Patio Door Aperture

[BiscuitThief]

Appreciate any advice/pointers on this because my Structural Engineer has sent calculations for a door and aperture that don't add up I think (and hasn't responded for 2 days).

We want to install sliding patio doors in the garden-facing back room which has an external width of 3430mm (location: London).

Room + Walls:
- One storey room with flat, timber + felt roof (no roof lights, only walked on for window washing/repairs etc)
- Boiler in the corner nearest the neighbour's extension
- Walls are 9"/228.6mm wide and solid (no cavity)
- Currently has a 1188.72 x 640.08 window (will be removed)

We asked that the door be located off-center so that it's closest to the rear-patio side.
This would leave approximately 895.2 mm on the side nearest the neighbour's extension which also has the boiler in the corner.

The S Engineer has returned drawings showing a 850mm gap on the rear-patio side and a door width of 2000mm.
If we use these measurements, the door overlaps with the boiler.
The only way to make a 2000mm fit is if the gap from the rear-patio corner is approx. 606mm instead of 850mm.
If we stick with 850mm then we'd have to get a smaller door or move the boiler which we prefer not doing.

Does the 850mm gap make sense and can it be reduced at all? Is there a way to still fit a 2000mm door without having to move the boiler?
Additionally, a builder mentioned that a Catnic Lintel Heavy Duty CH 90/100 is not right for a solid wall... can anyone advice on this?

 

[RonnyRaygun]

There are "deemed to satisfy" dimensions in Approved Document A for returns either side of an opening. The SE has probably tried to limit the dimension to 850 so that it complies with ADA (although then they're relying on contribution from next door's extension the other side, which isn't recommended).

However, the dimension could be removed and the door moved over - it would just mean doing maybe a wall panel check for lateral loads (ignoring the return) and taking diaphragm action of the roof into account.

 

[BiscuitThief]

a wall panel check for lateral loads (ignoring the return) and taking diaphragm action of the roof into account

Thanks @RonnyRaygun - in your opinion, this is something common/standard for a SE to do or would I be asking him something unusual.

 

[RonnyRaygun]

No, it's pretty standard to do this.

 

[jeds]

The pier marked 850 could probably be reduced to 665mm without calculation. (ADA diagram 14 and table)

 

[BiscuitThief]

The SE has emailed a new set of drawings.
He's reduced that pier/return to 250 internal / 500 external and swapped the Catnic Lintel for a UC Beam.
Hopefully he'll explain what's involved here... this looks like two columns at either end of the opening and a beam on top.

 

[BiscuitThief]

Hopefully he'll explain what's involved here... this looks like two columns at either end of the opening and a beam on top.

Sharing this in case it's useful for anyone else (or if anyone can see a problem with this): SE has explained "They are not columns but plate spreaders of the beam which is a part of the beam."

 

[freddiemercurystwin]

What's a 250x300x40 m/s plate? A spreader plate wouldn't be 40mm thick? What's m/s - mild steel? If your walls are 228 wide why has he specified a 250 wide plate?

 

[jeds]

That's a very large beam for such a small opening and a relatively small loading. A plated 152x152x23 would do it easily.

 

[RonnyRaygun]

Hadn’t even looked at the beam or bearings.

Also, usually there would be a few courses of masonry above the opening so I’d expect to see maybe a 150x100mm box section with a bottom plate picking up 100mm of masonry (maybe with snapped headers to mimic Flemish bond). Alternatively a solid wall lintel would do the job. Unless the beam is somehow hidden by a facia or maybe a rendered finish?

Whichever way, the spreader plates are complete overkill and 225mm bearing (likely even 150mm) direct onto the masonry would be fine for such a small span.

 

[BiscuitThief]

Thank you everyone.

If your walls are 228 wide why has he specified a 250 wide plate?

Had to get my tape measure out - I’m getting 10 inch/25.4mm wall thickness. For some reason the architect’s been saying 9 inches all along and I didn't question it. On this basis the 250mm spreading plate works?

A plated 152x152x23 would do it easily.

Does this apply to a 10 inch/25.4mm thick wall?

Unless the beam is somehow hidden by a facia or maybe a rendered finish?

The beam will be hidden behind render. Is this the sort of lintel you’re referring to - and with no spreading plates at all?

225mm bearing (likely even 150mm) direct onto the masonry would be fine for such a small span

I’ll ask the SE about this. Like the previous conversation with him, I'm guessing he'll say such a small bearing won't pass building control.
To confirm, you’re describing this scenario?

 

[freddiemercurystwin]

Surely you know how thick your wall is? Measure it and deduct 1/2" for plaster et voilà!

Try and stick to mm, you're measurements are all over the place.

 

[BiscuitThief]

Sounds ridiculous now but I never thought to measure before

Thanks for the tip!

 

[RonnyRaygun]

The beam will be hidden behind render. Is this the sort of lintel you’re referring to - and with no spreading plates at all?

I’ll ask the SE about this. Like the previous conversation with him, I'm guessing he'll say such a small bearing won't pass building control.
To confirm, you’re describing this scenario?

Yes, that's the type of lintel you would need.

The SE might tell you that a 150mm bearing won't pass building control but he would be lying. It could easily be calculated to prove it.

I don't like the idea of spreader plates extending beyond the end of a lintel as the eccentric load causes a rotation which means the load isn't even close to being evenly spread along the plate. Deeper pad stones work better but even then you need to keep the load within the middle third of the pad stone. Much better to just give the beam itself the required bearing length (which is what I usually specify) and then no pad stone or bearing plate is required.

If you are rendering then it's possible to weld some metal strips between the flanges on the outer face of the beam and tack some expanded metal lath on and render onto that, but the beam will need to be galvanised and there's more risk of cracking of the thin render in this location. I'd rather use a lintel supporting a couple of courses of brick or block and render onto that.

 

[jeds]

Does this apply to a 10 inch/25.4mm thick wall?

My bad, I was working on a 300 cavity. A plated 152 beam would be a bit tight. Ronny's suggestion is probably the best option.