I am about to start on a small extension to my house. The original house walls are rubble stone about two feet thick and as usual on this type of cottage have little in the way of foundations. The new wall will butt up against the old, and of course will have proper foundations. Any tips on how they should be tied together? I know you can get special wall plates to mount on the old wall and tie into the new, but these seem to be more for brick to brick rather than old stone to new construction. I am worried that there will be a difference in movement between the two - won't the old, lime mortar heavy stone wall with no deep foundation naturally move more than the new properly dug foundations and cause a problem on the join? Or is mastic used between them to allow for a little movement? Any ideas/suggestions/solutions?
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New walls butting on to old
- Thread starter geoff
- Start date
Gervase
Member
- Messages
- 1,500
- Location
- North London
when joining new to old you have to accept that the joint - for the first couple of years at least - will probably be sacrificial.
You generally will get cracking. Your old wall has had a century or more to settle and has largely finished moving, but now you've dug a gurt big trench almost up to its foot and poured a load of concrete into that. The dynamics of the soil have changed, and that old wall is going to shift a bit. Only a few millimetres if you're lucky, but some movement is likely.
Your new wall is also going to shift a bit. Assuming your foundation is twice the width of the wall on top, and that's a couple of metres high, imagine the loading on the soil. Unless it's on solid hard rock, you're going to get some movement in the new wall, too.
The movement can be minimised - or at least the movement of the two parts 'choreographed' - by using galvanised steel tie bars in the core of the new wall goiong through the face of the old, and by carefully matching new stone to old - matching courses wherever possible and setting the ends of the new into the existing wall (chipping out existing stone to accept the new stuff if necessary).
It's fiddly work, and you'll need to accept that it will probably need a bit of attention after a year to consolidate any cracks and tidy up the join once your work has settled.
All this is on the assumption that the new wall is also stone or stone-faced. If it's blockwork, then smaller tie bars - the flat perforated sort used to hold a ring-beam onto the walls - are one answer, grouted into the old stone with epoxy or a strong hydraulic mortar so they lie in the mortar bed of every second block. The gap can then be filled with standard brickies' expansion mastic as neatly as you can - or with a non-hydraulic lime mortar mixed with coarse aggregage and bits of crushed stone.
What sort of finish will the new wall be having?
You generally will get cracking. Your old wall has had a century or more to settle and has largely finished moving, but now you've dug a gurt big trench almost up to its foot and poured a load of concrete into that. The dynamics of the soil have changed, and that old wall is going to shift a bit. Only a few millimetres if you're lucky, but some movement is likely.
Your new wall is also going to shift a bit. Assuming your foundation is twice the width of the wall on top, and that's a couple of metres high, imagine the loading on the soil. Unless it's on solid hard rock, you're going to get some movement in the new wall, too.
The movement can be minimised - or at least the movement of the two parts 'choreographed' - by using galvanised steel tie bars in the core of the new wall goiong through the face of the old, and by carefully matching new stone to old - matching courses wherever possible and setting the ends of the new into the existing wall (chipping out existing stone to accept the new stuff if necessary).
It's fiddly work, and you'll need to accept that it will probably need a bit of attention after a year to consolidate any cracks and tidy up the join once your work has settled.
All this is on the assumption that the new wall is also stone or stone-faced. If it's blockwork, then smaller tie bars - the flat perforated sort used to hold a ring-beam onto the walls - are one answer, grouted into the old stone with epoxy or a strong hydraulic mortar so they lie in the mortar bed of every second block. The gap can then be filled with standard brickies' expansion mastic as neatly as you can - or with a non-hydraulic lime mortar mixed with coarse aggregage and bits of crushed stone.
What sort of finish will the new wall be having?
It'll be a block wall rendered to match the existing extension (single story) which the new bit is extending, if you see what I mean. I was worried about the stresses if I tied them together too firmly - in case the new wall, more firmly seated because of the foundations which building control will want, cracked if the old wall moved and its sheer bulk in moving caused problems. The existing extension (rendered block), curiously, seems to be on quite shallow foundations - it appears to be simply built on large stones laid in the ground like a stone wall would be. It was done about 40 - 50 years ago, and probably moves similarly to the old walls as they have similar foundations, hence no cracking. Building control would not let you do that now of course.
I think I'll specify it as tied in using the metal brickwork straps and see if they think it's a problem.
I think I'll specify it as tied in using the metal brickwork straps and see if they think it's a problem.