Block & Beam floor, max overhang into cavity.

[epsilonGreedy]

From previous threads here I understand that the beams of a block & beam floor are typically oversized and overhang into the cavity a little. How large can this overhang be before a beam end needs trimming with a large cutter?

 

The concerns I have are:

 

1. An overhang creates a ledge where dropped mortar can attach and lead to the creation of a cross cavity mortar bridge at which point moisture can transfer into the floor.
2. There is a reduction in cavity width for insulation and hence a larger cold bridge risk.

[Temp]

We had issues with mortar landing on wall ties. BCO spotted it and got the brick layers to knock it off.

 

 

[nod]

1 hour ago, epsilonGreedy said:

From previous threads here I understand that the beams of a block & beam floor are typically oversized and overhang into the cavity a little. How large can this overhang be before a beam end needs trimming with a large cutter?

 

The concerns I have are:

 

1. An overhang creates a ledge where dropped mortar can attach and lead to the creation of a cross cavity mortar bridge at which point moisture can transfer into the floor.
2. There is a reduction in cavity width for insulation and hence a larger cold bridge risk.

Your  below  dpc with the beams so shouldn’t matter about  bridging 

We laid a block on the BB floor then dpc   

[Mr Punter]

The beams don't normally span from outside wall to outside wall, so you may be able to lap them on the sleeper walls.  If you have rigid insulation going down past the beams it would be better to keep the cavity open and trim them back if needed.

[epsilonGreedy]

2 hours ago, nod said:

Your  below  dpc with the beams so shouldn’t matter about  bridging 

We laid a block on the BB floor then dpc

 

 

I will do the same but the beams also sit on their own dpc, the same is true for the blocks that bridge from the beams to inner wall and the 40mm slips on other walls. In effect the floor structure sits on its own ring of dpc then the inner walls and screeded floor has a second dpc/dpm as a backup. I assume the belt & braces design for preventing damp in a beam & block floor is because a house should last 100 years and we don't want a beam & block floor to develop concrete cancer after 40 years like a 1970's bridge over the M4.

 

My concern is that when the beam-end to cavity gap reduces under 50mm due to the overhang things are getting marginal.

[epsilonGreedy]

1 hour ago, Mr Punter said:

The beams don't normally span from outside wall to outside wall, so you may be able to lap them on the sleeper walls.

 

 

My beams sit on the inner walls plus sleeper walls in a few cases and if the whole design and installation was cut and positioned mm perfect I would have full 100mm cavities.

 

I noticed that the floor supplier rounded up my absolute as-built floor plan dimensions to the nearest 25mm so I assume block & beam floors are likely to overhang the inner wall in places. Someone here said this is to be expected.

 

The largest overhangs are where a sleeper wall runs mid room and shorter beams butt end-to-end. In some cases the 25mm excess in each beam is compounded to create a 50mm overhang.

 

I was just trying to establish if there was some industry tolerance beyond which the installer should trim back the overhang post installation.

[Oz07]

As an extra note I like to use a 9" dpc under beams, fold it back-up and then fold floor dpm down onto it and tape. Next course of blocks laid on top of dpm 

[nod]

2 hours ago, epsilonGreedy said:

 

I will do the same but the beams also sit on their own dpc, the same is true for the blocks that bridge from the beams to inner wall and the 40mm slips on other walls. In effect the floor structure sits on its own ring of dpc then the inner walls and screeded floor has a second dpc/dpm as a backup. I assume the belt & braces design for preventing damp in a beam & block floor is because a house should last 100 years and we don't want a beam & block floor to develop concrete cancer after 40 years like a 1970's bridge over the M4.

 

My concern is that when the beam-end to cavity gap reduces under 50mm due to the overhang things are getting marginal.

I ran a 600 dpc around the perimeter for the beams to sit on 

and once the block were in situ I simply pulled it over the beam ends onto the blocks I laid my first inner leaf block onto it Then I put a 600 Hyload dpc on top of the first block and left it flapping down 

One we were watertight I attached the Hyload to the floor membrane 

 

[Mr Punter]

13 hours ago, epsilonGreedy said:

The largest overhangs are where a sleeper wall runs mid room and shorter beams butt end-to-end.

 

Don't butt them, offset one bay by using a block widthways in the first row of one half, so the beams are staggered on the sleeper wall. You may end up with a few odd bits where you can't use full infill blocks but you can fill this with concrete.

[epsilonGreedy]

2 minutes ago, Mr Punter said:

Don't butt them, offset one bay by using a block widthways in the first row of one half, so the beams are staggered on the sleeper wall. You may end up with a few odd bits where you can't use full infill blocks but you can fill this with concrete.

 

 

We followed the design provided by the floor beam supplier which showed the short beams butted at the sleeper wall.

 

Apart from the overhang problem is there another reason to avoid butting beam? Avoid a straight flex line, expansion/contraction?

[Simplysimon]

13 hours ago, Mr Punter said:

 

Don't butt them, offset one bay by using a block widthways in the first row of one half, so the beams are staggered on the sleeper wall. You may end up with a few odd bits where you can't use full infill blocks but you can fill this with concrete.

i'd double width the dwarf wall and butt them, having had to do all the cuts by staggering them it took for ever. reckoned itwould take less time for the bickie to double the wall rather than cutting block to fit.