joining blockwork to solid walls

[rwa.martin]

I'm currently building a new, blockwork cavity wall extension that will butt up to my lime rendered stone house. I'm unsure as to the best way to bring the differing materials and construction methods together. How do I transition between lime and cement? Do I use a DPC vertically in the cavity at the join?
Any advice and guidance would be gratefully accepted so I can talk with some authority to my building inspector.
Thanks in anticipation.
Richard.

 

[Tom]

I'm sorry this doesn't really help you but why are you using blockwork and cement? Why aren't you building the extension using materials that match your house? Marrying the materials would be much better and you wouldn't risk damage in the future by the two structures moving independently. Just being nosey, that's all

Jaqy

 

[Pford75]

Jaqy - Understand where you are coming from, but some CO's insist that new additions to listed buildings are constructed in modern materials so that it is clear that they are not part of the original fabric (personally I think that's crazy, but i'm not a CO).

Friends of ours in Essex before we moved to Norfolk wanted to construct their kitchen extension in cob and thatch (same as the cottage) but were instead forced to construct in block & brick with tiled roof.

 

[Hatster]

Hmm - are any of the courses listed from the 'events' section on this website relevant?

 

[ljn]

I removed rather than adding an extension, returning a the doorway to the extension back to a window)
Building Control and the Conservation Officer were in agreement that blockwork would be appropriate. The wall was rendered, so the blockwork wouldn't be visible.
The new blockwork was tied to the old wall with some steel angle ties- apparently Building Control like these.

Where I rebuilt exposed well - the garden wall- they preferred the original bricks plus reclaimed ons to replace missing/broken ones. Mix of lime cement and send for mortar.

 

[Schoolmarm]

.................. and you wouldn't risk damage in the future by the two structures moving independently. Just being nosey, that's all

Jaqy

This is the most important thing.

 

[chuckey]

Are you going to run a new double skin block wall of at right angles to an existing stone wall? or continue an existing stone side wall, that will turn into a modern block wall? The other thing I was thinking about was if the existing building was 2 stories high and made of stone, thats a lot of weight which also has settled for 200 years, so if you tie your lightweight block skins to this building and the new foundations settle, you could end up with the new bit hanging of the old bit with horizontal cracks at every tie point. To overcome this I would go for double vertical DPCs to act as a slip membrane and tie the new wall at every course, so the new wall becomes self supporting.
The other big point is that with either case the old outer skin will become an inner skin in the extension so heat losses will be excessive unless suitablely insulated i.e. try to keep the "old wall" cold cos' 12" away it will be at the outside temperature anyway. Heat conduction through dense material is much higher then through a cavity even an old rubble one.
Frank

 

[rwa.martin]

Hi Frank. The blockwork will be at 90 degrees to the existing stone building. It's single storey only.
My plan was to use a standard wall tie system and not have any vertical dpc either flush against the wall at edge of the two cavities or cut vertically into the original wall to stop penetration of moisture. I was going to use lime at the joint with the old wall and standard cement based mortar for the rest.
However, it was also suggested to me that to try and separate the modern and old materials would be a good idea. This could be done by a vertical dpc sandwiched between the end of the blocks and the wall although it would be breached by the ties.
If I understand your suggestion correctly you're saying to run vertical dpcs between the end of the blocks and the wall as I'd planned but not tie it in; relying on cavity ties at each course in the new wall to provide support?
Thanks for your help - very useful.

PS the reason why I went for blockwork was cost, convenience and benefit of modern materials. Sorry!

 

[Tom]

PS the reason why I went for blockwork was cost, convenience and benefit of modern materials. Sorry!

Oh dear

 

[Schoolmarm]

PS the reason why I went for blockwork was cost, convenience and benefit of modern materials. Sorry!

In which case you might as well ask on a modern build forum. :lol:

 

[rwa.martin]

schoolmarm by name schoolmarm by attitude it seems I'm keen to preserve the structure of the stone building that's why I've come here, not a modern building forum where the knowledge and experience may not exist. Now, perhaps you could allow the thread to focus on the question in hand?

 

[MdB]

Our new wall was modern block also, for various good reasons. Stone construction just wasn't an option.

Linking to the old wall was an issue as the old wall could do with all the support it could get. We ended up filling all the gaps in the old wall with lime so there was a smoothish surface to build the new wall against. Long stainless eyelet screws (no idea what the correct name is) were then put into the old stone and cemented into each course of new block. The length of screws and the smooth surfaces means some that movement can be accommodated. It isn't a perfect solution though and I'm keeping an eye out for cracks. Mixing old style building methods with modern isn't ideal but sometimes it has to be done.

The previous wall, which we replaced, was also modern block and was just cemented to the old wall. When the render was removed, there was a gap between the old and the new from top to bottom where they had shifted apart. One of the reasons why we had damp.

 

[chuckey]

The reason I suggested a double DPC, was that you know exactly where the crack will appear, i.e. at the junction of the two bits of plastic. As this will only be a hairline, a bead of mastic wil be enough to keep the damp out, providing the render on the blocks is kept off the old stone part.
There is a tying systems that utilise, drilling the stones(or their mortar course) putting plastic plugs in, then a stainless bent rod that has a woodscrew thread on its end which is cemented into a course of the new masonary. I have used these and they OK for small areas, but if a new foundation settles the new wall will try to hang of these lightweight fixing devices and I not sure that they would be more resillient then the new mortar/block bond. There are also wallstarter kits. These are stainless steel strips with various tags that can be bent out to be cemented into the new block/brick courses. These strips are again plugged and screwed verttically on to the old wall.

Frank