A wall flashing detail for a steel shed using Trimdek profile fixed vertically.

A wall flshing question from a reader gets me to fire up my drafting program, as it is hard to answer questions like the one below from Phil in Palmerston, NT. with text alone.

Looking for info on fixing colourbond flashings at building corners when Trimdek used running vertically.  Flashing attached to Trimdek itself, not to girts etc.  All info I get from people (including Certifier) is "fix as per manufacturers specifications".  Can't find DTMC sheet covering it.  Can it be pop riveted or does it have to be screwed.
HELP, please.

(DTCM - Northern Territory Deem To Comply Manual).

My answer to Phil was along these lines.
Hi Phil,
Like any other flashing, it should be fixed to the same members that the sheeting itself is fixed to, and fixed through the crest of the profile.
So in the case of vertical sheeting on a wall, where the sheets are fixed to girts or noggins (in wall frames), then that is what you fix to.
With the same gauge screw (ridge type) as the sheeting in general.

In the case of "C" or "Z" girts at a corner, one wall's girts will be just about the full length of the wall and be close to the end, but the other wall's girts will have to be shorter by the depth of the girts used.
If the girts are 150 deep then you have at least 150 of nothing to screw to.  In a bad case with your Trimdek layout you may have to make your flashing something like 250 wide.
(Because with any profile, not just Trimdek you have to lap over at least one crest in the profile to weather the vertical joint).

steel wall cladding fixed vertically
Steel wall flashing - sketch of a layout using steel"C" purlins.

This makes for some big flashings at some corners and not at others, which looks crap if you are using this system on say a house.
So the answer is if you want to avoid odd width flashings on vertical corners in Trimdek, is to set out the wall sheeting bearing this in mind.
That is don't start at one corner with a full sheet and just hope it will be OK the other end.
Measure it up and adjust the position of your first full sheet slightly.
You might have to slide it say three quarters of a pan width one way or the other to get it so that both ends will be even.

vertical wall flashing
Steel wall flashing - A plan view of the same layout.

That being said, on most jobs this doesn't happen, the cladders just make bigger flashings and it is accepted, as the buildings are usually large industrial type sheds.

If you have already fixed the sheeting, and bought the flashings, don't fix with pop rivets. They should not be used, (although the flashings for the Custom Orb at Parap Village is full of them). Fix to the sheet with normal looking screws, in line with the other screws, but try to get something extra behind to screw to.

Phil gave me some more information.

Thanks for your advice, and by the way your web site has been of good use.
I've been fortunate, luck rather than good planning, that my shed project has ended up with the Trimdek being almost perfect for the 6m sides (8 sheets) with just one pan of extra overlap required on each side.  Therefore the flashings are really only of minimal size with the maximum flat area being less than 80mm then just the angle back over the ridges.

I gather that you are saying that the screws have to be on that flat area and not on the angled back section.

Due to the fact that the Trimdek had to fastened to 4mm thick steel the screws for this have been 14-20x22mm hex heads with climaseal washers, but I've had to predrill each hole as even those screws are reluctant to easily get through the steel.  These screws are rather large just for attaching the flashings to the underlying Trimdek so I'm hoping that I can use something a bit smaller like 10-16x16mm hex heads with washers so that they are less visible.

So it seems that Phil's flashings look something like the small one in red in the sketch above.
He is not using 'C" purlins though, as the material is 4mm thick! The sheets won't blow off that too easily.

  • We are talking about a shed here, so I would say that the less than 80mm wide flashing here would be OK, given that there is a profile crest under it to weathe the joint of course. (as drawn above).
  • I'd definitely use the 10g screws, and as Phil implies, I would fix though the side slope and not the flat.
  • I'd try a couple, and if there was any sign of the screws not pulling the underlying sheet tight to the flashing, I would be pre-drilling to try to get them pulling up snugly.
  • The reason for this is that fixing to an end of a sheet, and more so if that end isn't supported, then the force of driving in the tek screws distorts the end of the sheet under, making it sometimes a less than neat joint.

Note! Building certifiers seem to have different ideas on things like this that are not documented or detailed.  Some don't mind the use of pop rivets on these smaller flashings and others stick to the full size screws.   So there are a couple of options.

  • The best being to ask the certifier what he wants to be done when he is making a previous inspection.
  • The other way is to just go ahead and do it as we have discussed, neither too small (pop rivets, or the larger guage screws).  If the certifier wants it changed it is easy enough to do, nothing is being hidden or covered up.

Also Note! I show in the sketch above that the large flashing is going over two crests. This is mandatory on roof sheeting, but not of walls like this, the inner crest could quite easily be missing, but you must lap over one crest.

Leave this wall flashing page and back to home.

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