100mm lintel seats, a target or barely acceptable?

[epsilonGreedy]

I am putting together an order for the all the lintels required in my house. There are no fancy steels,  just basic window and door 100mm cavity lintels in a 2-story brick & block house.

 

So far in my order I have added 200mm extra to each window width for a 100mm lintel seat each side but then I thought is 100mm just a barely acceptable minimum that I should try to exceed?

 

Background: My inner block walls are 3.7N medium-light Fibolites and the rural farmhouse heritage look is achieved through narrow aspect windows where most are 1248mm or narrower. There is a french door opening that is 1810mm wide.

 

 

Edited February 11, 2020 by epsilonGreedy

[Russell griffiths]

Make sure you are talking about opening sizes and not window sizes, as allowing for fitting will leave your lintels without enough bearing. 

[bassanclan]

When you look at tge manufacturer info it will sometime specify a 150mm bearing on the larger lintels. 

You can't go under 100mm bearing, so for your 1248 window you would order a 1500mm lintel

[PeterW]

Size up to the next 50mm usually so as @bassanclan says it would be a 1500mm for the windows, 2100mm for the French doors. 
 

Don’t forget your cavity trays and weep vents on all of them too. 
 

You also need to look at the bearing load on them too - if you’ve got less than 2 full block courses on the inner leaf before a floor or ceiling joist point load then consider going to heavy duty lintels to stop any cracking or bending. Not much more expensive but certainly worth the peace of mind. 

[Temp]

What's the loading over the patio door? The required bearing area can be calculated from the load in Newton's divided by the area in mm^2. That needs to be less than the crush strength of the blocks. What I'm not sure about is the safety margin that should also be applied? Factor of 2?. 

[MikeGrahamT21]

All lintels generally require a minimum 150mm contact area on each side

Edited February 11, 2020 by MikeGrahamT21

[epsilonGreedy]

11 hours ago, Russell griffiths said:

Make sure you are talking about opening sizes and not window sizes, as allowing for fitting will leave your lintels without enough bearing. 

 

 

Yes got that covered but thanks for cross checking my thinking. I am taking masonry aperture dimensions off the technical drawing for building control and the brickie team has started creating openings using the same dimensions.

[Mr Punter]

Has anyone used the IG Hi-therm+ lintels for a brick/block build?  They are meant to be good at reducing thermal bridging.  I was going to use them on a project but the lead time was too long.

[epsilonGreedy]

15 hours ago, PeterW said:

Don’t forget your cavity trays and weep vents on all of them too. 
 

 

I purchased a box of 50 for the brickie team after calculating how many for the window and door lintels but for got the 50 or so also needed around the cavity tray at the DPC perimeter.

 

15 hours ago, PeterW said:

You also need to look at the bearing load on them too - if you’ve got less than 2 full block courses on the inner leaf before a floor or ceiling joist point load then consider going to heavy duty lintels to stop any cracking or bending. Not much more expensive but certainly worth the peace of mind. 

 

 

A point had I missed completely. Fortunately the architectural technician produced a couple of house cross section diagrams showing every brick, block and floor joist, so this will be simple to to check. As a general point my brickies call these modern cavity lintels "tin lintels". I can see their point because they seem too thin to last 100 years even from the major brand names in the lintel business.

[PeterW]

Fully galvanised and painted they don’t really get any exposure. 
 

If in doubt I would always go HD or XD anyway for the extra £20 each.