Pan head or countersunk screws

Typically, we wouldn't put the steel trunking direct on the wall - it would be more usual to put up sections of channel first, and fix that for the load ratings to the wall, then fix the trunking to the channel with dome heads (it has the advantage you can also pass some services behind the trunking


If you do go "direct to wall" then panhead is the way to go, as trunking has insufficient thickness to use countersunk (without dimpling the trunking into the fixing hole)


Regards


OMS

Hi OMS


I would always use a large flat washer regardless of what screw type I was using to stop the dimpling effect that you described. My query was more regarding what screw head type would be best to avoid cable damage as the way dome head gutter bolts are used to join trunking together and the bolt head should always be on the inside of the trunking

Sure - but using washers actually makes the problem worse - dimpling tend to crate a natural countersink - the washer just leaves the "raw edge" of the screw head showing to potentially drag or damage cables


Regards


OMS

The ‘raw edge’ of the screw showing would in my opinion be no sharper or pose any more of a risk than the cross slots of a gutter bolt. On a large steel trunking (4x4) or larger, I wouldn’t like to be relying on a screw, regardless of type) without a washer to hold the potential weight of a full trunking. Trunking naturally has lots of ‘sharp’ edges to snag cables on no matter how careful you are when cutting and filing such as the lip of the trunking, raised edges of manufactured bends and the rotating clasp of the lid. I know these aren’t that much of a problem apart from the last one

OK - I've suggested that you select panhead screws in favour of countersunk screws  - back in the day that's what we were taught as apprentices and it seems perfectly reasonable to do so based on a reasonable view of what you are trying to achieve. Certainly I wouldn't look that favorably on anyone belting in PZ 2's with countersink heads as it's the wrong screw to be using in sheet metal, full stop


You'll be telling me next that it's OK to drill straight through the metal trunking with the masonry SDS ?


I guess you pays your money and takes your choice


Regards


OMS

Fair enough. That was the question that I asked and you have given your opinion, and it is a good one. I was merely pointing out that a washer with a countersunk screw doesn’t really pose much of a danger to cables in my opinion. Yes the screw head does sit proud of the washer but it is not so sharp as to damage cable insulation and that a washer would be beneficial to void the screw head pulling through the hole.

Im grateful for your opinion, thanks

HI Baldyhugh


If I had to fix trunking to a wall - I'd certainly use uni strut and then fix the trunking to the uni strut with a gutter bolt and zebedee. I actually have loads of uniformly cut uni strut just sitting in a crate, already filed smooth and painted with galv paint on the ends; its really easy to level up the trunking as you go then too. Fit the end caps on the uni strut and it looks the business. 


But But But - I'd never install three compartment trunking supplying office desks for example (Metal or plastic) on uni strut. I'd then fix direct to the wall with a round head or torx screw and washer. Horses for courses I guess.

Hi Tatty


This recent job was a mixture of suspended unistrut for the trunking and the rest was trunking fixed directly tothe wall. I used frame fixings and washers for this part. When you mentioned torx screws were you referring to the countersunk head type that screws directly into the masonry?

I must admit I have never quite understood the use of gutter bolts and square nuts (on the outside) with trunking. "Proper" nuts have not been square since the 19th century, and why they are favoured by some electricians appears to be hidden in the long lost past. The usual reason for gutter bolts (sometimes as long as 50mm) seems to be that the job is being made as difficult as possible, in that it is almost impossible to hold the head still when tightening to a suitable torque, without the use of pliers or more complex tools, and electricians cannot be bothered to source bolts of the correct length. 5mm long conventional hex bolts and nuts have little or no thread exposed and can be fitted and tightened with a spanner and socket, using an impact driver if you have one, about 10 seconds each. If you want a bit more length tolerance simply use nylock nuts. Trunking cables should not be "drawn in" anyway as there are still sharp edges to bends and cut lengths, but placed in carefully, when no cable damage will occur. Proper bolt threads and ends are not at all sharp, and star washers under the nut will make much better electrical connections, probably with 6mm bolt length. Countersinks are impossible in sheet metal of 1 or 1.2mm thickness unless very tiny bolts (m3) are used and there is no drilling tolerance available. The question is why use gutter bolts (at huge prices from the wholesaler) when the proper engineering ones are cheaper and readily available (although Screwfix does not have the short lengths, although fixings wholesalers do). A Brummie screwdriver for trunking work is a bodgers tool!