Concrete heads: which side up?

Chrizfitz

Hi,

Building atm and have an argument you guys might be able to answer. my brickie put the heads on the windows etc with the rough side down. My architect said they should be the other way but it doesnt really matter. Whats your opinions and could you link to some sources?
Brickie says rough side down for the plaster etc to stick.
Architect says you scud the smooth side for windows.

I haven't a clue.

Regards

sas

Mine were rough side down as far as I remember.

Slig

If they are Pre-stressed lintels(which most pre-cast lintels are) then the "common" method should be rough side up. This gives a key to the blockwork (slightly more important that this sticks rather than the plaster) but also, more importantly, these lintels are cast with a slight upwards bow produced by stretching the reinforcing, casting the concrete and releasing the tension. This is what gives such a small profiled element its impressive strenght but means that there is a certain quantity of blockwork required over the lintel to produce a composite structure and straighten out the lintel.

There is more about pre-stressed lintels and composite zones in the homebond manual if you want to look it up. So just to sum up, dont rely on the plasterer to make decisions on the structure of your house.

Dupont

the ones we use from a local supplier has top stamped on them so you cant really go wrong(i think that leaves the rough side down)

the bolt

never seen anybody put the smooth side down yet

YouTookMyName

Rough side down. The plasterer will have a night mare trying to finish the reveal otherwise.

Slig

Ask the manufacturer, ask a structural engineer or even an architect. Just ask someone that knows about structural masonry before the job goes too far, not a plasterer looking to make their own job easier or anonymous contributors on the internet.

the keen edge

Slig wrote: »

If they are Pre-stressed lintels(which most pre-cast lintels are) then the "common" method should be rough side up. This gives a key to the blockwork (slightly more important that this sticks rather than the plaster) but also, more importantly, these lintels are cast with a slight upwards bow produced by stretching the reinforcing, casting the concrete and releasing the tension. This is what gives such a small profiled element its impressive strenght but means that there is a certain quantity of blockwork required over the lintel to produce a composite structure and straighten out the lintel.

There is more about pre-stressed lintels and composite zones in the homebond manual if you want to look it up. So just to sum up, dont rely on the plasterer to make decisions on the structure of your house.

The second point is correct and explanation is correct.
I've seen/fitted hundreds of concrete lintels, and the correct way is obviously with the camber springing upwards.

However I'd have to disagree with your first point.
Fisrtly, I have never seen a lintel cast with the rough side being the upperside of the camber. I'd imagine that the manufacturing process of pre cast concrete lintels can not allow them to be cast this way.
I can say that in the west of Ireland and the UK the rough side of a pre-cast lintel is on the underside of the camber. Maybe, though I doubt it, lintels are cast otherwise where Slig is located.

Secondly regarding giving a key to mortar joint above the lintel. This is not necessary. Blockwork is routinely built to six metres in height with the first course sitting on a damp proof course, completely eliminating any key between the blockwork and footings(read lintel).
(Obliviously there will be other structural measure in 6 metre high walls, but I just want to make the point that above the lintel a key for the mortar joint is not required).

The smooth side of the lintel could be scudded, as in-situ concrete is, however the rough side of the lintel provides a excellent key.

Echoing Sligs comment check with your engineer, he is the guy signing the work off.

sydthebeat

ps

a significant amount of lintels will have stepped dpcs over them anyway....

henryporter

Chrizfitz wrote: »

Hi,

Building atm and have an argument you guys might be able to answer. my brickie put the heads on the windows etc with the rough side down. My architect said they should be the other way but it doesnt really matter. Whats your opinions and could you link to some sources?
Brickie says rough side down for the plaster etc to stick.
Architect says you scud the smooth side for windows.

I haven't a clue.

Regards

Never in my life heard such preposterous claptrap from an 'Architect' - rough side is always down to form a render/plaster key. And it does matter which side is up/down as other posters have already indicated - its to do with the pre-stressing of the lintel.

You sure your 'Architect' is an 'Architect'??

Dupont

slightly off topic but why are heads always more than your 100mm block work, its not so bad on cavity wall you can step it in a few mm extra but inside single leaf walls is a problem espically where a door is situated against a wall with only a 150mm nib,you cant get a true reading to plumb the block on top of the head as the head is sticking out

saltandpepper10

heads always rough side up,damp course bedded BOTH sides,and slightly off topic never use fairy liquid for ANYTHING on a building site,IMO of course

mickdw

Lintel must be bowed upwards. That is the important point. This leads to the rough side being down in most cases. Now that is great for plastering etc but IMO the lintel would be better if it had a roughness on the upper surface too to enable a better bond between it and blockwork over. I know the casting will only allow one rough surface but a light scabbling would be a great solution but will never be done.

the keen edge wrote: »

Secondly regarding giving a key to mortar joint above the lintel. This is not necessary. Blockwork is routinely built to six metres in height with the first course sitting on a damp proof course, completely eliminating any key between the blockwork and footings(read lintel).

This is not correct. The lintel only gains it strength when built in as part of a composite structure. It must (to gain proper strength) be bonded to blockwork overhead. This is not a situation that can be compared to blockwork sitting on dpc at floor level.
Additionally, someone made the point that the dpc trays regularily break the joint between lintels and the blockwork overhead. This should not happen. I have condemned this detail in the past. There is no reason to build in a dpc in this way. Much better to build in the dpc at a higher level - at least a block above. In cases where this is not possible, the dpc can be just nipped into the joint sufficiently to hold it in place (inner leaf). There is no valid reason to have it built all the way into the inner leaf. Blocklayers often dont under stand the function of the tray and think it needs to cut off the inner leaf completely.
So to sum up, lintel arched up.

the keen edge

I was only using that example to disprove the point made by a previous poster that the rough edge on the lintel was need to give a key to the blockwork above it.

I also wasn't suggesting that the dpc above lintels should extend over the lintel itself.

The primary function of the dpc above lintels is to divert outwards, water drips that may fall from blockwork wall ties. So as mickdw points out, the dpc only needs to be nibbed into the blockwork joint.
Obviously dpc in footing details are there to eliminate rising damp, therefore the dpm must cross the joint fully.

Correct fitting of dpc is very important, and I'd go further that mickdw and say most blocklayers don't know, or at least aren't bothered of how to fit dpc's correctly.

The most common practice I see blocklayers doing is placing the dpc directly and completely over the inner lintel and then down through the cavity which is then cut away, usually roughly, by the plaster when the window reveals are being done.

Loburn

O.K I am a structural engineer, this Pre-stressed Lintel thing has been annoying me for a long time. A pre-stressed Lintel forms a composite beam, meaning the Lintel and the block above act together, the more blocks above, in general, the more capacity the beam has. For a per-stressed Lintel to perform as required structurally it needs to bond to the block above, therefore rough side up. I would presume the main reason that Lintels are being installed rough side down is because not enough engineers are paying regular trips to site and secondly because we have no building control in the county so the plasterer is making the incorrect call to suit himself and has being doing so for so long that everyone now thinks that rough side down is the right way to do it. In relation to DPC's, if the Pre-stressed Lintel is designed to perform with 3 blocks above, the DPC should not be installed in that zone, that should be indicated on the drawings. Lastly because the majority of openings are small, the pre-stressed Lintel works fine as non-composite (Acting on its own without the blocks above contributing) So in that case structurally it does not matter which side is up, but that decision is for the structural engineer to make and not the plaster.

A fella called fish

Loburn wrote: »

O.K I am a structural engineer, this Pre-stressed Lintel thing has been annoying me for a long time. A pre-stressed Lintel forms a composite beam, meaning the Lintel and the block above act together, the more blocks above, in general, the more capacity the beam has. For a per-stressed Lintel to perform as required structurally it needs to bond to the block above, therefore rough side up.

Structural Engineer here too - I fully agree with the above. The purpose of a rough side is to provide adequate frictional resistance (bond) between the blocks/mortar/lintel as it forms part of a composite structure.

The rough side is a structural consideration. Not something to make the plasterer's life easier. However, if you're letting the plasterer call the shots from a structural perspective then I think you've got bigger problems!

muffler

I have unapproved a post by Max Moment (its there but only mods can see it) and as these posts are flying in the face of our forum charter Im locking this now.