Are windposts required?

[MJ6]

Hi All,

I'm helping a friend to build a single story 6x3.45x2.6m (LxWxH) extension to their house. The cavity wall consists off bricks, cavity and blocks, 100mm each.

The architect wanted windpost design for the extension, now I'm not sure whether it's required by the Building Control Officer (BCO) or the architect asked for it from previous experience.

Looking at Approved Document A, there's no clear statement to when a windpost is needed (i.e minimum brick wall length or height).

Also, I found that the proposed plan doesn't match the Document A requirements of having a minimum of 665mm return (like in red circles 1 & 2), and the minimum dimension between 2 openings (P2 in the second snapshot). Probably why they asked for windposts?!

So far, I more inclined that windposts are not needed, especially that it's a mid-terrace house with fences from both sides, with the left-hand side fence being very close to the proposed extension, but probably the followings should be met:
1. Minimum of 665mm returns are built, with appropriate wall starter system
2. Appropriate wall ties are used between wall leaves & around openings (225mm long, 225mm apart vertically)
3. Appropriate straps to tie walls to roof

Therefore;

1. Am I thinking in the right direction?
2. Do you think windposts will be required if the minimum returns and P2 are not met?
3. Is there a rule of thumb to when windposts are required?
4. Does the 3.45m wall works as a buttressing wall for the other two side walls?
5. What are the calcs needed to justify not using windposts? (e.g. Masonry wall panel design to EN1996?)

Many thanks!

Snapshot from Approved Document A

 

[tony1851]

You don't need windposts in that design.

Where it shows a problem is the wall on the right-hand side, which is effectively unrestrained by return walls at either end.

A possible way around this is to:

1) make the stud wall at the back of the wardrobe into a buttressing wall by diaphragming both sides of the studs with plywood or OSB before fixing the plasterboard. This stud wall must be firmly screwed to the floor, wall and ceiling, after which it will provide lateral restraint to that end of the brickwork panel.

2) form a brick/block return at the outer corner (adacent to the door). It needn't be 665 (3 bricks) - 2.5 bricks (550) would be OK (having no return at that point might in any case give you a problem with the bearing of the lintel).

If the roof has a closed ceiling, that would be better structurally because the ceiling will form a horizontal diaphragm which will give lateral support to the top of the wall, once the 30 x 6 straps are fixed.

The diagrams you show from Approved Doc A are only 'deemed to satisfy' provisions and are often overly-conservative. Some architects and SEs tend to over-engineer these things, which is not necessary in this case.

(For the structure, the fencing is irrelevant and would not be taken into account in any formal assessment).

 

[freddiemercurystwin]

What if the next owner rips out the stud wall Tony?

 

[tony1851]

What if the next owner rips out the stud wall Tony?

Stud walls are not normally diaphragmed with boarding each side. If some future owner started taking it apart and noted that it was unusual in its construction and the way it was fixed, that would set alarm bells ringing.

But ultimately, not the OP's problem. Future occupiers need to satisfy themselves that whatever they do to the property, they are responsible for ensuring that they comply with the regs.

 

[MJ6]

Thank you for the thoughts Tony, appreciated.

Where it shows a problem is the wall on the right-hand side, which is effectively unrestrained by return walls at either end.

Apparently the BCO asked the architect to add sufficient returns, otherwise to explore the need for windposts in the design.
My understanding from Approved Document A is that if you don't have a buttressing wall/chimney/pier and/or sufficient returns, then an alternative solution should be explored, which I think is the WPs here. Is that right?

Your first point suggests creating a buttressing wall using a diaphragm, but I think the architect wants to keep that wall as it is, 100mm thick, due to the limited space. Is there a minimum requirement for the OSB diaphragm?
As for the second point, I clarified the need for the 3 bricks returns, or at least bigger returns than the proposed ones to accommodate a minimum of 150mm for the lintel bearings.
Therefore, and considering the useless fencing and climate change effects, this suggests that WPs are required, no?

If the roof has a closed ceiling, that would be better structurally because the ceiling will form a horizontal diaphragm which will give lateral support to the top of the wall, once the 30 x 6 straps are fixed.

The roof is hipped and to be vaulted with a straight middle area under ridge to accommodate light bars.

For the left-hand side wall, this has a 90cm return at each end, would it be enough to not use WPs?

As for the 345cm wall, considering a long lintel is installed over the door and window with stuffiest tying to the other walls, that should provide enough stability, right?