What size RSJ?

[martyc]

Hi Static...my architect did.
I wont go into the detail here but he has let me down a few times and there are flaws in the design that im only realising after getting planning.
Im in ireland and the only person that comes to inspect a build is the bank to make sure you havent squandered their money before they give you some more!
Yeah you can hire a SE but i know what he'll say...dig everything up and lay new footings...i had told my architect that i dont have the cash to do this and he came up with the option of supporting off front and back wall (which have good footings)...however the purlin he suggested running the 7M seems V light and im looking @ other options as i want it to last.
Marty.

 

[Static]

Ah good old Architects...
To get planning permission your scheme doesnt have to work structurally, it just have to look acceptable.

Ok let me see if ive got this, single storey extension, timber roof with loose pitched rafters onto a midspan purlin with ceiling joists.

Well first of all the ceiling joists are inadequate for the spans, they should either increase to 47x145 or reduce the centres to 400mm. Timber purlin size is woefully inadequate should be more like 75x300 or a 203UC46 as others have suggested. You will also have delivery and lifting issues with such a long member. The other problem is
Have you considered doing the roof in trusses.. get a truss manufacturer to do the calcs and plans. Of course that option means you support the roof from the side? walls and not the gable which i believe are your "good" ones.
What makes you think these footings are inadequate? The purlin doesnt completely remove all load from these walls by the roof.

 

[martyc]

Your spot on...planning here is more about how it looks and fits in with surrounding buildings.

Its a single block building @ the minute...and the single block has supported a V heavy asbestos roof for 50 years...so I have no reason to believe the footings are inadequate.
its 24' * 10'6" in all(rafters running the 10'6" beam or whatever running the 24')

Now as you know "planning" is what i have to adhere to more so than any building codes(yet i want to make sure this is sound and safe)..so the outline drawings must be adhered to....bottom line or it will all have to come down...the structure inside i can do what i like!
This includes moving an existing gable window to a set height for my new roof to abut just below @ its ridge to give me adequate pitch.
The other side of the "lean to" is a parapet arrangement with the guttering in line with my joists...so my joists dont run fully to this boundary wall...this is why/where i need my support.
If i change any of this to get my joists to the "good" wall it will effect my pitch(which is already only 20 deg.)...a bigger rafter would even change my pitch as i cannot raise the ridge due to "planning"!

I have looked @ all my options now and im heavily leaning towards building a timberframe load bearing inner leaf wall(gutter goes between inner and outer leafs)...and then no need for purlin.
Im hoping the existing foundation for the current outer load bearing wall comes inside the building enough to give a goot footing for the timberframe...although i wont dig to it ill just put the frame on the floor, i will get the drill out tonight and see what concrete i have!

Trusses not an option as i want the small attic space as storage for bulky items...mostly surf boards and sails etc..all v light but bulky stuff.

I have looked @ lots of other roofs around and 4*2 rafters @ 600 c/s is common practice...in fact my own house has 4*2 rafters and joists and its 50 years old and its perfect up there!

Anyway someone else had said i should go 400 C/s on the timberframe wall as it will be load bearing...does this automatically mean the joists and rafters should follow the 400c/s ?

Also i hope i havent offended any architects...there are some great ones out there but when you live in a country where there are no BCO's its inevitable youll have a few dodgy guys and i obviously got one of them!

Marty.

 

[freddiemercurystwin]

It always suprises me why people expect an architect to know about structures, don't get me wrong, this guy sounds like a knob and should have advised you that you needed an engineer which you now know anyway (but are still not prepared to do). You wouldn't go to an engineer and ask him to get you planning permission. One of the reasons the tables that allowed people/architects to size their own timbers were taken out of the regs in the England was because there were too many failures. It would appear that you haven't actually spoken to an engineer and are just sumising what they may or may not say, in reality theres more than likely a practical and cost effective solution which you may not have even considered but you'll never know if you don't ask.

 

[Static]

In the end of the day its upto you what you put in..

So the "purlin" is at the low end of the lean to wall and completely support 1/2 the roof? IF you go for a support there make it a steel (203UC), timber wont cut it not without intermediate support.
Stud wall is an option yes, i would at the very least double up the studs, make them 400c/c.
Without mid support your roof rafters are quite small. 4x2 rafters is common practice.. well yes it is but they also have purlins at mid span that break the span down to about 2m rather than the 3.2 you have. The size your specifing is too small for that span and will sag over time. The smallest i would use would be 38x125 @ 400c/c (grade C24).

 

[martyc]

One of my mates is a SE.
He has looked @ it and told me what any other SE would...dig the whole lot up, put down new footings and build a cavity block wall, which aint practical @ this point in time.(especially as it would involve tearing up some of my neighbours nice new drive)

This is why i am asking here...looking for practical advise/solutions from builders etc.
At the end of the day an engineer will only advise you what they 200% can guarantee and i know that myself...bit of a belt and braces approach!
Im an engineer myself (electronics yawn!!)so i understand why they must take this approach and thats fine...but more often than not they are "over engineers"

The roof on my house has no purlin and the span must be ~15 foot.
Ill measure it when i go home just to see and ill report back...thinking of it though i think the centres may be more like 400.
The timberframe wall id like to build will be totally sheeted on one side with 18mm OSB so should be quite a strong unit.

Yes the purlin was at the low end of the rafters and essentially the design has the joists hanging from the rafters and not supported from below @ the low end.
The purlin would support half the weight..the pole plates/ridge boards would take the rest.

Cheers,
Marty.

 

[Static]

Good luck.. sorry i couldnt help

 

[KKnocker]

hi martyc

i'm interested in the regulations in Eire, i know that irish building regulations exist but from what you say there is no-one inspecting to enforce them.

Do you need to get any certification to say the works are built to satisfy the regulations?

Is an engineer or architect needed to do this or does a fire officer have any involvement?

 

[martyc]

Hi knocker,
The main problem here if building is planning...can be v hard to get and often with no meaning or rational reasoning or consistency.
My mate spent 5 years going thru the motions appealing to the v highest level(an boird pleanala) on a site while similar houses popped up either side of him...sometimes its just ridiculous.

Regarding regs for building...the only one that looks @ your house is the bank!..you do need perculation test done though prior to getting planning.

You can of course involve an engineer to sign off on things but this is rarely the case for a self build...it would be your own undertaking.
Most SE work as far as i know would be with contractors.
I havent done a self build myself yet and believe this to be the case from listening to others who are building.

Anyway drilled a few holes in my foundation and in my concrete where my timber wall was to go...2" concrete and a 3" foundation under the current load bearing wall!!
So im going to cut away a foot of the floor...leave the existing block wall as an outer leaf/cladding wall put in a foundation for my new inner wall and build up...safest bet,
Thanks all for the tips tho!
Cheers,
Martin.