Structural advice - approx costings

[ban-all-sheds]

I'm planning a single storey extension on the rear of my house (semi-detached), pretty much like this (grey walls = existing house)

My question is about options to do with the existing opening shown with the dotted red line in the sketch. In the other, deeper half of the extension the existing opening for the back door will remain a doorway, but on the side with the wider opening, which currently has patio doors, it will end up a through room and I would like to have minimal intrusion at the side from a pier, and nothing protruding below the ceiling.

Viewed from the inside, the elevation of the existing rear wall and opening is this:

The opening is essentially as it was when the house was built nearly 60 years ago. I've not removed the plaster to check, but from my experiences of trying to drill into the wall above the opening I reckon there's a concrete lintel there.

Above the opening is the rear wall of the upper floor, which is 9" brick. and then a hipped roof.

What I want is to do away with the lintel, and part of the return on the LHS, and have a steel put in:

The idea being that the joists (which do run in the right direction) would be extended and would sit in the web of the steel, thus giving me a ceiling running from the existing room through to the extension with no visible beam across it.

SO - questions at last.

1) What, approximately, would this be likely to add to the cost of construction compared to doing nothing, leaving the opening as it is, and putting up with a 420mm protrusion below the ceiling? Not looking for free design work here, just trying to decide if I think it is worth doing, and therefore if I want to engage an SE to do the calcs.

2) The existing joists (6x2s) would need extending - not sure by how much (it's been a while since I had the boards up in the room above), but at a guess by around 300mm. Is it feasible to do this by sistering? Would not relish having to have them replaced.

 

[jeds]

Providing the masonry is sound this should be straightforward. Extra-over for taking the brick down is half a days work and making good. Couple of hundred maybe plus materials? Cost of making the beam flush with the ceiling is probably the same. It's more fiddly and a bit of messing about but on a small beam like that it won't be that bad. I'd be surprised if many BCOs would want a full set of calcs for such a small beam. The O/A loads are going to be so small it's only going to be a 127x76 or a 152x89 at most. As for extending the joists, it depends on the span but in most situations you'll need to overlap about 750mm and double bolt. Then add a load of Paslode nails as well. I would also add solid blocking as close to the ends as possible to reduce torsion.

 

[ban-all-sheds]

Thanks.

No reason to think there's anything wrong with the brickwork.

Joist span is about 3.7m.

I guess once I've got more ducks in a row I could run it by Building Control to see if they want calculations - I'm assuming the same will apply to the small return on the side wall.

 

[Jason1790]

BC will probably want SE calcs for the small return and possibly the lack of return on the centre wall so worth a chat with them first. I did something similar (2 storey semi) and BC said they wouldn't need calcs as long as I used 7" RSJ's and minimum 650mm returns.

 

[ban-all-sheds]

Lack of return on which wall, this one?:

Hadn't thought about that.....

 

[Jason1790]

Lack of return on which wall, this one?:

Hadn't thought about that.....

Yes that one, it'll reduce your patio doors down to about 1680 maximum. I had a similar problem with mine, though the gap was smaller. I ended up with a wide door and glazed unit. BC might pull you up on the amount of glass you want, ours didn't want more than 25% total wall space, but if you can up the U values elsewhere they might listen.

 

[ban-all-sheds]

OK - maybe it's time to decide on Plan B.

Firstly, we are really hoping to re-use the existing french doors - they look nice, they are good quality, and were far from cheap. Hopefully we'll be able to reach an agreement with BC what U-value to assume for them.

However, and this is where Plan B comes from...

They actually look like this:

i.e. there is a narrow window each side (couldn't have the doors any wider). At the moment the returns each side are where the curtains hang when pulled back, but clearly with the plan above this would be a problem - where would they go? So we had been thinking that, sadly, we might have to lose them and just have the doors. (Sadly mainly from a £ POV, as it would mean having to have a new frame made, but we'll tell ourselves that it would probably not have been possible to get the old one out undamaged anyway )

So......

If we only use the doors that would narrow the new opening and allow a return each side:

...650mm returns.

Where is that measured? Are the returns in my drawing above 420mm or 740mm?

 

[Jason1790]

The returns on your drawing are 740mm.

650mm from the furthest corner so the minimum internal length of the return on a 300mm cavity wall would be 350mm if that makes sense. As I said I have no SE calcs or regs to back me up, just that my building inspector was happy as long as I adhered to his stipulations. I should add that the 7" RSJ was two bolted together with spacers for 300mm wall.
Shame about the doors, perhaps you could sell them to offset the cost of the replacements?

 

[theoldun]

A P5 pier is a return nib on a 90 degree corner Without being to technical a P5 pier should be a minimum of 665mm external (three bricks) and not exceed 2100 in height..
All your return piers appear to comply with the possible exception of the internal one, how ever read ADA page 29 and you can drive a coach and horses through the regs regards internal angle pier.
On the information posted, no chance of giving approx costing.
Regards oldun

 

[ban-all-sheds]

All your return piers appear to comply with the possible exception of the internal one

Which internal one?

 

[wedginald]

The 25% rule is for additional glazing over the existing unless things have changed recently so the doors should be fine, you could always get a rated units to fit the frame if they cause a fuss. If they have been installed in the last couple of years I would hope that they would be a rated anyway

 

[Jason1790]

All your return piers appear to comply with the possible exception of the internal one

Which internal one?

The only one I can see from the measurements you give is the one you want to reduce from 580mm to 385mm on your existing wall. If you changed your proposed new stud wall to a 300mm cavity you would achieve the return you need.

 

[Justin Passing]

read ADA page 29

ADA?? I Googled but couldn't find it

 

[tony1851]

read ADA page 29

ADA?? I Googled but couldn't find it

Google 'Approved Document A'.

 

[ban-all-sheds]

The only one I can see from the measurements you give is the one you want to reduce from 580mm to 385mm on your existing wall.

Sounds like you're talking about this one?

If you changed your proposed new stud wall to a 300mm cavity you would achieve the return you need.

But the new stud wall is on the other side of the opening...

The one ringed above is the rear wall of the house - on my neighbour's side the opening is the original, so externally the section indicated by the dotted pink line is 1100--1200mm.