Steel lintels

[Kenc]

Hi, I am starting to plan my garage conversion and need to install a lintel just below ground level to span the door opening to carry 10 brick courses. Can I install a steel one that’s corrosion protection coated with a lapped over DPC? Or do I need concrete?

[markc]

Why do you need a lintel below ground? And if you did concrete would be the way forward

[Nickfromwales]

You usually dig out for a footing, trench fill, and the BCO has normally already stated this before you start works. What's written in your spec?

[Kenc]

So I’ve been recommended to not dig down to the footings and trench fill as not necessary for a 8 course load especially and there are drains running through the garage and it’s a tarmac drive with a French drain running across the garage entrance so didn’t want to disturb all that. So quickest and cheapest was a lintel across the door opening just below ground level and leave the drain etc in tact. Treated steel was recommended as lighter to man handle.

[Iceverge]

You could chuck in some rebar and pour one in place. Very cheap. 

[Kenc]

Can I do that? Will the building inspector accept that?

[Iceverge]

Just now, Kenc said:

Can I do that? Will the building inspector accept that?

 

I have no idea. If you have a SE engaged then ask them for a required spec.  Similarly if you ask the BC for a spec for the lintel they expect then there's some online designers that would allow you to design a pour in situ beam.

[Gus Potter]

On 23/04/2023 at 22:47, Kenc said:

Hi, I am starting to plan my garage conversion and need to install a lintel just below ground level to span the door opening to carry 10 brick courses. Can I install a steel one that’s corrosion protection coated with a lapped over DPC? Or do I need concrete?

Welcome to the world of garage conversions.

 

I do a few of these and BC often ask how do you support the outer leaf of brickwork even the ten or so course below what is often a window above. Many of the jobs I do are on relatively new builds. Now the obvious solution is to get a hold of the "as built plans" that show whether the found is running continuously under the garage door opening. In practice this can be a challenge. If you can get this info and it shows a continuous found then the easy way is to excavate down and build off the found if niot too deep and it gives you more of a vertical zone to get drains past. But you may have a piled found.. can be a little more complex but best to try and find out now.

 

If you can't then you can often span the gap with a lintel as you say.

 

One secret of garage conversions is to make it easy for the builder to get stuff off the shelf. Steel lintels are problematic as say Catnic lintels are not design for below ground use.. so you need hot rolled steel.. then maybe an SE to design / check it and it needs to be hot dipped galvanised. That all adds to the cost and how much running around the builder needs to do.

 

Ok lets put some numbers to this. How heavy is ten courses of non load bearing brick? The window is supported on the inner leaf.

 

A standard course + mortar is 75mm high. 10 x 0.075m = 0.75m. Brick density 20 kN/m^3, say facing brick 102.5mm wide. The working load weight you need to hold up is 0.75 x20x 0.1025 = ~ 1.5 kN/m run of wall.. say 1.6 kN/m. Now if the door opening is 2.7m wide tops...the total weight of the wall is 2.7 * 1.6 = 3.2 kN ~ 320 kg just for a bit of context.

 

For fun .. a 15 stone person weighs about 100 kg so we are talking about three folk standing on the beam. I think a Sumo wrestler weighs more but so does an elephant. "Interestingly" SE's have a number of bibles and an adult group of elephants impose a load on a floor say of ~ 320 kg per square metre of floor.

 

 

Below is an extract from a concrete lintel manufacture, Robslee. The values shown below are what is called the permissible load, a working load, thus I have not added safety factors to the calculation above.

 

 

 

A clear spanning type C concrete lintel (145mm deep) can carry a uniformly distributed load of 6.53 kN/m which is greater than the 1.5 kN/m we have calculated above. Now that looks like it works, provide that you drains are not clashing with the lintel as you can cut a hole in that.

 

Ok the weight of a concrete lintel that long. Well you'll need 150 mm rest each end so the lintel you need to buy is going to be 3.0m long so weighs about.. 27 m of lintel = 1000kg. Thus a 3.0 m length will be about 3/27 * 1000 = 112 kg.

 

In terms of price and availability the concrete lintel now looks attactive, easy to get a hold of and something the builder is used to working with.

 

The good thing is that the concrete lintel generally works below ground.. provided the ground is non aggressive.. not full of acid soil and sulphates from power station ash say. . I'm just making caveats in case folk start to use my example for things other than a typical garage conversion say.

 

@Kenc Things to check..

 

1/ Your ground levels and DPC level.

 

2/ Where does the existing floor slab of the garage stop and can you put the inner skin on that.. requires another post.

 

3/ Do you have a gas membrane.. BC are certainly tightening up on this in Scotland so you need to make sure that if you have one it is detailed out.

 

Hope this helps.

 

 

 

 

 

 

 

 

[Kenc]

Thank you for all the information. I am lucky that I have access to a number of SE’s I can call upon to get calcs done etc in my workplace. 
 

the house is 3 years old and whilst I can’t get the original drawings I have had the builder confirm footings run under the opening but they are 3ft deep ish so not too deep. However above the garage is a bedroom and en-suite so the drains run out the garage to the man hole in the drive. I assume above the footing. Hence the suggestion from the SEs at work that it’s easier to just go one course below GL and put a lintel in. The garage is double skinned and the floor is up to the inner skin then a concrete cap over type 1 compacted back fill. So my simple hope was to lift the cap, bed mortar and a lintel on the type 1 across the gap into the brickwork either side and come up 8 courses as the window will match the height of the rest of the ground floor which are quite large. The SEs had suggested a catnic to support both inner and outer skin: this is not sounding realistic now though…