Box Bay Window

[Lift span]

Hi,

 

I want to build a box bay window out of timber and I was looking for a few tips regarding construction.

 

Here is the existing window:

 

 

 Here is what I’m hoping for:

 

 

 

 

Here is what I thought: 

6x2 timbers bolted to the wall vertically with wall anchors.

6x2 and 4x2 timbers as shown below.

2 or 3 gallow brackets, again with wall anchors into the brick work.

 

Then insulation, plywood, covered in membrane then clad.

 

Caldding would be either renderboard plus render, aluminium, or wood.

 

 

 

The aluminium window weighs 143kg and it would also need to be strong enough to sit on.

 

Internally it would look like this:

 

 

 

Here is the existing internals:

 

 

 

Does any body have any tips on construction or cladding?

 

 

 

 

Thanks!

[Oz07]

I'd be looking to incorporate some angles pieces in those sides. Going from bottom corner closest to house to top corner away from house like a hidden gallows bracket. One or two in centre as well

[Russell griffiths]

All looks reasonable, but I would re draw it with your gallows brackets to get a better idea. 

I would also use resin anchors as with no expansion they will not displace the brickwork. 

[PeterW]

Like @Russell griffiths says, I would get the gallows drawn up first. 
 

If there are no side windows then bring the gallows bracket inside the timber frame and secure to both the inner and outer brickwork and blockwork. As you’re building quite a deep external without a matching internal recess then  you have space to hide more steelwork. 
 

I’m assuming the whole house is getting cavity wall insulation at some point as part of this..?

[Mr Punter]

I have some of these oriel windows in timber frame, but 2300mm wide and 330mm deep.  Your cavity wall looks about 275mm, so you will have quite a deep cill and reveal with your 400mm add-on if you set the window near to the outside.  If you clad the sides with OSB it will act as racking resistance.  Ours are just timber frame panels and they don't have gallows brackets.

[Lift span]

Thanks everyone! That’s a great help.

 

I’ve removed the current windows and the existing rafters above have L shaped timber brackets on to form a triangle. The windows were screwed into these. I reckon I can use them too.

 

How about the design below:

 

It has 6x2 timber for the verticals, the middle horizontals and the “noggins”. The rest is 4x2. I’ve doubled up around where the windows will go.

 

 

 

 

 

 

This would then be clad in 11mm OSB with insulation in the spaces above bracket level:

 

 

 

 

 

 

How about these box section brackets below with resin fixed bolts into the wall.

Maybe I would only need two brackets?

 

 

 

 

 

 

[Russell griffiths]

Two things, don’t like those gallows brackets, I would look at the angle iron version available from most builders merchants. 

 

I would look  at the brickwork and find how stable it is to take fixings that bit under the bay window looks a bit suspect, according to this it may pay to get a couple of brackets fabbed up so you can get fixings lower down to pick up better brickwork. 

 

I have just just had a couple of posts fabbed by a local steel company and they where cheaper than expected. 

[Lift span]

Thanks Russell. I think you are right . The brickwork is better lower down and the wall ties don’t start till about a metre down as well.

 

 

[Mr Punter]

From your photo it looks possible that you may have asbestos board around the existing window.  You can get it checked really cheaply and it will give everyone peace of mind.  Some can be removed DIY and some is v. dangerous.

[Mr Punter]

I would set the cill at 450mm from FFL so you can use it as a window seat. The window as drawn looks a bit wide and squat.

[Lift span]

Very observant Mr Punter, it’s asbestos cement board.  The Building Control Officer noted this and was very helpful about how to dispose of it. I’m fine though ... I got the Mrs to bash it off ?.

 

Nice idea about the window seat. However, I can’t see anyone ever sitting there apart from the dog. He likes to perch there and watch the world (well pigeons) go by. A low seat means he couldn’t. So, I’m really doing all this for the mutt!

 

Here’s how it looks now:

 

 

 

 

 

Regarding the brackets, I spoke to a local chap and he said get a structural engineer. I asked him if he was to over engineer it what would he do and he reckoned 80mm box section with longer verticals as shown below:

 

Looks a bit over the top to me but might be the quickest option!

 

 

 

 

 

Edited October 22, 2019 by Lift span

[Russell griffiths]

I personally think that 80mm is very over the top, but that’s for you to decide. 145kg is the weight of two men sitting on top of those brackets plus all other timber. 

 

Your original brackets where going to be 40x40 

 

i think i would go with 50x50 box 3-4 mm wall thickness 8mm plates to wall with 13mm holes to except 12mm studs chemical fixed avoiding brick joints 

 

your latest drawing looks good, I would notch those side verticals a bit deeper to carry more load on the brackets. 

Treated timber c16 not that crappy csl 

[Oz07]

Just don't go washing her overalls for her @Lift span

[Lift span]

Thank Russell.

 

Chap I spoke to was a local fabricator. He did say the 80mm section would hold the house up!

 

I like your 50mm better! How about the flanges though?

 

This:

 

 

 

or this:

 

 

 

 

[Russell griffiths]

First version, second is hard to drill through and inject the resin, with the tabs on the sides you can see what your doing. 

[Lift span]

Here you go...

 

Timberlok and resin anchor heaven!

 

 

 

 

 

Even though I’ve tied it into the rafters, I’m wondering how much weight a cavity wall can take.  I may well end up with some steel supports below. Brackets are 60mm box section; 24hr turnaround from a chap up the road!

 

 

 

Edited October 30, 2019 by Lift span

[Mr Punter]

The wall could take lots more.  The cantilever is not very big, and neither are the loads.  It would be good to see some more fixings on the bracket lugs, especially at the top.

[MikeGrahamT21]

I'm going to keep an eye on how this turns out, very interesting. I currently also have a bow window, but would like to replace it with a normal flat window, which is hung further out of the brickwork, partly to keep it a little more in line with external wall insulation, and partly to keep the deeper cill.

 

I'm curious though, would this type of thing have to go through planning permission? I know for a proper bay window conversion you have to have planning, but I wonder if this would be covered under PD?

[Lift span]

Hi,

I had to get planning permission for our front extension as it was beyond the front building line. The planning officer suggested squaring off the existing bay window ... good idea said I!

 

However, that was amendment to the existing approved application so I had to pay £35 to get this amendment through. Things work very slowly in the planning world. I had no idea a minor amendment could take two months to pass; and it was his idea!

 

 

Now with 18mm ply (heavy!) added:

 

 

 

 

Thanks Mr Punter!

 

I did wonder about adding more resin fixings at the brackets. I’ve only gone into the outer skin with with existing fixings. However, my main concern has been the turning moment from the whole structure pulling the whole wall away from the house. I’ve mitigated this by fixing the upper part of the frame to the roof rafters. I’ve added extra 4x2 timbers to make a triangle with the hypotenuse formed by the rafters. It seems pretty solid ...I’ve been hanging off it most of the afternoon!

Edited October 31, 2019 by Lift span

[MikeGrahamT21]

Thats a real shame it requires planning permission, i can understand it for a proper bay, as it will have foundations, but didn't think this would need it