Building Newbie overwhelmed by extension

kalinka · 16 Nov 2015

Hi All

What a great forum. I am a complete newbie here and in the world of building. Without boring you too much, me and hubby bought or first home and decided to extend at the back to take advantage of the 6m allowance (single storey only). Due to the purchase we are on a very very tight budget and as he works in construction we decided to self build with the help of his best mate and dad (they too have a lot of building experience but in another part of the world.) I read all the building regs and and did the drawings etc. My only question to which I cannot find an answer is do we need a RSJ beam to support the flat roof or not (extension is 6m x 5.40m wide). Asked building control and he said ask your architect...... If I do need one how do I calculate what size do I need. Thanks a ton
Kate

Brad Ninch · 16 Nov 2015

Unfortunately the good old days of "deemed to satisfy" tables are a thing of the past. Your Building Control officer will almost certainly want to see structural calculations by a qualified engineer. An architect may be able to do it but so much hangs on Professional Indemnity insurance these days the architect may not be covered for structural calculations. The structural engineer will not only let you know what size steel is needed but will also ensure that the foundations of the wall and the pads that the steel sits on are correctly specified. These professional services are pretty hard to avoid under current Building Regulations and one has to take comfort from the fact that they are ultimately for your safety. Best wishes.

tony1851 · 16 Nov 2015

If you are talking about supporting the flat roof, you would just run 9x2 joists across the width (ie the 5.4m width).
That way you would not need a steel beam.

If you are putting a lantern in, you could still use timber joists for the trimmers, but ususally they have to be doubled (and, depending on the span) occasionally tripled up.

If you are taking out the existing rear wall to open up into the extension, that is a different matter and you would need some
steelwork there.

Cabbyfan · 16 Nov 2015

Calcs are not that expensive to get done and if you are lucky and get on with your Building Control people sometimes they will advise the size. Mine did when I knocked down the wall between my kitchen and dining room. Just make sure they email it so you have a record, or at the every least email to them confirming what they agreed.

tony1851 · 16 Nov 2015

Cabbyfan said:
Calcs are not that expensive to get done and if you are lucky and get on with your Building Control people sometimes they will advise the size. Mine did when I knocked down the wall between my kitchen and dining room. Just make sure they email it so you have a record, or at the every least email to them confirming what they agreed.

An inspector's job is to check compliance with Building Regs, not to freely offer advice on structural matters.
Yes, occasionally inspectors do this, but it's not their job, and any inspector taking on this responsibility is going outside the council's powers.
If something went wrong, where would the blame lie?

Cabbyfan · 16 Nov 2015

Hi Tony, quite agree but on some occasions they are happy to advise and as they are the ones who do the sign off I am not going to argue.

RonnyRaygun · 16 Nov 2015

Download the Steico joists technical manual and use their load span tables to size your roof joists. The supplier will also size trimmers for you if you send them drawings.

cAtLeYx · 17 Nov 2015

There are many previous threads debating if a BCO can or can't offer structural advice. I personally have never met one that would put his preverbal on the block and is only ever there to ensure the works are compliant with designs / ACOP's. Whereas others have advised otherwise. Perhaps its a regional thing or acceptable on small domestic projects.??

RonnyRaygun · 17 Nov 2015

cAtLeYx said:
Perhaps it's...acceptable on small domestic projects.??

That's probably what it is - some BCOs probably actually want to help out the home owner or small domestic builder, so where they have seen something a hundred times before they will give assistance with design. On large developments less likely as the developer will have their own engineer / consultant who should take responsibility for the design.

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Building Newbie overwhelmed by extension

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