What steel sheet to use

[trevorbayliss]

I need to use a small piece of steel pad to sit over my cavity wall construction.

An oak beam will sit on the top. This will be 300 x 300mm in size and span 3m.

I need the weight to be spread between the brick and block work so thought a steel pad is the best way.

What thickness steel do i use and should it be stainless or can i just hammerite a standard steel pad ?

 

[John D v2.0]

usual caveat about that your SE should have specified exactly what, but it won't have to be stainless. For ours we had 3mm specified (bottom of a narrow lintel) but it was much lighter load (and the builder decided it was enough and welded a massive flange all the way along)

 

[trevorbayliss]

I only got calcs for the beam size. I only need a 250 x 150 beam over this patio door, but i am going to use a 300 x 300 because it is chunkier and will look nicer i feel.

I thought 3mm will be ok, but i will check with the BR guy as well.

 

[John D v2.0]

I only got calcs for the beam size. I only need a 250 x 150 beam over this patio door, but i am going to use a 300 x 300 because it is chunkier and will look nicer i feel.

I thought 3mm will be ok, but i will check with the BR guy as well.

If the beam is already full width then it's just providing a stable surface, ours was needed to ensure the beam didn't fall down the cavity!

Ps When we had our Oak columns done at the front, there was a lot of oils coming out of them afterwards, so just make sure that's not going to cause an issue.

 

[trevorbayliss]

I think oil in the oak can cause problems, so i will check if hammerite paint is ok to use. Or it will have to be stainless.

I thought i would go with a 300mm wide beam as opposed to 150mm because everything would be resting on the new external brick leaf, plus i think a chunkier timber will be nicer to look at. It will cost be an extra £200 for the bigger size, but it is worth it.

For the 3 oaks spanning the room and holding up the joists and flat roof above i was thinking a 100 x 300mm x 3mm plate would be fine. I may get these in and end up using classic concrete padstones anyway. Cost for the 8 plates from the black smith is £40, so not a great deal.

 

[tony1851]

If the oak beam is supporting only a flat roof, do you need a bearing plate?

 

[^woody^]

Why would you want to create a cold thermal bridge across a cavity?

Bearer plates tend to be 8mm upwards

 

[trevorbayliss]

I think the weight of the beam is about 300kg. Green oak weighs 1161kg per cubic m. So my cubic size is 0.3cubm. = 387kg. It has been air dried for 3 yrs, so i reckon it will weight 300kg.

The oak beam will sit on the external reclaimed brick course which is 110mm in depth and 60mm of the blockwork. So i can sit the beam without the bearing plate, but i thought a bearing plate would just do a better job.

Would a plate really make that much difference to thermal performance ?

 

[endecotp]

Would a plate really make that much difference to thermal performance ?

Steel has a thermal conductivity about 300 times greater than wood. So - for example - if you had a 300mm timber on a 1mm plate, half the heat would pass through the wood and half through the steel. With a 3mm plate, 3/4 of the heat would go through the steel, i.e. the heat loss would be 4 times greater than with the timber alone.

 

[tony1851]

Steel has a thermal conductivity about 300 times greater than wood. So - for example - if you had a 300mm timber on a 1mm plate, half the heat would pass through the wood and half through the steel. With a 3mm plate, 3/4 of the heat would go through the steel, i.e. the heat loss would be 4 times greater than with the timber alone.

Are you sure??

 

[endecotp]

What do you doubt Tony? I may have made a mistake somewhere but I think the basic nunbers are right.

I was going to add a bit about the geometry but decided to keep it simple.

 

[bernardgreen]

I think oil in the oak can cause problems,

It can. It was rexommended to use stainless steel screws in new oak or old oak that could become damp. The previous owner of this cottage had put steel screws into oak joists and floor boards and these screws were very badly corroded

 

[John D v2.0]

Steel has a thermal conductivity about 300 times greater than wood. So - for example - if you had a 300mm timber on a 1mm plate, half the heat would pass through the wood and half through the steel. With a 3mm plate, 3/4 of the heat would go through the steel, i.e. the heat loss would be 4 times greater than with the timber alone.

I haven't checked your calcs, but that's only kind of true - it may be true if whatever is at either end is able to conduct heat in and out 300 times quicker too, but in reality if for example you were to chuck a bit of paint on it then straight away it would make a much bigger difference on the steel than the timber. Even the air/steel or air/timber boundary has some thermal resistance.

 

[endecotp]

Yes, surface emisivity etc. all affect the overall conductivty. I'm just addressing the question "would a metal plate really make much difference" by pointing out that metal is a vastly better thermal conductor than wood, and perhaps by a greater margin than our intuition might tell us.

 

[John D v2.0]

Yes, surface emisivity etc. all affect the overall conductivty. I'm just addressing the question "would a metal plate really make much difference" by pointing out that metal is a vastly better thermal conductor than wood, and perhaps by a greater margin than our intuition might tell us.

Yeah totally agreed.
In fact that's not even the worst thing, as one you get a cold enough surface you'll get it covered in condensation and mould.