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How to hide steel lintel and complete square set finish on its underside?

Hi folks,

I’ve had our 40-year-old timber windows replaced with aluminium ones, as a result wooden architraves were removed and the galvanized steel lintel was exposed on the indoor side of our window surround.
I’d love to hire someone to render the opening properly and a square set modern finish is what I’m looking for. The only tricky part is how to treat the underside of the lintel so there won’t be any cracks due to movement or expansion.
GPT advised to either screw a piece of FC board in and render over it as normal, or to slurry with some adhesive compound and render on top. Love to hear what you would recommend to do before I blindly turn to someone.
Thanks so much!

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JacobZ

Hi @ohce,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

If you were doing the work yourself, I'd say that attaching some fibre cement over the top and then rendering it would be the way to go, because you can be confident that the render would stick to the fibre cement. In saying this, considering you are planning to have a professional do the work for you, rendering directly over the lintel also seems possible, assuming the correct technique and materials are used.

I can't say I am hugely experienced when it comes to rendering, so I couldn't confidently tell you what the best method would be.

My suggestion would be to have a couple of renderers come out to quote on the job and ask them questions about how they would address the issue. In construction, there is rarely only one way to do things, so having an open discussion about their proposed method during the quoting stage isn't off limits. If they are an experienced and reputable tradesperson, they will be able to explain what their plan is and why it would be the best way to go. As long as they are comfortable doing the work and can explain why they would do it that way, then I think it is reasonable to trust what they say.

Allow me to tag @Nailbag, who is an experienced handyman, to see if he has any thoughts.

Jacob

Nailbag

HI @ohce

All you need to do here is provide a suitable surface for a renderer to work their magic.

1. Using the instructions in the image below, fix 90mm x 45mm pine to the underside of the door frame using liquid nails and a couple of self-drilling metal screws. Drill pilot holes through the timber first. As noted, the 90mm batten wont be the same depth as what it is from the brick edge to the door frame. But the cement sheet will cover this.

2. Use cement sheet brads to secure a strip of cement sheet to the face of the batten. Then the 2nd flush along the front of the batten back to the door frame.

The area is now ready for rendering. The renderer will fit the external aluminium corner edging angle so, you don't have to worry about that. I do always think it's a good idea if unsure though to discuss what you propose to do with your renderer as @JacobZ also suggested. This is just in case they have another solution to offer.

I have included a photo of window and old box AC unit I removed and prepared for rendering to give to some idea. The main difference here was it was to have a flush finish with no square edge requirement.

Nailbag

Screenshot 2025-12-15 at 3.43.16 pm.png

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