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 

[Bonner]

The architect has probably suggested windposts due to the lack of buttressing by appropriate return walls. I don’t think the end wall qualifies as a buttress even assuming there is a lintel and masonry above the door, it won’t give the same stiffness as a full height wall. Normally they would say windposts or buttressing ‘to be confirmed by structural engineer’. Have the drawings been submitted to building control for approval yet?

[nod]

Your Architect is probably prompting what BC will as for 

Wind posts are incredibly easy to install 

We had a similar extension on our last home I intended putting WPs in 

But our engineer designed an elaborate steel plate system bolted to the house 

I go with your Architects suggestion 

[George]

The 345cm end wall isn't a buttressing wall - doors and windows are not structural components. 

 

It sometimes is possible to deign a masonry wall to span top to bottom, but this has much less lateral capacity and requires a lot of fiddly detailing. much more robust is to use a windpost.

 

The building regulations are a bit out of date on minimum return length. A wall with a cavity o 75mm needs 665mm. But many walls now have cavities of 100mm+ - this really needs a return of 780mm.

 

 

Quote

 

it's a mid-terrace house with fences from both sides, with the left-hand side fence being very close to the proposed extension

 

That's fine until the beast from the east MkII rolls through, blowing over the fences. Structural design for domestic houses needs to consider a 50 - 60 year time span. 

 

In summary:

1. Am I thinking in the right direction? Yes
2. Do you think windposts will be required if the minimum returns and P2 are not met? Yes
3. Is there a rule of thumb to when windposts are required? Masonry panels require support to be able to resist lateral loads. Most common is buttress wall in accordance with Part A. Designed solutions almost always require a windpost.
4. Does the 3.45m wall works as a buttressing wall for the other two side walls? No
5. What are the calcs needed to justify not using windposts? (e.g. Masonry wall panel design to EN1996?) EN1996 would suffice but it is unlikely to work without a windpost anyway.

[MJ6]

9 hours ago, Bonner said:

Have the drawings been submitted to building control for approval yet?

I spoke with the architect, the drawings were submitted for approval and it was the BCO who asked to explore the windposts option.

 

I agree with your point about the small wall not being considered as a buttress, that's why I asked the question just to confirm.

 

Any thoughts on the left hand side wall?

[MJ6]

8 hours ago, nod said:

Your Architect is probably prompting what BC will as for 

Wind posts are incredibly easy to install 

We had a similar extension on our last home I intended putting WPs in 

But our engineer designed an elaborate steel plate system bolted to the house 

I go with your Architects suggestion 

Yeah found out WPs are suggested by the BCO anyway, so calcs/justification are needed for both options.

[MJ6]

8 hours ago, George said:

It sometimes is possible to deign a masonry wall to span top to bottom, but this has much less lateral capacity and requires a lot of fiddly detailing. much more robust is to use a windpost.

I see your point, plenty of other requirements are needed to justify not using a windpost, even then, a windpost might still be needed.

 

 

8 hours ago, George said:

That's fine until the beast from the east MkII rolls through, blowing over the fences. Structural design for domestic houses needs to consider a 50 - 60 year time span. 

Good point. Climate change effects need to be considered.

 

8 hours ago, George said:

In summary:

1. Am I thinking in the right direction? Yes
2. Do you think windposts will be required if the minimum returns and P2 are not met? Yes
3. Is there a rule of thumb to when windposts are required? Masonry panels require support to be able to resist lateral loads. Most common is buttress wall in accordance with Part A. Designed solutions almost always require a windpost.
4. Does the 3.45m wall works as a buttressing wall for the other two side walls? No
5. What are the calcs needed to justify not using windposts? (e.g. Masonry wall panel design to EN1996?) EN1996 would suffice but it is unlikely to work without a windpost anyway.

As for point 2, would BCO be ok for not having these in place anyway?

And if sufficient returns are applied, will WPs still be required?

 

Also, what are your thoughts on the left hand side wall? Considering it only has a small bathroom window with sufficient returns at both ends.

[Bonner]

17 minutes ago, MJ6 said:

Any thoughts on the left hand side wall?

I can’t see any dimension of the return wall but I would say there is an argument that WP’s are not necessary on that side. You will need a SE anyway so see what they come up with.

[nod]

1 hour ago, MJ6 said:

Yeah found out WPs are suggested by the BCO anyway, so calcs/justification are needed for both options.

Most of the Architects are pretty tuned in to what BC will ask for

It just saves time to get in there first 

[George]

55 minutes ago, MJ6 said:

As for point 2, would BCO be ok for not having these in place anyway?

And if sufficient returns are applied, will WPs still be required?

 

Also, what are your thoughts on the left hand side wall? Considering it only has a small bathroom window with sufficient returns at both ends.

Any wall panel which complies with Part A rules is fine and won't need additional windpost/justification.

 

Anything that doesn't comply will need a structural engineer's input to satisfy BC. I've rarely ever made a (non-Part A compliant) wall panel work without putting in a windpost or reinforcement. Windposts are a bit pricey but fundamentally are just a steel section with a bolt top and bottom.

 

Left hand side appears fine to me. Only 6m long and at the bottom corner the two walls buttress each other. House construction is all about building boxes!

Edited January 2, 2023 by George

[MJ6]

1 hour ago, George said:

Any wall panel which complies with Part A rules is fine and won't need additional windpost/justification.

 

Anything that doesn't comply will need a structural engineer's input to satisfy BC. I've rarely ever made a (non-Part A compliant) wall panel work without putting in a windpost or reinforcement. Windposts are a bit pricey but fundamentally are just a steel section with a bolt top and bottom.

 

Left hand side appears fine to me. Only 6m long and at the bottom corner the two walls buttress each other. House construction is all about building boxes!

I think I have a better understanding of Part A now, thanks to your comments. So if you don't have a buttressing wall/chimney/pier and/or sufficient returns, then an alternative solution should be explored, which is WPs in our case, otherwise to reinforce the wall.

 

As for the left-hand side wall, this has both 90cm start/return, so this is Part A compliant. But in general, is there a maximum length of the wall to comply with this rule, considering openings are less than 3m?

As for the 345cm wall, considering a long lintel is installed over the door and window with stuffiest tying to the other walls, would this provide enough stability? What about the lack of return (P5) in the right hand corner?

[George]

53 minutes ago, MJ6 said:

I think I have a better understanding of Part A now, thanks to your comments. So if you don't have a buttressing wall/chimney/pier and/or sufficient returns, then an alternative solution should be explored, which is WPs in our case, otherwise to reinforce the wall.

 

As for the left-hand side wall, this has both 90cm start/return, so this is Part A compliant. But in general, is there a maximum length of the wall to comply with this rule, considering openings are less than 3m?

As for the 345cm wall, considering a long lintel is installed over the door and window with stuffiest tying to the other walls, would this provide enough stability? What about the lack of return (P5) in the right hand corner?

 

Maximum wall length covered by Part A rules is 12m. Although usually you start to break the rules (due to multiple openings etc) when you go over around 6m.

 

No a lintel won't provide lateral stability. Walls are panel structures and panels need linear supports else they will span in two directions. For masonry this rarely works.

 

A lintel will only support it at a 'point' (in the top corner). The difference looks a bit like this:

 

 

Orange zone being at risk.

 

I'm afraid we're into the realm of 'step 1: attend university and do a civil engineering degree' but fundamentally, the layout proposed does need a windpost. Best person to speak to next is an engineer who can specify what you need. Potentially a windpost manufacturer would do the calculation for you as well. 

 

What I would do is put a windpost at point 'W' and make the studwork wall a shear wall a point 'S' (this would be an extra layer of OSB and some additional strapping to the wall and the floor).

 

 

 

Edit - actually what are the red blobs? Are they the proposed windposts or something else? 

Edited January 2, 2023 by George

[Gus Potter]

21 hours ago, MJ6 said:

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.

Good for the sole to help out.

 

My thoughts are:

 

That extension is quite long 6.0m and will pick up a bit of wind on the long sides thus you need to stop it moving sideways in the plane of the main rear elevation of the existing house.

 

You starting point here is to ask.. can we transfer all the sideways wind load to the existing building and how do we do that.. and is the existing building able to take the extra sideways loads. It looks like a terraced house. One commonly accepted principle is that if you live in a terrace your house has to take it's "share" of the load. In other words you can't shed load onto your neighbours.. because if everyone starts doing it you can destabalise the whole terrace of houses.

 

The back door introduces a significant break in the load path, that is an anomoly particularly as it is next to a pier which will probably be carrying quite a lot of load from the main roof.. it will protest if you try and use it to hold an extension still.

 

My initilal thoughts looking at the geometry is that you'll need a goal post (steel portal frame) along the front elevation of the extension. If it only extended some 3.5m from the house then you may get away with that masonry return on one side.

 

I would recommend that you speak to an SE.

 

 

 

 

 

 

[MJ6]

1 hour ago, George said:

No a lintel won't provide lateral stability. Walls are panel structures and panels need linear supports else they will span in two directions. For masonry this rarely works.

Sorry, as for the lintel, I was referring to the 345cm wall, I know it won't stabilise the long wall. So just saying that one side would have a 90cm (P5) return but the other won't, which as I understood is non-Part A compliant, hopefully I got that right!

 

1 hour ago, George said:

Best person to speak to next is an engineer who can specify what you need.

I spoke with a couple, who advised no WPs are needed, if the 3 points suggested in my original post are followed! But as I wanted to be sure, I asked the question here.

 

1 hour ago, George said:

Edit - actually what are the red blobs? Are they the proposed windposts or something else? 

Yes they're, but now we know they're not needed for the left wall, only the right hand one.

 

Your suggestion of adding a WP at the bottom right corner would serve both walls I think, but as for the shear wall behind the wardrobe, can't this be replaced with another WP either at the start or the middle of the wall (where I put red "?")? - I understand that full calculations still need to be provided here.

 

Thanks again George, and sorry if it's a lot.

 

[Bonner]

9 hours ago, MJ6 said:

but as for the shear wall behind the wardrobe, can't this be replaced with another WP

Quite sure you can, I think the suggestion was to save cost. A shear wall will probably work out cheaper than a WP

[IDSE]

Hi there, I have a similar issue, with the difference that it's for the main house, not extension, for semi-teraced house. Currently there is 300mm to the party wall on one side and 605mm to the end of the wall on the other side. Now, the opening next to the party will be widened (from a small window they are making 3000mm bifold door), but the opening will be starting from 605mm from the party wall line. Will I need windpost at that opening or can I omit it as I am actually adding more buttressing masonry on that side? The house is older, so I believe there is no windpost there currently.

Looking forward to hearing your thoughts on the above!

Thanks!