get BCs approval before going down this road. If you do go this route and you want 450mm overhang then rafters will need to be 4500 long. You might even have to clunk a bit on the tails, depends how neat your chippy is. You can get them that length, but will have a fair bit of searching.
We mentioned concern in the beginning with the pitch. If your new extension has a standard 2400 storey height, a rise of 1592 at 23 degree pitch, rafters at 225 deep with 75mm deep heel leaving approx 175mm HAP, battens and tiles 50mm, lead apron 150mm. = 2400 + 1592 +175 + 50 + 150 = 4367 from DPC to top of lead apron.
Now if your house is a standard house with 2400 ground floor storey 200 joists and 2400 first floor storey height = 5000. Knock say 300 of for your soffit level gives you 4700m so your new roof will scratch in but means you have no windows on your first floor rooms other wise they will all be in the new roof space at that pitch.
No point in going any further until you confirm whether any windows in back wall above new roof. Let us know and will finish it of for you ref treatment, the Longhorn Beetle Myth and fixing ledger plates and wall plates. etc.
Read this through a dozen times and you will get the gist of it. Do not think our figures are wrong.
Regards oldun.
Catlad. You do write a load of ****e.
Jeeeezus ... do you own shares in a pine forrest or something
Whatever happened to building economically and expediently?
Those are monster joists for a floor, let alone ceiling joists and even worse for rafters. But there's more ... putting them in a 400 centres and then having them at 4500 long, makes this an OTT roof, much too heavy, much too costly and a job that Popeye would baulk at.
TBH, catlad does have a point. There are 1000's of houses with 3x2 rafters and nothing at all wrong with the roofs.
We have a bizarre situation where todays span-tables say that all these 1000's of roofs are over-spanned, and yet why aren't they all falling down or spreading?
There is nothing wrong with 3x2's if the supports are there, but I'd agree that 4x2's tend to be the minimum spec'd nowadays, but that's probably just our laziness and wanting to get drawings passed rather than spend time calculating and proving that a 3x2 rafter roof will work
Quote Jeeeezus… do you own shares in a pine forrest or something
In timber, No. Other commodities. Yes. Very many.
Quote. Whatever happened to building economically and expediently?
Went out the window 1667 when the London Building Act was introduced and even more so in 1936 with introduction of the Public Health Act. Will not mention 2010 Approved Docs introduced 1985.. Get too wound up. Possibly you are not old enough to know or remember.
Quote. Those are monster joists for a floor, let alone ceiling joists and even worse for rafters. But there's more ... putting them in a 400 centres and then having them at 4500 long, makes this an OTT roof, much too heavy, much too costly and a job that Popeye would baulk at.
Agree, monster ceiling joists and rafters, but if you was to read the post correctly and understood roofs, then you may just see the reason why they are slightly oversized. Rafters at 4500 long.. Okay, so you design roofs without overhang for facia and soffit. We do not feel BC would be very happy with rainwater running down middle of cavity, but will give it a try , and say that woody said it is good practice. OTT roof. Prove it. Do a cost on increased baulk sizes, as against omitting doubling up 4 no joists, purlin struts and wrap round purlin. and labour on purlin Weight of the roof. What’s that got to do with it? Founds will take it. A job that Popeye would baulk at. Quite apparent then woody that you have never done any joisting or heavy roof framing. As you evidently do not know the weights of timber. Will tell you 4500x50x225 weighs in at approx 26.75kg.
Quote, catlad does have a point. There are 1000's of houses with 3x2 rafters and nothing at all wrong with the roofs.
Agree. We have seen a few. Those that we have had to strengthen and take the sag and bow out of often at great expense to the house owner.
Quote. We have a bizarre situation where todays span-tables say that all these 1000's of roofs are over-spanned, and yet why aren't they all falling down or spreading?
We have not been in 1000’s of houses that have these so called 50x75 roofs, so we can not comment, we can only comment on those that we have been asked to strengthen and take the bows out of. However, we do note a lot of sagging roofs in our walkabouts. Possibly they are the ones you mention.
Quote. There is nothing wrong with 3x2's if the supports are there, but I'd agree that 4x2's tend to be the minimum spec'd nowadays, but that's probably just our laziness and wanting to get drawings passed rather than spend time calculating and proving that a 3x2 rafter roof will work
Agree span tables are well loaded and in our opinion over the top, but question is woody, how do we get BR’s approval if we do not work to span tables? Could we say that woody said 50x75 are acceptable. Your answer should have been, get a quote for half trusses with a bottom toe, then again that was not the OP question.
Regards oldun.
In timber, No. Other commodities. Yes. Very many.
Quote. Whatever happened to building economically and expediently?
Went out the window 1667 when the London Building Act was introduced and even more so in 1936 with introduction of the Public Health Act. Will not mention 2010 Approved Docs introduced 1985.. Get too wound up. Possibly you are not old enough to know or remember.
Quote. Those are monster joists for a floor, let alone ceiling joists and even worse for rafters. But there's more ... putting them in a 400 centres and then having them at 4500 long, makes this an OTT roof, much too heavy, much too costly and a job that Popeye would baulk at.
Agree, monster ceiling joists and rafters, but if you was to read the post correctly and understood roofs, then you may just see the reason why they are slightly oversized. Rafters at 4500 long.. Okay, so you design roofs without overhang for facia and soffit. We do not feel BC would be very happy with rainwater running down middle of cavity, but will give it a try , and say that woody said it is good practice. OTT roof. Prove it. Do a cost on increased baulk sizes, as against omitting doubling up 4 no joists, purlin struts and wrap round purlin. and labour on purlin Weight of the roof. What’s that got to do with it? Founds will take it. A job that Popeye would baulk at. Quite apparent then woody that you have never done any joisting or heavy roof framing. As you evidently do not know the weights of timber. Will tell you 4500x50x225 weighs in at approx 26.75kg.
Quote, catlad does have a point. There are 1000's of houses with 3x2 rafters and nothing at all wrong with the roofs.
Agree. We have seen a few. Those that we have had to strengthen and take the sag and bow out of often at great expense to the house owner.
Quote. We have a bizarre situation where todays span-tables say that all these 1000's of roofs are over-spanned, and yet why aren't they all falling down or spreading?
We have not been in 1000’s of houses that have these so called 50x75 roofs, so we can not comment, we can only comment on those that we have been asked to strengthen and take the bows out of. However, we do note a lot of sagging roofs in our walkabouts. Possibly they are the ones you mention.
Quote. There is nothing wrong with 3x2's if the supports are there, but I'd agree that 4x2's tend to be the minimum spec'd nowadays, but that's probably just our laziness and wanting to get drawings passed rather than spend time calculating and proving that a 3x2 rafter roof will work
Agree span tables are well loaded and in our opinion over the top, but question is woody, how do we get BR’s approval if we do not work to span tables? Could we say that woody said 50x75 are acceptable. Your answer should have been, get a quote for half trusses with a bottom toe, then again that was not the OP question.
Regards oldun.
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Yes they do develop character and sag a bit with age but most still work.
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