Single storey extension help

[cruddler]

Hi,

I'd really appreciate a bit of advice on the feasibility of our project and any approximate cost if possible.

We are looking to install 23m2 (6.6x3.5m) mainly glass single storey extension on the back of our French stone farmhouse as per the pics if I managed to include them correctly.

First of all, with regards to opening up the back wall of the house, ideally we would like to take out what is currently a patio door and a window to create a full width opening spanning 5m. We have had 2 local Macons quote for this and both believe that we will need a central supporting column of about 60cm diameter. Is this really the case? Would a correctly sized 5m RSJ/UB really not be sufficient to support the back of the house? For comparison sake, roughly how much would labour and materials for this aspect cost in the UK?

Likewise, for the garden side of the extension, would a single RSJ/UB be suitable to span the approx 6.5m above the patio doors? And also one of 3.3m for the return side? How large would the column have to be to support these 2 beams where they meet?

As for the roof, from what I've seen online, a standard warm flat roof procedure in the UK is something along the lines of 150mm joists followed by timber deck board, a VPL, 150mm ish of PIR followed by a membrane etc amounting to a depth of no more than 35-40cm. Does this sound about right and is that suitable for an extension 6.6x3.5m? Would the joists sit within the RSJs/UBs?
The main stumbling block here and the reason for the questions is that Carpenters don't seem to want to consider any kind of flat roof and the stone masons want to use concrete block resulting in a roof depth of around 70cm all in which is far too thick for our project.

Finally, what would be the rough cost for installing the 6.6x3.5m concrete slab in the UK?

Sorry for all the questions but any help would be most appreciated as we are on the verge of abandoning our project out of frustration.

Thanks

p.s for anyone wondering, we originally proposed a stone extension in keeping with the original house but the planners would only permit a contemporary structure with as much glass as possible..

 

[tomfe]

Anything is possible with the correct people and the correct amount of money!
6.5m is long but it's not that unusual in the UK to open the back of houses up and span larger distances than this. Depending on the construction of the original house it may be a pita to do especially is it's rubble infill etc.

Columns don't need to be that big as the weight is in compression but you'll need to make sure you have enough lateral stability but as both ends of the beams as be restrained you should be alight.
Your warm roof is fine and the joists can sit in the ub minimising the head of the windows/door.

6.6x3.5 slab around 1k but you'd need foundations, insulation, base hardcore etc.

 

[tony1851]

both believe that we will need a central supporting column of about 60cm diameter.

Yikes!..... that would support the Titanic with more to spare.

A 5m span within the rear wall is nothing - one of the 203- or 275- deep UC sections would do.
One thing you will have to watch is the bearings - stone walls are notoriously unpredictable in workmanship and you need to ensure there are suitable padstones.
You really need to speak with a local SE - they will know the local building methods.

 

[Benway]

It's dooable. 7.5m and 5.5m openings.

 

[cruddler]

Thanks for all the replies and sorry for the delay in responding.

I'm glad to hear the consensus is that a 5m beam will support the back of the house without a central column. The walls are 50cm stone but I can't guarantee that they aren't rubble filled so I did wonder about their ability to support the beam. Would padstones be enough or might framing the beam with steel uprights be better? (if I've understood what I've read correctly...)
And yes we are looking to engage a structural engineer to give it the once over but they are hard to find in darkest Burgundy.

Just so that I can get a rough idea of what this would cost in the UK, does the following look about right?

Purchase of 5.5m, 6.5m and 3.5 Beams - About £1k?
Taking out wall and installing 5.5m Beam - About £2-3k?
23m2 slab flooring (I have 10cm of Celotex already on site) - About £1k?
Pillar to support at junction of 6.5m and 3.5m beam (in concrete?) - About £1k?
23m2 warm flat roof materials and construction (15cm Celotex already on site) - About £2-3k?

Is there anything I've forgotten? I have various quotes for the windows around the €10k mark

Thanks again

 

[tomfe]

Without SE input it's hard to say as slab might end up being a lot more.
The UBs over here would cost around £500 if there is any fabrication then the 1k as you said.
To install them probably a fair bit more than 3k as its stone not brick.

Roof will cost a lot more depending on covering.

 

[Grantx]

The cost of glass extensiomns is enormous. Do searches on google for English sites/installers. Have you got architect's drawings?