concrete lintel above doors instead of catnic

[johnhenstock83]

morning all,

 

I'm asking this for a friend.

 

single storey rear extension, PD, approx. 6x3.5m, council building control.

 

there are 2 openings in the extension: one 1.8m for french doors, another one for a normal door, less than 1m wide. construction is breeze block both skins, 150mm cavity, rendered on the outside.

 

BCO is being difficult about the lintels, questioning why a catnic hasn't been used, talking about damp (lol) and other non-sense. may I please double check my understanding that there is no legal requirement for a catnic lintel to be used? I know they are different products, but they both serve the same purpose and given the very small span and the fact that they're literally only holding several courses of breeze blocks and a flat roof, there's nothing that can go wrong. it's like asking why I used a cheaper brand of insulation instead of going with celotex, when they both have the same properties.

 

what is the actual requirement and why are they picking on something that's not even proper structural stuff? there's nothing above it, the doors are small, there are 2 lintels, one on each skin, resting on engineering bricks etc. ...

 

can someone advice what to do in this situation?

 

thank you

[nod]

There’s nothing to stop you 

 

[johnhenstock83]

that's what I thought too. what should my friend tell the BCO to make sure they are left alone?

[kandgmitchell]

Structure wise as long as the concrete lintels can carry the load then there is no problem. Ask the merchant who the manufacturer is, jump on their website and get the load capacity at that span and show the BCO. Providing proof normally closes these things down. What about a cavity tray, has that been provided over the lintels; is that what the query about damp was about?

[johnhenstock83]

a cavity tray has indeed been installed, BCO is happy with that. his opinion was that the lintel, being made of concrete, will 1.) absorb humidity from the outside (what???) and 2.) not be as well insulated as a catnic one, despite the builder filling the space between the two lintels with celotex (everything else is mineral wool).

 

I'll tell my friend to get the specs for that product, hopefully that will be the end of it. should they insist on structural calculations, is this something that can be done online, quickly? I know such websites exist for RSJs, what about concrete lintels?

 

thanks

[Big Jimbo]

Ì did the roof on my both my daughters extensions with jji joists. because i was having bi-folds on one, and large french doors on the other, both 2450 high. I just used a jji on the inner and outer leaf to take the flat roof load on both of them. Both building inspectors were happy. Yours is being a knob IMO. On another, i had 4 internal soil pipes. All with air addmittance valves in the draughty loft. Building inspector said i needed One of them to outside air. I told him that as i was a middle connection, in a row of 10, to bugger off. (All the roof had been finished. I also refused to fit crappy trickle vents to any of the windows. (stating that the house was draughty as, and that with 5 people the front door was opened atleast 10 times a day. I told him that i would get an airtest if he wanted to show how leaky the house was, but that if i was right, i would expect him to pay the cost. He called me a grumpy git and signed me off. Sometimes they just like to throw a bit of weight around. Nothing wrong with your Two concrete lintles.

[mr rusty]

If it's any help, conc lintels are shown on NHBC https://nhbc-standards.co.uk/6-superstructure-excluding-roofs/6-1-external-masonry-walls/6-1-12-lintels/

 

My local supplier provides all the BS numbers and span tables for the lintels they supply - I'm guessing yours are not much different. https://www.cwberry.com/prestressed-concrete-lintel-100-x-65mm

[kandgmitchell]

Never heard about absorbing humidity - whatever next! Easy calculation - flat roof load plus some blockwork gives the total load carried by the lintel then the manufacturers load/span table tells you what load those lintels carry over a particular span. As long as the lintel capacity is bigger than the actual - job done.