Issues with DPC between lintel & block work

Nativespeaker

I have a question.
I have been told that if DPC is put between the blockwork and the lintel, over an ope (ie if there is no bond between the lintel and the blockwork), it loses it's strength and is treated as negligible therefore making the overall structure unsafe.

My question is: Can DPC be put between the lintel and the line of blocks above IF IT IS THE INNER LEAF OF BLOCKWORK? Also can DPC be put on some of the lintel, let's say half way in between the lintel and blockwork, so that there is still some bond between the two?

Hope someone can help me out.

No6

Was it because blocklayer couldn't be bothered putting in a dpc over the window head and needed a reason? By the same logic it is therefore structurally insane to put a dpc at floor level as the bond is broken. Someones trying to pull a fast one IMO.

Bb4sure

The inner leaf is where the dpc goes, I put 4 inch on top of the lentil rolling down by the window jambs down to where the sill will sit to stop moisture bridging from the outer leaf. Then its good practice to go along on top othe lentil with brick after that. Get wide damp (12 14 inch) and sit it on top of the brick, covering the width of the window plus past the jambs 6inchs. Nearly every house Ive done I done like this also my boss who taught me.

Mellor

Bb4sure wrote: »

The inner leaf is where the dpc goes, I put 4 inch on top of the lentil rolling down by the window jambs down to where the sill will sit to stop moisture bridging from the outer leaf. Then its good practice to go along on top othe lentil with brick after that. Get wide damp (12 14 inch) and sit it on top of the brick, covering the width of the window plus past the jambs 6inchs. Nearly every house Ive done I done like this also my boss who taught me.

Maybe you just worded this poorly, but the DPC (cavity trap) goes over both leaves.


OP, it doesn't reduce the bond, the main strength of blockwork bonds is due to the weight and compressive force

mickdw

The DPM must always be keep a few courses above the lintel. If we are talking about prestressed concrete lintels, they only achieve their strength as part of a composite structure... i.e. when bonded with courses of block/brick above. The lintel becomes stronger the more courses built on it up to about 500mm, after that it is being loaded.
There is no reason to fit the dpm just about the lintel, just use a wide enough role and start it about a block above the lintel. This also ensures a good fall from inner to outer leaf.

This case cannot be compared with the practice of fitting dpc between floor & wall construction. The difference here being the the lintel must form a composite structure to achieve its strength whereas, blockwork on blockwork does not.

There would be no problem if the lintel was an rsj or reinforced concrete beam with its own approved structural loading but not with the prestressed conc. lintel.
By way of explaination, an rsj or a reinforced concrete member can be used as stand alone structural beams whereas a prestressed lintel cannot and would simple snap under a tiny load without its composite strength. IMO, placing the dpc on the lintel creates the situation where you expect the lintel to perform without it being a composite structure. All of the strength would not be lost due to compression from the loading above but alot of it would be.
One of my mates in college did some tests on this as part of a structural design thesis and iirc, one test was as follows:

1/ 100mm wide prestressed lintel, 65 high with 235mm poured concrete over. (300mm total composite height)
2/ 100mm wide prestressed intel, 65 high with 235 poured concrete over with slip layer over lintel.
3/ 235 poured concrete... no lintel
4/ 65mm high prestressed lintel

I think he found expected results for no 1 above..... adequate strength
No 2 results were only marginally better than the combined results for 3 & 4 above... i.e very low reading
Now the slip layer was a low friction material compared to the dpc but it certainly proves the theory.

Additionally, doubling up of prestressed lintels is a no no also. 2.5m or so is the max span for a 65mm high lintel in most cases with its proper composite structure over. placing 2 on top of each other doesnt improve this and no manufacturer or structural engineer will give an indication of any safe increase in span/loading due to doubling up. Ive actually seen many cases of cracking between lintels when these are used on an overly wide span

Nativespeaker

Firstly, thank you to everyone who responded - even though there is some variation between the opinions, I have found them quite helpful and I'm getting a clearer picture of the situation. I also had a look in the Home Bond book and I've seen what it has to say also.
The thing is, my house is currently at wall-plate level and this issue has been brought up by a structural engineer who was checking another steel-related issue on-site. As you can imagine, he takes a very dim view of the current situation and has insisted emphatically that this needs to be rectified asap.
As you can probably gather I'm not a builder and have little or no experience in this area.
I chose a builder who has a good reputation, has built many houses over the years and who would appear to have had nothing but good said about him. I extended him the courtesy of trusting that he knows what he's doing, but now this has come up and well...

I'm not sure what the solution is or what the consequences of not finding a solution might be. What is the worst case scenario? Small cracks in the plaster / final render? Windows breaking? House falling?
I'm very grateful for your input so far and would be interested in hearing more views, especially as regards finding a solution that does not involve pulling down the house.

sinnerboy

http://www.cavitytrays.co.uk/pdf/18-Type-C-Cavitray.pdf

would require disruption to external leaf only


.

Carlow52

Nativespeaker wrote: »

Firstly, thank you to everyone who responded - even though there is some variation between the opinions, I have found them quite helpful and I'm getting a clearer picture of the situation. I also had a look in the Home Bond book and I've seen what it has to say also.
The thing is, my house is currently at wall-plate level and this issue has been brought up by a structural engineer who was checking another steel-related issue on-site. As you can imagine, he takes a very dim view of the current situation and has insisted emphatically that this needs to be rectified asap.
As you can probably gather I'm not a builder and have little or no experience in this area.
I chose a builder who has a good reputation, has built many houses over the years and who would appear to have had nothing but good said about him. I extended him the courtesy of trusting that he knows what he's doing, but now this has come up and well...

I'm not sure what the solution is or what the consequences of not finding a solution might be. What is the worst case scenario? Small cracks in the plaster / final render? Windows breaking? House falling?
I'm very grateful for your input so far and would be interested in hearing more views, especially as regards finding a solution that does not involve pulling down the house.

Re The thing is, my house is currently at wall-plate level and this issue has been brought up by a structural engineer [ what issue exactly? ] who was checking another steel-related issue on-site. As you can imagine, he takes a very dim view of the current situation and has insisted emphatically that this needs to be rectified asap. What needs to be rectified?

We can guess at the problem but it is better if we dont have to:)

Nativespeaker

The problem is that the DPC HAS been put between the lintel and the row of blocks sitting directly above it in most of the windows in the house and in both doors in the garage. I'm looking for a solution.

Carlow52

Nativespeaker wrote: »

The problem is that the DPC HAS been put between the lintel and the row of blocks sitting directly above it in most of the windows in the house and in both doors in the garage. I'm looking for a solution.

You are seeking a solution to a problem which, since you have not spelled it out, I perceive to be an issue raised by your signing-off-engineer that he does not agree with the dpc between the lintel and the block-work.

With all due respect to your signing-off-engineer who

As you can imagine, he takes a very dim view of the current situation and has insisted emphatically that this needs to be rectified asap.

, as it happens I cant imagine what his problem is unless the lintel spans are such that perhaps he relied on a deep beam design for the opes and he omitted to make that clear. or that there is only one or 2 rows of blocks above the lintel, with massive point loads mid span.

In my case when I am signing off engineer I always propose the solution required to a problem and don't sit there asking for it to be rectified: what happens if he unhappy with the rectification: more

he takes a very dim view of the current situation and has insisted emphatically that this needs to be rectified asap.

:mad:

In relation to what else has been posted here:

mickdw: The DPM must always be keep a few courses above the lintel. If we are talking about prestressed concrete lintels, they only achieve their strength as part of a composite structure... i.e. when bonded with courses of block/brick above. The lintel becomes stronger the more courses built on it up to about 500mm, after that it is being loaded.
There is no reason to fit the dpm just about the lintel, just use a wide enough roll and start it about a block above the lintel. This also ensures a good fall from inner to outer leaf.

So is it

always be keep a few courses

or

start it about a block above the lintel

Make up ur mind time:mad:

Bb4sure

Nativespeaker wrote: »

The problem is that the DPC HAS been put between the lintel and the row of blocks sitting directly above it in most of the windows in the house and in both doors in the garage. I'm looking for a solution.

The only solution you could do is take it back down and re-build which in my opinion is a absolute waste of time. If the walls are all plumb and level the load on top of the lentils isnt going to "fall" in or out due to the dpc. There wont be any damp getting through because the dpc is still higher than the outer leaf opening.

mickdw

Carlow52 wrote: »

You are seeking a solution to a problem which, since you have not spelled it out, I perceive to be an issue raised by your signing-off-engineer that he does not agree with the dpc between the lintel and the block-work.

With all due respect to your signing-off-engineer who , as it happens I cant imagine what his problem is unless the lintel spans are such that perhaps he relied on a deep beam design for the opes and he omitted to make that clear. or that there is only one or 2 rows of blocks above the lintel, with massive point loads mid span.

In my case when I am signing off engineer I always propose the solution required to a problem and don't sit there asking for it to be rectified: what happens if he unhappy with the rectification: more :mad:

In relation to what else has been posted here:



So is it

or

Make up ur mind time:mad:

I shall clarify, Typically you would have bricks above the lintel so when I said a few courses, I meant for example 2 brick courses which would be a 2/3 block upwards . I hope this clarifies what I meant. It has to be a composite structure which it is not if dpc is placed directly on the lintel.
I have seen the laboratory tests and also practical examples on 2 occasions on site where the lintel sagged when dpc was placed directly on it.

sydthebeat

ive read this thread 2 times and i still cannot find the issue??

block courses are pretty much exact in compression... very very bad in tension...

whats the problem?? where is the tension happening... is the eaves block the only block over the dpc.. thus the scepticism...??

im not a structural engineer but.... common accepted conventions mean double skinned leaves with 'adequate' tensioning;.. (wall ties).. equates to a structural element...

its only when this structural element lacks tensioning that there is any case of failure....

sinnerboy

OP - your certifying engineer has found a defect . Mickdw has in several posts explained in very clear detail what the defect is .

Your lintols will sag . You may find if hard to open windows and doors and your glazing may even crack .

Your builder - reputation notwithstanding - has let you down , your engineer is doing his job . Thank him for that and have the defects corrected by the builder

Be grateful the defect is not covered over for you to suffer it later .

sinnerboy

Bb4sure wrote: »

The only solution you could do is take it back down and re-build

which in Sinnerboys opinion is all there is to it .

sydthebeat

... maybe i shouldnt be reading threads at 2.57 am on a sun morning...

Carlow52

op: Will bow to the others here re having no DPC if that is the majority view.

Rather than ball and chain the current structure, if there are concerns about long term deflection due to shrinkage/creep/poor workmanship etc perhaps it would be possible to conceal steel props in the window frames where the span exceeds 1.2m.

What is very important is what bearing has been provided for each precast lintel: generally 150mm min up to 1.2m-1.4m span and 225/250 mm beyond up to about 2.4m/3m after that steel/rsj.

If the block wall above the lintel is twice the span arching action can be taken into account which will reduce the deflection.

mickdw

If the house is at wallplate level and no works started on the roof, it is no big deal to whip the heads off the windows & doors and put them back properly. Please pay attention to your Engineer in this case no matter what the builder says.
Even the homebond book has details in relation to height of composite structure required for lintels to achieve strength. In your case, you have zero composite height. Your Engineer will be walking away from this if not changed. He certainly couldnt certify it as he would be just walking straight into a court case imo and where would the builder be then? ...... blaming the engineer I suspect

Nativespeaker

Thanks to MickDW, Sinnerboy and Bb4sure and others who have given this some time and attention. I'll be talking to builder and engineers tomorrow and I'll see what solution can be found.
I've checked all the windows and as far as I can see, the DPC has been installed correctly over some windows and incorrectly in others, suggesting that some of the construction crew know how to do it the right way and others perhaps not, though that's pretty irrelevant now.

Just on that:
1. Does the span of the windows have any bearing on the issue? Very few of the windows are any more than 100cm wide and a number are about 70cm wide (The windows are mostly tall and narrow or else square-shaped rather than the wide rectangular windows that were common in many houses built in the 80's & 90's)
I'm probably clutching at straws here, but some construction people I've spoken to outside the Boards.ie forum have said that this is a consideration that might help to mitigate matters.
What do you think?

Nativespeaker

Thanks for your reply MickDW. (I had my earlier reply sent before I read your most recent response)
You say there that: "If the house is at wallplate level and no works started on the roof, it is no big deal to whip the heads off the windows & doors and put them back properly."
That sounds encouraging. just a few questions on that.
1. Is this a thing that happens from time to time on sites?
2. I'm a bit happier to hear you use the term "whip the heads off the windows and doors" as it sounds like something quite routine - or at worst just an inconvenience.
How delicate or complicated a procedure will that be in reality, do you think? Is it a common enough sort of amendment that I could expect my builder to be familiar with it and be able to manage it?
As a non-construction person myself, it seems like a huge undertaking as I couldn't really imagine how it could be rectified without ripping down walls etc.

sinnerboy

Native - this is no big deal for a contractor - he may try to make out like it is - but it is not . Mickdws turn off phrase is apt .

Nativespeaker

Sinnerboy,
I see. Would you be able to outline roughly the steps involved?

mickdw

Depending how long the blocks are built, it could be easy to just knock off the blocks. Also, this should only involve removing the closing block on top (if used) & the blocks in the inner leaf. The dpc over head should be only built into the inner leaf and draped down over the outside of the window frame (when fitted) therefore not interferring with the outer leaf. Now in your case, if the builder has happened to build the dpc into the outer leaf somehow, well that would have to be dismantled too.
Its a messy job and certainly not ideal but given that nothing is done with roof etc, its not the end of the world.
Imagine if the whole roof structure had to be supported, windows taken out, etc if the works had progressed further, then it would be a massive job. I suggest you make that point to the builder just to "soften his cough" so to speak.
In the case of a very small window, it might be possible to get away with it (up to Engineer) but then again, why should you have to.