Mortaring above a lintel

[Lobster]

Probably a daft question but what the hell...

If you are fitting a lintel over an opening, to support several tons of masonry above, how do you get an adequate amount of mortar into the (hopefully small) gap above the lintel and below the bricks it's supporting? Do you have to just force the stuff in with a trowel and hope you don't have any airgaps inside? Presumably if there is insufficient set mortar in there, at best you run the risk of the wall "settling" once the acros are removed... what's the secret?

David

[Mike Taylor]

In theory you should dry pack if you do not build the bricks onto the lintel. To dry pack you leave a thin gap above the lintel and wedge in the gap slate to pin the bricks up. You then push/spread as much mortar into the gaps left. It wont go anywhere

[BigWallop]

Leaving a gap between the lintel and the masonry will mean the bricks are not supported properly and will drop onto the lintel. Dropping bricks, even by a slight amount, is not what you want. The lintel is put there to hold the bricks in one place so should actually be resting on the lintel with nothing between them. To tidy up the edge you can point some mortar along it, but this shouldn't be necessary if the lintel is being boxed in correctly.

[Lobster]

Thanks (and to Mike). So basically, when the lintel slot is cut, you should be aiming for as tight a vertical fit as possible, and if there is a gap, pack with slate? I hadn't realised that mortar in the gap above the lintel is not intended to be loadbearing.

By the way, has anybody used these 'Jackall' props, available from HSS? (see

they easy/safe to use, as an alternative to conventional needles/acro props? I'm not entirely clear how they work, looking at the picture.

Cheers David

[Tony Bryer]

I'd prefer to see a gap of 15-25mm between the top of the lintel and wall over which is then dry-packed: 1:1 cement and sand with just enough water to hold it together when you squeeze it. This needs to be rammed into the gap with a suitable piece of wood and hammer.

[BigWallop]

And if the mortar crumbles ? Down comes the lot above it. No Thanks.

[Peter Taylor]

BW, dry packing as Tony describes really is the proper way to do things. Any structural engineer would recommend it. It is also the method used for packing between old foundations and new underpinning. The big benefit is it that the mortar moulds itself to the bottom surface of the brickwork above, giving a solid overall bearing, rather than bearing on just a few high spots.

The mix has to be strong (I would say 1 cement to 2 sharp sand by volume), and fairly dry - just wet enough to give proper hydration of the cement. This would form a cohesive ball in your hand if you squeezed it. And you must kill any suction from the brickwork by wetting it first. The props should be left in position for at least 3 days, a week is better. If you have experienced crumbling mortar then something was obviously wrong.

Peter

[Tony Bryer]

That's the point: we are *not* talking mortar. Drypacking uses

*sharp* sand and cement, just enough water so that when you squeeze a ball in your hand it holds together but no excess water comes out. After a week it's like rock. It's what they ram in between column base plates and foundation bases on steel buildings.

[Frank Davis]

I had a similar problem a few years ago. My brother (who claims to be an engineer) recommended against the use of cement based mortar, on the basis that even if packed well into the gap, the mortar will shrink. He recommended shrink proof grout - it is similar to cement but no sand is added, and it is forced into the gap in the same way as mortar. I sourced a bag in a specialist cement manufacturer. Cannot remember the name but as I am in Ireland so it would be of no use to you.

I cannot remember the correct terminology but a search on Google gave the following

It might be worth a call

[Dave Liquorice]

Erm, it's not. It's really only supporting the triangle of bricks directly above it. It will have some of the load of the full wall but not as much as one might expect. You can take out the triangle of bricks above an opening and the wall won't collapse, unless the redistributed load to the sides of the opening becomes to great for those bricks to carry. It might not be "safe" though...

[BigWallop]

Well I've learned a lot from this thread. I'm glad I came in with my two pennith now, because it shows what knowledge you can get when you disagree with people a little. I just hope you have all your answers now David ? :-))

[Lobster]

Yep, looks like I'm all set now. Glad I didn't rush off and do the job after reading the just first two replies!! :-) Many thanks to all.

Anyone care to comment on my follow-up about these new 'Jackall' props?

[BigWallop]

The tinyurl link won't work for me, but if you mean these:

they are OK for small jobs that only require holding for a short space of time. Best to get solid props if you have to leave them in place for a long while without movement.

[PoP]

So as a matter of curiosity, what sort of weight are we talking about on a load-bearing wall? I have always assumed it must be measured in tons, but I've never seen any figures.

PoP

Sending email to my published email address isn't guaranteed to reach me.

[Lobster]

Funny, the tinyurl still works OK here. The HSS link is actually

you can see why I went for a tinyurl link! Doesn't look the same as the ones in the jpg you indicated; but I don't know how those work either!

David

[fred]

In article , Lobster writes

I get: "LoadPageLanguage - Error -2147217865: [Microsoft][ODBC SQL Server Driver][SQL Server]Invalid object name 'Pages_'. LoadPageButtons - Error

3709: The connection cannot be used to perform this operation. It is either closed or invalid in this context." Are you getting it from your cache?

[Toby]

I think the HSS Jackall props are the C shaped fair where you knock a hole above and below the lintel, stick in the support and wind it out, very good for a long way up an outside wall. The standard acrow props are a lot less to hire, and should be available from the local friendly independent hire shop. Alternatively second hand at ~£10 each.

[Peter Taylor]

The link works OK for me (weird or what???) although the picture shows a few Strongboy heads on ordinary Acrows, not Jackalls. The hire price for the Jackalls is £12.00 a day, ordinary props are £6.50 plus the Strongboy heads £7.50, so Jackalls are cheaper than Strongboys at HSS.

Last year I had to sort out a building in London W2 where a builder had supported two sides of a 3 storey rear addition on 6 Strongboys while he altered the basement walls. Because the load is offset from the prop, the Strongboy head creates a bending force on the prop, and these 6 had bowed like bananas. As a result the rear addition had fractured and moved away from the main building about 50mm at the top. We had to evacuate it and the BCO slapped a dangerous structure notice on it. So it was a good reminder that Strongboys aren't all *that* strong.

Peter

[Toby]

'kin ell. Any building that has people in it that has moved 2 inches already seems highly likely to move a lot further rapidly downwards. I guess that also means a fairly comprehensive rebuild.

[BigWallop]

And wapping big insurance claim.