Repairing Overkill Chases in Blocks

[Gaf]

Have two different issues with chases done to blocks.

 

#1: Single leaf internal wall (100mm) holds one edge of a precast concrete floor. Chased to within an inch of its life. One 1.5m section has seven vertical chases plus a 100cm x 10cm horizontal chase at mid height. Chases are 30mm deep. One chase was done back side also so they left only 4cm of block in that section. 
 

#2: Inner leaf of external walls got some funky chases (walls are 100mm block, 150mm cavity, 100mm block). By funky I mean a 10cm wide x 3cm deep chase floor to ceiling on a 40cm end of wall pillar to accommodate for just a single cable, a really messy handheld kango chase at a 30 degree angle that’s 4cm deep in places and 20cm wide, and a more normal looking neat but curved chase floor to ceiling. 

 

Main issue is repair versus replace versus leave them.

 

One structural engineer said he’d pass them based on his visual inspection but he was there for a different reason and it was a 10 second look at them. Second structural engineer initially said the same, but he did calculations and said they don’t pass so need to be repaired. He provided a repair method using a certain repair mortar but the manufacturer said they don’t recommend the use of their repair mortar for chase repair because the 30-40 N/mm3 is too high compared to the blocks so risk of debonding and cracking.

 

There doesn’t appear to be a low N/mm3 repair mortar, just general M4/M6/M12 general purpose mortars that I believe aren’t suitable because they can shrink too much. Engineer wanted a low shrink repair mortar to ensure no debonding.

 

Engineer now agrees with the manufacturer and said best just replace all chased blocks since other repair mortars are even higher N/mm3.

 

Blocklayer who has been doing it since he was in the womb said he doesn’t recommend replacing the blocks as all of the sections are load bearing. He said toothing in new blocks will never be as strong as original blocks, one reason was there will be no compression on newly toothed blocks so they don’t form as strong a bond. Also said he’ll never get the blocks up as tightly under the precast floors as the original install and he’ll be hammering in steel shims that will never be a good enough job and may lead to settling / cracks when the acrows are removed. Blocklayer saying repair surely is better method. Engineer was somewhat differing to the blocklayer on how to get the job done, so we’re a bit stuck on how to proceed.

 

Wondering if anyone has been in a similar position and how they repaired chases or replaced blocks in load bearing walls. The house is fully built at this stage.

 

 

[Mr Punter]

Not sure what the blocks are, but it may be best to render over these.  You may need to prep by cleaning off dust and giving a coat of rend aid.

[Gaf]

Standard 100 x 215 x 440 concrete blocks. 

[ProDave]

And this is why I dislike "plastered on the hard" forcing cables to be chased in.  Much better with a timber batten service void and plasterboard.  

[Iceverge]

Mix up mortar 4:1 sand cement and trowel it into the chases where they're too deep.

 

Maybe easiest done when the electrician is fitting conduit and push the conduit into the wet mortar. 

 

I know you're concerned but I wouldn't be too worried so long as everything is plumb. 

 

 

 

 

 

 

[Spinny]

I had to have a stone block replaced at a corner after the builder broke it with an SDS drill. The load on the steel lintel above was propped and then the stone block removed and replaced.

 

It sounds like you have different answers from different pros. On another issue I had the builder, structural engineer, and architect all giving different guidance with an issue - it is very annoying as a customer. As it was a new build issue I wrote to the architect setting out the different viewpoints and defining the different options proposed. I also stated my priorities - in my case certainty over the builder being able to do it successfully, and structural strength over loss of insulation. The architect then replied agreeing to compromise the insulation and proposing some insulation on the outside. So we had a documented way forward.

 

As your issue is NOT new build, I'd suggest a formal written statement/agreement from your structural engineer is needed covered by their insurance. What you don't want is a structural issue later and no recourse because it wasn't signed off by a qualified structural engineer.

 

I am not saying it is happening here, but sometimes tradies just talk their own book e.g. quick and easy job to fill, PITA costly and delaying job to replace blocks.

 

Obviously it is standard practice to insert lintels to create whole new openings underneath. Yes there can be some shims involved, and someone who is a builder can comment on how well blocks bed down onto the top of the lintel. I think there can be some settlement ? Has the structural engineer considered the insertion of a lintel or two into the wall to bridge the compromised blockwork ?

 

PS I aint a builder or structural engineer, just one builders mug = customer.

 

Make sure you get a solution that should stand up for an insurer with an SE's name on it.

 

[Gaf]

On 08/04/2026 at 11:00, Iceverge said:

Mix up mortar 4:1 sand cement and trowel it into the chases where they're too deep.

 

Maybe easiest done when the electrician is fitting conduit and push the conduit into the wet mortar. 

 

I know you're concerned but I wouldn't be too worried so long as everything is plumb. 

 

 

 

 

 

 

 

Yeah the engineer past it on visual inspection. The repair method at that stage was simple (fill chase with repair mortar) and the chases were exposed so we reckoned why not just get confirmation and do the repair now. Then the repair approach became difficult.

 

Those chases are there now 1.5 years with partial loading above (bathrooms all tiled). No cracking was visible. No floors, sanitaryware, or furniture installed.

 

Spoke to second manufacturer who said same as other one. Repair mortars have too high N value and risk cracking/debonding. Recommends the repair advised by engineer just not with that high an N.

 

[Gaf]

On 08/04/2026 at 12:33, Spinny said:

I had to have a stone block replaced at a corner after the builder broke it with an SDS drill. The load on the steel lintel above was propped and then the stone block removed and replaced.

 

It sounds like you have different answers from different pros. On another issue I had the builder, structural engineer, and architect all giving different guidance with an issue - it is very annoying as a customer. As it was a new build issue I wrote to the architect setting out the different viewpoints and defining the different options proposed. I also stated my priorities - in my case certainty over the builder being able to do it successfully, and structural strength over loss of insulation. The architect then replied agreeing to compromise the insulation and proposing some insulation on the outside. So we had a documented way forward.

 

As your issue is NOT new build, I'd suggest a formal written statement/agreement from your structural engineer is needed covered by their insurance. What you don't want is a structural issue later and no recourse because it wasn't signed off by a qualified structural engineer.

 

I am not saying it is happening here, but sometimes tradies just talk their own book e.g. quick and easy job to fill, PITA costly and delaying job to replace blocks.

 

Obviously it is standard practice to insert lintels to create whole new openings underneath. Yes there can be some shims involved, and someone who is a builder can comment on how well blocks bed down onto the top of the lintel. I think there can be some settlement ? Has the structural engineer considered the insertion of a lintel or two into the wall to bridge the compromised blockwork ?

 

PS I aint a builder or structural engineer, just one builders mug = customer.

 

Make sure you get a solution that should stand up for an insurer with an SE's name on it.

 

 

Thanks for this. Useful to hear from someone who has been through a bit of repairing. This is a new build (but this was noticed only when it was close to final completion. What you're saying had crossed my mind... since there have been plenty of renovations going on where walls are knocked and lintels/rsjs inserted, can it be that difficult to get a tight enough fitting of new blocks at the top.

 

I hadn't considered your idea of the lintel. I'll run that by him and see.

 

We're considering a hybrid approach but will need engineer approval. A crew who specialises in heritage building repairs suggested we use helibars and resin across the cahses, as well as the repairing of the chases.