Plastering onto steel

[Frank Z]

Hi folks, I need to plaster over a steel lintel over an open fore recess. Its visible surface is 6" x 31" . It is 1/4" thick steel. The steel will get fairly hot (I guess about the pemperature of boiling water but not much more) because it is in close proximity to a fire. That means the steel may expand and contract somewhat. I'll be using one-coat plaster from B&Q, and the plaster will be about 6mm thick over the steel. What is the best way to ensure the plaster stays stuck to the steel? One idea I had is to rivet a piece of perforated sheet-metal to the steel before applying the plaster. Any other suggestions would be appreciated.

Thanks

Frank Z

[Andrew Gabriel]

I think the plaster will crack big time if fixed to the lintel or fixed to anything fixed to the lintel. I would look into having a sheet of plasterboard fixed to the wall around the lintel, and maybe loosly fixed to the lintel so sideways movement of the lintel can occur, but you won't get plasterboard and plaster in 6mm depth, so it would step out in front of the wall.

Otherwise look for some other way to finish the lintel. Maybe face it with wood so it looks like a wooden lintel from the front or with brick fronts to look like a row of soldier bricks?

[John Armstrong]

Maybe expanded metal lathing fixed to the wall around the lintel. Not sure how easy it is to get some wide enough but in small quantities. Like you say, movement of the steel could be a problem, but presumably if it expanded that much, it would also crack the masonry around it.

[Frank Z]

but presumably

Thanks to both respondees. I'm not sure how much a 31" length of steel will expand. My guess is less than 0.5mm. To stop the masonry cracking at each end of the lintel, I stuck some foam draught excluder onto each end of the lintel so it will simply be compressing the foam, not the masonry. I guess plaster will expand when hot too. With a bit of luck, it'll expand the same amount as the steel - but that may still not prevent cracks occurring.

Perhaps I'll follow Andrew's idea and stick something onto the lintel, like brick tiles or something... not sure...

Frank

[Dave Baker]

0.1mm per 10 degrees C

My guess is less than 0.5mm. To stop the masonry cracking

The CoE of metals is many times higher than that of building materials. The data is freely available on the web.

- but that may still

Dave Baker - Puma Race Engines

"How's life Norm?" "Not for the squeamish, Coach" (Cheers, 1982)

[BigWallop]

Perforated plywood is good for this. Make a frame around the lintel and cover it with the plywood. The perforations lets the plaster ooze through and when it dries it grips like mad. It's just like an old lath and plaster wall in a way..

[Frank Z]

Excellent tip - thanks. I came to a similar conclusion last night, while mulling over Andrew's plasterboard coment. I was thinking to fix a piece of 5mm cement fibreboard over the lintel - fixed to the wall each side of the lintel - then plaster over that. Actually, I think I may use the cement fibreboard rather than plywood, since I feel it will be unlikely to shrink the way plywood might. Also, the bottom edge will be close to a flame, so it needs to be something fire-proof. Drilling holes in it is a very helpful tip. Thanks!

Frank

[jerrybuilt]

The coefficient of thermal expansion of mild steel is about

0.0000126/DegC. Assuming a possibility of a 100 degree temperature change, that's 0.03906", near enough 1mm.

I don't understand your setup. Have you used a square section tube, or what? Can you supply an ACSII cross-section?

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[Simon]

galvanised steel mesh using a 2 part construction adhesive (Epoxy, PU or Acrylic) to adhere it to the lintel....... and for extra heat security ... use a fibre reinforced heatproof screed (available online from BES) with a plasticiser, and Don't use plaster ......