Oak clading for steel framed orangery

[PhilC]

Hi, first post and need your advice.
I am building a steel framed orangery but I would like to give it the general appearance of an oak framed building so I will be cladding the steel posts (galvanised) in oak and face applying the glazing as if it were an oak frame.
for the inner cladding I am planning to use kiln dried approx. 38mm x 156mm and the outer air dried. The question I have is which method would be best to joint the inner cladding, either option 1 or option 2 shown below (please excuse some details and sizes on the diagrams, I borrowed it and edited it from another site)?

Option 1 would have a long mitre and an inserted plywood spline. I'm tending towards this design as I think it would make the covering look more like a solid beam having the join on the corner but I am a bit worried that it might open up in the future?

for both options I intend to glue them and use a second fix nail gun to hold them together which I should be able to hide the ends of the brads (stainless steel) easily enough. And fix the oak to the steel posts with stainless steel self drilling timber to steel fasteners covered with an oak plug.

Look forwards to your suggestions and any ideas would be most welcome.

 

[Random Orbital Bob]

Caveat no 1, I've never done this myself but......my mother in law had several RSJ's clad in an extension to her old oak cottage and the chippys used your rebated method precisely because even a splined mitre of that length and in a room with that much humidity and temperature variation, might open up over time.

They added a detail which you don't have which looks lovely mind which is they stop chamfered the length til about 6 inches shy of each end.

Here's a piccy of what I mean but on an outside patio planter to give you the idea.

Also make sure your stainless fixings are good quality or the tannins in the oak might show through where the fixings are.

 

[dickm]

Does it have to be steel framed? Doing the whole thing in (green?) oak might work as well?

 

[RobinBHM]

I use a trend mitre lock joint spindle moulder block for making orangery post covers, available from wealdon tools.

Most orangeries we make are painted but we do make some in oak. The simplest method is to build separate window and door frames and join them together with hardwood posts say 150mm wide x the thickness of the frames. Then clad in and out with oak boards. Cladding steel posts adds up to virtually the same amount of timber, and you will still need some form of cill and head to finish off the design as drawn.

If the orangery is under building regs, the square hollow section posts may need to be celetex clad to avoid thermal bridging.

Where are the steel posts to be? - cast into the foundation and coming up through the cavity (or resin fixed) or sitting on the outside skin? Up through the cavity is pain, its usually inaccurate and means big corner posts. Sitting on the outside skin may need steel pads to spread load

If lateral stability is a consideration, both steel and timber posts can work. A header plate can be a glulam beam if you use timber posts.

 

[PhilC]

Thanks for the reply's. Dick, the reasons we went for a steel frame are mainly because of the cost. An oak clad steel frame works out to be approx. 20% to 25% of that of a green oak frame. Also not having any green oak means we should avoid any movement which means less chance of leaks and less chance of any glass damage. Also, If we opted for a green oak orangery it's not something that I could comfortably build myself.

Robin, thanks for your very informative reply, I did look at mitre lock joints but they seem to be only available in smaller sizes than I was planning to use. As my steel posts are 80mm the oak thickness to give the look a solid oak post would need to be approx 38mm and I haven't come across jointers that would do that.

We did look into having separate frames made to sit inside the steel frame but these worked out quite expensive, almost the cost of an oak frame and also it doesn't give the clean, face applied look we are after.

We are under building regs and they haven't mentioned about thermal bridging, maybe because of the thickness of the oak we intend to use? after all there wouldn't be a requirement to insulate a separate oak window frame (thoughts?) I was thinking about filling the hollow posts with closed cell expanding foam not so much for insulation but more to stop condensation on the inside of the posts.

The posts will rest on the outer skin with a foot going back onto the inner skin (cast ring beam/pad stone) and resin bolted.

Any other comments or advice most welcome.

 

[memzey]

Caveat no 1, I've never done this myself but......my mother in law had several RSJ's clad in an extension to her old oak cottage and the chippys used your rebated method precisely because even a splined mitre of that length and in a room with that much humidity and temperature variation, might open up over time.

They added a detail which you don't have which looks lovely mind which is they stop chamfered the length til about 6 inches shy of each end.

Here's a piccy of what I mean but on an outside patio planter to give you the idea.

Also make sure your stainless fixings are good quality or the tannins in the oak might show through where the fixings are.

That's a really nice planter Bob! I have promised SWMBO that I will make a couple for the kids this Saturday and looking at your impressive efforts here has given rise a question or two I hope you won't mind helping me out with?

1. What is the material you used to line the inside and where can I get my hands on some?

2. Did you leave any drainage holes in the bottom? I'm not sure if this is good practice or not.

I don't intend to make them as snazzy as yours (my skills are sorely lacking I'm afraid) but I do want to make something that is fit for purpose and built well enough to last.

 

[RobinBHM]

The mitre lock joints do restrict thickness, which is 28mm maximum on the block we have. However you cant see the thickness of the timber, so you could make the U sections to the outside dimensions you want and pin on packers to suit inside. If anything a thinner section is likely to be more stable and you could add stress relief grooves. You dont want the timber tight to the steel SHS. Leaving a gap allows options if the steelwork is a bit out.

Which ever way you do it, I would always opt for the join to be at the corner, which after assembly you can run a router down for a small chamfer. If you choose option 2, it will always be obvious that it is a cladding and not solid.

You are right about avoiding a green oak option, Ive seen quite a few of these and they do create their own problems. The first is that the oak is both the structural element and the finished surface, so you have to content with water staining, and wet trade damage all to be cleaned off afterwards. Also there will be long term movement.

The softwood packer which covers the front of the post probably needs to be wider leaving an 18mm rebate for the glass, there is no real advantage having more glass cover.

 

[PhilC]

Robin, many thanks once again for your useful advice.
I guess that if I did make the U sections out of i.e. 28mm that would leave a small amount of room for packing with rigid insulation too. I have seen a router cutter that works up to 36mm but the problem is that either way I would need to find someone who would be able to do the cuts for me as I just don't have the facilities. That's why I was contemplating the splined mitre joint which I could do in house.
RE the softwood packer, yes I agree but the pic I found to illustrate the options was from a green oak supplier and they would need to cater for greater shrinkage/movement.

While I have your attention can I pick your brains/experience with regards to the EPDM tape to seal the glass? Have you got any preference to the lipped version or unlipped? I was thinking that If I did have a lot of glass cover I might use both the lipped on the edge and then the unlipped behind or is this overkill? I saw an American oak framed house, albeit with much larger glazing with at least 5 strips of tape!

 

[RobinBHM]

If you make the U sections thinner, you might as well make them from ex 32mm and finish at 25mm.

I wouldnt try a lock mitre joint with a router, its difficult enough to set up with a large spindle moulder.

If you use a splined mitre, assembly will be tricky since you are not cramp in the direction of the spline. I would try a test in softwood first.

It is difficult to cramp the corners square, you could cut some 'noggins' to the exact internal size to hold it square.

Lipped EPDM is a neater finish and guides the water away. You could double up the seal although more importantly I would do a double row of screws -into the packer not through to the steel. If you fit these oak covers with the smiley face (end grain) towards the steel posts they will naturally cup towards the glazing.

Before putting the in glass I would be tempted to do a bead of silicone in the back corner of the rebate behind the inside epdm seal -that will stop any water getting beyond this point. Also a bead of silcone in the bottom corners just before putting the glass protects the joint from water. Any water that gets in at the sides will run straight down to this point.

I would leave a minimum of 5mm air gap for the glazing.

Where are you finishing the outside covers to at the bottom? -if you cut these tight to a cill, you will get water ingress up the cover, Id cut these short if possible or seal really well with end grain sealant. All depends how you are coating the oak, if at all.

 

[PhilC]

If you make the U sections thinner, you might as well make them from ex 32mm and finish at 25mm.

I wouldnt try a lock mitre joint with a router, its difficult enough to set up with a large spindle moulder.

If you use a splined mitre, assembly will be tricky since you are not cramp in the direction of the spline. I would try a test in softwood first.

It is difficult to cramp the corners square, you could cut some 'noggins' to the exact internal size to hold it square.

Lipped EPDM is a neater finish and guides the water away. You could double up the seal although more importantly I would do a double row of screws -into the packer not through to the steel. If you fit these oak covers with the smiley face (end grain) towards the steel posts they will naturally cup towards the glazing.

Before putting the in glass I would be tempted to do a bead of silicone in the back corner of the rebate behind the inside epdm seal -that will stop any water getting beyond this point. Also a bead of silcone in the bottom corners just before putting the glass protects the joint from water. Any water that gets in at the sides will run straight down to this point.

I would leave a minimum of 5mm air gap for the glazing.

Where are you finishing the outside covers to at the bottom? -if you cut these tight to a cill, you will get water ingress up the cover, Id cut these short if possible or seal really well with end grain sealant. All depends how you are coating the oak, if at all.

What can I say but thanks, lots of useful info.

When you say smiley face towards the post is that the same as heart of the tree facing out? As I have seen and been told conflicting ways which the oak is likely to cup. Curve of the grain towards the post or facing out?
I was thinking of going as large a section as I could to make the spline easier/stronger, I think I'll have to try a few before I finally decide.
I could also make a buck from a spare post with some spacers to mould and clamp the U section around.
There isn't going to be a sill as the windows are floor to ceiling and any water running off the bottom of the window will go down the horizontal cover and onto the ground.
I planned on the vertical covers finishing on a similar horizontal cover over a packer the same depth. The bottom would have about a 15 deg downward bevel on it to match up with a similar bevel on the horizontal cover which again would help water to run off. Not planning on any coating for the oak.

 

[RobinBHM]

Yes is suppose there would be conflicting views, it depends on whether you consider long term the timber will shrink or swell compared to its starting position

Timber shrinks tangentially, so think of the rings trying to straighten out, then decide which way!