Building directly off boundary wall

kev1234

I am at the tender preparation stage of planning for an extension and refurb of a 3 bed semi detached in Dublin. The neighbours house already has an extension built to their rear which seems to be onto/over the boundary. It looks as though they built the extension and then a wall right up against it with a parapet.

This parapet wall is on the boundary, i.e. comes onto our property. Therefore in building our extension could we build directly off this wall as opposed to putting up a new wall next to it.

See attached image of the neighbours wall.

https://imageshack.com/a/img924/3795/kz46KJ.jpg

I want to know if we could potentially plaster the existing dash wall on the boundary rather than put up a new wall inside of it. Putting up our own new wall means stepping in from the existing house which I would prefer not to do as it it lost space. Any advice or experience with this?

BryanF

Thermal Bridge at top of parapet to deal with

kev1234

BryanF wrote: »

Thermal Bridge at top of parapet to deal with

I wouldn't be building up to the very top of the wall though so would this still be an issue? My flat roof would be built to just under the parapet. Top of the parapet is at 3.23metres. I would then look to tie the new roof into that existing wall. Need to look at the detailing of this though.

I'm not sure of the legal obligations to building directly off this wall though. I've had an architect and structural engineer out to discuss the proposed works and both have stated I might be able to build directly off this but neither were fully committed to saying I definitely could or couldn't so still a bit confused.

BryanF

kev1234 wrote: »

I wouldn't be building up to the very top of the wall though so would this still be an issue? .

yes

kev1234

BryanF wrote: »

yes

How would there be a thermal bridge at the parapet if I construct the new roof beneath it though? What if I plaster and insulate the existing wall and then install a warm flat roof adjoining into it below the parapet?

BryanF

If you dryline you can avoid the thermal bridge.

James 007

You could also run your roof timbers parallel to this wall thus ensuring no load is transferred onto this wall.

Gumbo

Confirm the wall build up and fo7ndation to ensure it’s capable of taking your extension. Engineer on site during trial hole exploration.

Are you building single storey?
What’s the proposed roof style?

I would personally insulation the wall too.
In theory this should be 100% possible and will look a lot better than if both sides built on their own side. Just make sure your working methods don’t damage, disturb their finishes internal through wall plate fixing or roof rafters fixing etc

kev1234

kceire wrote: »

Confirm the wall build up and fo7ndation to ensure it’s capable of taking your extension. Engineer on site during trial hole exploration.

Are you building single storey?
What’s the proposed roof style?

I would personally insulation the wall too.
In theory this should be 100% possible and will look a lot better than if both sides built on their own side. Just make sure your working methods don’t damage, disturb their finishes internal through wall plate fixing or roof rafters fixing etc

Thanks all for the advice.

Yes, I'm building a single storey extension with a fibreglass flat roof system.

I am going to dig a trial hole and have the structural engineer investigate. Also I am going to speak to the neighbours to see if they have details of the wall build up. It looks like they built their extension and then added an external block wall with parapet to this. I will post back when I have these further details. Thanks.

Calahonda52

http://www.irishstatutebook.ie/eli/2009/act/27/enacted/en/print#part8-chap3

Chapter 3

Party structures

Interpretation of Chapter 3.

43.— In this Chapter, unless the context otherwise requires—

“adjoining” includes adjacent;

“adjoining owner” means the owner of any estate or interest in a building or unbuilt-on land adjoining that of the building owner;

“building” includes part of a building;

“building owner” means the owner for the time being of any estate or interest in a building or unbuilt-on land who wishes to carry out works to a party structure;

etc

tradesman

I would be inclined to build independently of this wall as if it was not there. Own new foundations & just flash the new roof into the boundary wall. At least you know you are building on a solid foundation

kev1234

tradesman wrote: »

I would be inclined to build independently of this wall as if it was not there. Own new foundations & just flash the new roof into the boundary wall. At least you know you are building on a solid foundation

Yeah I know in building our own wall there is the security of mind in ensuring the building integrity but its annoying me that if I go with a 350mm cavity wall on the extension and will need some working space between the existing wall (presumably at least 100-200mm?), that the wall of the new extension steps in approx. 500mm from the existing internal wall. It's a fair amount of space to lose and narrows the area of the extension more than I would have liked.

I'll see what the trial hole and discussions with the neighbour brings up with regards to structure.

Calahonda52

James 007 wrote: »

You could also run your roof timbers parallel to this wall thus ensuring no load is transferred onto this wall.

OP
I don't know if you picked up on this suggestion.
If you read the detail in the link I posted earlier, the "talk with my neighbour" may not end well: there may be a row as to where the wall is built and they may claim its all their wall: .... I have the jersey...I have seen cases where fixings on such a wall were limited in depth to the boundary line

The trial hole may not reveal anything useful, it may show evidence of a 1/3..1/3..1/3 traditional foundation, but even if it does, it may not support a further wall on the 1/3 on your side, so in fact your foundation may start another 300mm in.....

I would not sign off on an additional load wall on such a 1/3, I value my PI..

Depending on the span I would suggest using slightly over sized full web I joists to limit the deflection and twisting you will get with normal graded timber
eg
http://www.woodconcepts.ie/store/i-joists

kev1234

Calahonda52 wrote: »

James 007 wrote: »

You could also run your roof timbers parallel to this wall thus ensuring no load is transferred onto this wall.

OP
I don't know if you picked up on this suggestion.
If you read the detail in the link I posted earlier, the "talk with my neighbour" may not end well: there may be a row as to where the wall is built and they may claim its all their wall: .... I have the jersey...I have seen cases where fixings on such a wall were limited in depth to the boundary line

The trial hole may not reveal anything useful, it may show evidence of  a 1/3..1/3..1/3 traditional foundation, but even if it does, it may not support a further wall on the 1/3 on your side, so in fact your foundation may start another 300mm in.....

I would not sign off on an additional load wall on  such a 1/3, I value my PI..

Depending on the span I would suggest using slightly over sized  full web I joists to limit the deflection and twisting you will get with normal graded timber
eg
http://www.woodconcepts.ie/store/i-joists

Thanks for the advice. Yeah I know that talking with the neighbour does not always go well but I figure it is best to try this first and aim to have open discussions with them to find a suitable solution. 
The house on that side is actually a care home for kids owned by a department/company belonging to the Mater Hospital. We had preliminary discussions with them to notify them of our planned works and they had no objections so hopefully they are receptive to our proposals on this aswell, time will tell.
I don't fully understand your comment on the foundations? I am looking to plaster and insulate the existing wall on the boundary and use this as the internal wall of our extension. I am not looking to add a new wall onto their existing foundations. Perhaps I've misinterpreted your comment here though.

Also can you explain what 'full web' I joists are? Thanks.

Calahonda52

open web http://www.roof-truss.ie/Engineered_Open_Web_Joists.html
full web you have already
okay re using their wall as the internal, just need to ensure it has a dpc

its only if you were building a new wall, you would need your own foundation, starting maybe another 300 in from the wall