Why You Shouldn't Cut, Drill or Notch your Trusses!

Why You Shouldn't Cut, Drill or Notch your Trusses!

A frequent question that we are asked at Minera Roof Trusses is, “Can I drill holes in my trusses/ joists?” Most commonly, we’re asked this for plumbing and electrical reasons such as fitting pipes and wiring through the trusses after the holes have already been drilled and it has been picked up by a building inspector on site. 

These trusses have had holes drilled through the bottom chord so that wiring could be fed through – A big ‘No No’ in the rules of trussed rafters. 

We’re also asked on occasion whether customers are able to cut our trusses/ joists and this has been in cases where plans have changed later down the line in projects, decisions have been made to install a roof light or to cut trusses/ joists to fit into spaces where measurements have altered from what was originally supplied.

The answer to both of these queries is unfortunately, you’re NOT able to do this and we would like to explain why…

These trusses have been notched, metal plating cut and a length of timber removed so that they would fit between the wall plates. To be quite frank, it’s reminiscent of something we have seen on Nightmare Builders! 

The calculations which are provided for you are designed to negate the necessity of a structural engineer (a cost saving for you – you will thank us later) and can be used in their entirety for building control approval. Therefore, any alteration, no matter how small, risks invalidating our designs. Any alterations can negatively impact on the ways in which forces are transferred into the timber and plates.

This is another example of holes being drilled through the trusses, this time through the centre of a bottom chord where the most deflection can occur. The trusses have been designed to account for possible deflection to a certain degree. By drilling holes here, the integrity of the design has been compromised and therefore invalidated. 

Put simply, we kindly ask that you do NOT cut, drill or notch your trusses unless otherwise agreed with Minera Roof Trusses. Our metal web joists are also designed specifically to accommodate services in between their webs so there’s really no need to get that saw out. You can find further benefits of using metal web joists here. We go to great lengths during the design phase to ensure your trusses and joists are accurately stress graded and meet with EC5. 

This image is of someone who has had to take off the metal plating to make unauthorised adjustments to their trusses. We have plenty of timber in our yard but we use metal plating for a reason – please don’t try this at home!

If you’re in doubt, we say to always ask us before it’s too late! We’re on hand to help and give the best possible advice during design and for your installation. We will do everything we can to meet your design requirements but making adjustments afterwards that were not intended is a risky business!


Equally, the use and placement of metal plating is strategic – they’re not removed and replaced with DIY wooden ‘plates’ like this case shows!

Occasionally, it may be possible to carry out a remedial detail on site to the affected roof trusses/ joists by our Engineers but this is only achievable in rare circumstances, cannot be guaranteed and would incur an additional cost from ourselves, not to mention mammoth labour costs on site to reinstall or make adjustments. What a nightmare that would be!


This is an extremely rare case where we were fortunately able to carry out a remedial repair so that the trusses would be brought back up to compliance and pass building control regulation. The customer of this case was lucky and managed to avoid having to replace the trusses entirely. 

All of the images included in today’s news story are aimed to give you a better idea of situations where our designs have been invalidated due to adjustments being made on site after delivery without consultation. These have not only been costly errors but dangerous mistakes that we wouldn’t want you to fall foul of. All cabling and pipes should be situated in the void spaces at either side of the trusses. If in doubt, please contact us!




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