Reinforcing joist holes in the UK

[Sparrowhawk]

In the states they've got products on the market to let you make holes bigger than 65mm / 0.25x the depth in solid joists like this Joist Repair Hole Reinforcer 2810HR By Metwood or this one from Skyline.

 

Over here in the UK I've seen one for I-Joists so you can make large holes in them, like the I-Joist Hole Support from Simpson Strong-tie but nothing for solid joists. Are there any products approved for use here?

Edited October 8, 2023 by Sparrowhawk

[Temp]

5 hours ago, Sparrowhawk said:

Are there any products approved for use here?

 

I doubt they can blanket approve something like this as too many variables. I suppose it's upto your BCO to approve it and they might want to see the design justified by a SE to cover their backside.

 

Normally joists are sized to allow some holes without reinforcement. These should be on the vertical centre line and a diameter no more than a quarter of the joist height. There is also a limit of where they can be along the joist.  Not sure if these are the latest rules..

 

https://www.diydoctor.org.uk/projects/Notching_joist.htm

 

 

[SteamyTea]

Would it be possible to drill a large diameter hole, say 70% of the depth, then bond in a steel tube, with an ID ≤ 60% of the depth.

A project for @Gus Potter to ponder.

 

[Gus Potter]

On 08/10/2023 at 09:40, Sparrowhawk said:

Are there any products approved for use here?

Hiya.

 

The reinforcement options like the Simpson etc have their limitations and these are reduced further when tackling solid joists.

 

One reason for this is that the engineered type joists are less variable in terms of the local strength.. which is where we want to put nails / screws etc.

 

If you only want to hole one joist with a big hole then look at trimming the joists out.. like a stair well and double the joist up either side for example.

 

If you can't do that then we may plate the joist with flat steels each side but these steels need a hole also and thus up to a point the larger the hole the thicker the plates. At some point the hole becomes too big and even the steel plates won't work.

 

Now the plates will be long. A starting point would be a steel flats 1.0 m long each side and be about 10mm to 15mm less than the depth of the joist to account for timber shrinkage. Here the joists shrink in depth, the steel doesn't so it can pop you floor and or a ceiling.

 

An important thing to remember it that timber is graded at the size it comes out the mill based on the complete section depth and width. Now timber has knots, shakes in the grain etc. It's sods law that where you want to cut the hole will be right where there is a big knot or defect in the remaining grain... and now all the theory can be invalid.

 

Each case needs considered on it's merits. Can you tell us a bit more about what you want to do?

 

On 08/10/2023 at 15:42, SteamyTea said:

Would it be possible to drill a large diameter hole, say 70% of the depth, then bond in a steel tube,

 

This concept works for steel beams where we hole the webs reasonably close to the middle of the section and weld in a tube to transfer the stress around the hole. But I don't think it would work for timber as not least when the timber shrinks it would stress the glue... and then we would need a glue that can really bond timber to steel for structural applications.. and also be able to be used under site conditions?

 

 

 

[Alan Ambrose]

The American products are interesting - they suggest that a 50% hole can be supported with something ~40cm long with a 40mm flange made from 2mm steel and screwed.

 

FWIW it wouldn't be a big deal to get those custom made. Besides destructive testing, I wonder if they can be modelled?

[Gus Potter]

15 minutes ago, Alan Ambrose said:

Besides destructive testing, I wonder if they can be modelled?

Yes you can in say a finite element analysis but in real life and on site it often does not work out that way, thus we need to apply some serious safety factors.. and then you find solution is often not cost effective and not easily buildable.

 

I often embark on ideas like this.. and sometimes get things to work.. but then I come up against fire protection / buildability issues for example and I'm back to square one.. but it was fun looking at it..

[Sparrowhawk]

11 hours ago, Gus Potter said:

Each case needs considered on it's merits. Can you tell us a bit more about what you want to do?

 

While I've got the floors up I want to install MVHR ducting. There are ground floor rooms where there's no vertical drop from the loft, and the joists run across the direction the ducts need to go. So I started looking at how I could cut big holes in joists. FWIW the first floor joists are 7 inches deep.

Edited October 11, 2023 by Sparrowhawk

[JohnMo]

You really need to investigate coanda nozzles for MVHR, makes the whole process way less complex. Basically put the outlet or extract points anywhere that is convenient and let the air flow and it's tendancy to stick to a surface such as a ceiling do the hard work.

 

Also research cascade ventilation strategies with MVHR, which can tie in to the above.

[Gus Potter]

On 11/10/2023 at 08:29, Sparrowhawk said:

While I've got the floors up I want to install MVHR ducting.

Can you run rectangular ducting from the attic void under the ridge within the depth of the rafters insulate the duct then drop that down to the first floor ceiling?

[Sparrowhawk]

On 13/10/2023 at 21:14, Gus Potter said:

Can you run rectangular ducting from the attic void under the ridge within the depth of the rafters insulate the duct then drop that down to the first floor ceiling?

Yes I think I could. Currently there's 100mm gap and 50mm PIR between the rafters, so if I used a 60x200mm rectangular duct, had a 40mm layer of PIR and packed either side of the duct with insulation that would do the job. I'm not going to be able to insulate it properly where it passes over the wall, but that's a short max 0.5m length that will be cold.