What type of joists?

[osmononame]

Hi all,

 

I am in the process of finalising the joists for our build. I have looked at the cost between i-joists and posi joists with the difference being circa £4k. Wanting to save where I can those who have gone with a MVHR do you regret not getting posi joists or was it not much of an issue with the i-joists? 

 

The house is over 3 floors - ground, first and second. The MVHR unit will be on the second and BPC design shows that most of the rooms will require 2 runs of ducting to them.

[nod]

We used I joists and they are absolutely fine 

I counter battened with 3-1 on the underside To stop floor board creeks 

Though Posi are handy for the MVHR 

Which I decided against 

[PeterW]

2 hours ago, osmononame said:

the cost between i-joists and posi joists with the difference being circa £4k

What is the span and the size of joists ..?? And roughly how much in total area do you need ..?
 

I- beams aren’t that much cheaper than Posi joists, are you sure the posi quote doesn’t include the noggins/clips and boards too ..??

[MJNewton]

I retrofitted our house with MVHR and have I-joists. We were lucky in that I only had to cut through one as I could work with the direction for most runs. The one I did have to cut happened to need it right at the end, outside the safe zone, and so I had to reinforce it with a Simpson bracket designed exactly for this - was dead easy to use but around £25 so could start to add up if you had loads to do. 

[osmononame]

The JJI's are coming in at £5,416.50 without any brackets or fixings - this is for 245mm height at 400crs. The max spans are around 5.4m's. With the posi's the prices I have been quoted is £6,298.19 for the posi-joists and £568.02 for the solid joists making it £6,866.21 but these are 304mm deep (so losing a bit of height) but at 600crs. I was told by the manufacturer that the posi's at 400crs would be 20% more so that takes it to about 3k more - my mistake on saying 4k - I only sat down to do the proper arithmetic now!

 

The way they have designed the floors are using the posi's is longer spans - some up to 10m in length! The builder is really pushing the JJI's and I am conflicted. If the MVHR is not going to be an issue running the 2 runs of ducting to each room I am minded to save the money but honestly in the grand scheme of things is not a lot extra to push for the posi's!

[Russell griffiths]

You need to look at the standard deflection rates, then specify you want it better, I wouldn’t want anything at a 10m span, and I would also really look at avoiding 600 centres. 

You are only going to get one chance to do that floor, spend your money on that and take £2000 out of your kitchen budget, you can always upgrade that, but you will hate a bouncy floor forever. 

[PeterW]

18 minutes ago, Russell griffiths said:

I wouldn’t want anything at a 10m span

Oddly, if it is spanning over a load bearing wall it may improve the structural strength as you will transfer the tension in the chord past the wall into the next room. 
 

I would still be checking the pricing though as the variation sounds too big. Some builders like JJI joists as they can be adjusted on site - a luxury you don’t get with a posi joist. 

[osmononame]

58 minutes ago, PeterW said:

Oddly, if it is spanning over a load bearing wall it may improve the structural strength as you will transfer the tension in the chord past the wall into the next room. 
 

I would still be checking the pricing though as the variation sounds too big. Some builders like JJI joists as they can be adjusted on site - a luxury you don’t get with a posi joist. 

 

It is (spanning over load bearing wall).

 

I have specced the deflection at 8mm or better on the JJIs at 400 crs (joists require to be doubled on some of the longer spans). Not got to that discussions with the posi guys yet but if I go with them I will be making sure that its specced for decent deflection rates i.e. 8mm or better. We have 600 crs where i am just now and the bounce is more than noticeable so will definitely try to avoid that.

 

I have tried just about all the suppliers in Scotland - MiTek and Wolf system manufacturers but they have all come in at a decent bit over the JJI equivalent - unless my builder is getting really good rates for the JJI's (he has got that quote and I can substitute it with my own joists and pay the difference - he will allow that) I don't know why I am finding there to be a bigger cost difference than you guys!

[Gus Potter]

Hi all.

 

For the self builder here are a few practical observtions. Can anyone else chip in?

 

If you are self building your frame will often be out in the rain a lot longer. Engineered joists require a lot of other stuff to make them work and you need to know how you supervise that to ensure you have all the other bits that make a long span Engineered Joist work. One reason they are cheep in terms of spanning is that they they are working a lot harder / efficiently, they rely much on bracing, stiffeners and so on. In summary, they are very much less forgiving than a solid bit of timber.

 

There is a big difference between the self build and the major house market. If you are self building it maybe takes you longer. Also, you could be employing a general local builder rather than an experienced specialist kit erector with all the site / quality controls in place... yes there are many large house builders that don't deliver on this.

 

Say you have an Engineered joist where the web is glued to the flange... then although the manufacture's say they use water proof glue they caveat this a lot in terms of what you need to do to look after them during construction. Perhaps take a 1.0m length off cut of a glued Engineered Joist, soak it in a barrel a few times to reflect a self build application, take it out and hit it with a hammer... then decide if that is the right thing for your application. You may find that the flanges separate from the web pretty easily. The ones with the metal webs are essentially mechanically fixed and seem to be a bit more forgiving in terms of the self build where things might be out in the rain longer.

 

Russell makes a good point on the deflection /vibration so consider his advice. A 10.0m span is not small so be very careful!

 

In summary, sometimes for the self builder perhaps look at your own circumstances in terms of budget, programme and then match the materials to give you the best outcome and reduce your financial risk?

 

[osmononame]

Thanks guys - I have emailed the local posi supplier (who had the best quote) to see if there was any movement on the price. It's good to know that if I go with the JJI's the MVHR will still work but perhaps I will need to strengthen in a few bits.

[trialuser]

Have you had a quote from Stevenson and kelly. I got my easi joists from them, best price, responsive to questions, delivered on the stated day.

304 x 147 at 400, spanning 6.3M, 7mm deflection. Best bit was I'm too fat to fall through the gaps :-)

I asked for 3 quotes, one never replied, one dried up when I started asking questions about reducing deflection (posi), the best one got the order.

[dpmiller]

going through our TF quotes, the extra cost to go from timber to posijoists was 2-3k, over 120m2. Spans all around 5m.

[Gus Potter]

Hope this helps when choosing your joists, maybe thinking about deflection / strength of floors / cost and engineered type joists or solid timber.

 

Roughly, if you weigh 100kg the joists will bend by X amount when you stand on them.  If you put on a 50kg rucksack they will bend by about 1.5 times X  roughly 50 % more.

 

Remember that they will have bent a bit during construction due to their own weight and the floor / ceiling / services you install hence my approximation.  But very roughly if you have joists at 600mm spacing and you reduce the spacing to say 300mm you will reduce the extra deflection due to walking about and so on by about half. The cost will increase as you need more of them and possibly more strutting / bracing etc.

 

To get the best out of things.. Imagine you are working in the pricing depatment of a timber frame outfit. You may do tens and more quotes a day, many of them small and with not much financial motivation. You could get a bit bored after quoting the tenth domestic floor job? However, if someone gets in touch that has clearly put a bit of effort into understanding what they think they might need, phones up and says they need help but have tried their best to get to this point, then often the flood gates will open and you will get a lot of help, maybe a better price too! We are all human after all.

 

For the very keen..

 

Maybe think about the benefits of having a slightly thicker floor in terms of cost and practical space to get all your services in.

 

As an alternative you can increase the depth of the engineered joists / or thickness of the flanges and this too reduces deflection. All other things being equal you can do a qualatative analysis just to give you a feel for things.  You don't need to know / calculate the loads etc you just need to know what is called the second moment of area of the joist. You can often find this in the manufacture's property tables for engineered joists.

 

The second moment of area (units often mm^4 / cm^4 or m^4, does not matter so long as you compare like with like) is partly a function of the depth of the joist.You will be able to see how it rapidly it increases with a small increase in joist depth or introducing a thicker flange on an engineered joist. The higher the second moment of area roughly the less the deflection. There is another kind of deflection called shear deflection but we are just trying to get a rough feel for things at the start.

 

If you are looking at the tables just make sure you have the axes correct. Under the British Standard notation the value you want is the Ixx direction. Under the Eurocodes which most data is now presented in the vertical direction is called the Iyy direction.

 

Again and roughly if you increase the second moment of area by say 20% you will reduce the deflection proportionally . There a few other things to consider but at the early concept stage this can be a very useful tool to give you a feel for things.

 

 

 

[Mr Punter]

On 12/07/2020 at 22:58, osmononame said:

I have specced the deflection at 8mm or better

 

You have done the right thing!

 

Bear in mind that with solid joists at 300 centres they are tricky to drill.  Also beware of strongbacks, steels and trimmers that get in the way of ducting.

[osmononame]

On 13/07/2020 at 00:30, trialuser said:

Have you had a quote from Stevenson and kelly. I got my easi joists from them, best price, responsive to questions, delivered on the stated day.

304 x 147 at 400, spanning 6.3M, 7mm deflection. Best bit was I'm too fat to fall through the gaps ?

I asked for 3 quotes, one never replied, one dried up when I started asking questions about reducing deflection (posi), the best one got the order.

 

It's Stevenson and Kelly that are the best posi quote I have - they haven't come back to me on a final price and I suspect that they might not having perhaps already given me the best they can offer. I will have to wait and see and when it's time to order one or the other I might call them to see if they can do anything and if not just go with the JJI's.

[eandg]

On 12/07/2020 at 14:07, nod said:

We used I joists and they are absolutely fine 

I counter battened with 3-1 on the underside To stop floor board creeks 

Though Posi are handy for the MVHR 

Which I decided against 

Any tips? Through an oversight on my part we have I rather than Posi joists and everything I can find online refers to using web/posi joists for MVHR. 

[PeterW]

36 minutes ago, eandg said:

Any tips? Through an oversight on my part we have I rather than Posi joists and everything I can find online refers to using web/posi joists for MVHR. 

 

Do you mean JJI type with the OSB web ..? They can be drilled - manufacturers guidelines tell you where and by how much

[osmononame]

In the end we ended up with i-joists and it wasn't an issue drilling to run the mvhr pipes. The manufacturer has guidelines for where and how far part you can make the openings. The cost together with the builder preferring the I joists ended up being the deciding factor for us and I have no regrets with the decision we made. 

[MJNewton]

54 minutes ago, PeterW said:

 

Do you mean JJI type with the OSB web ..? They can be drilled - manufacturers guidelines tell you where and by how much

You can also get reinforcement plates that allow you to remove the entire web to quite a hole size (full height and at least 30cm a cross). Pricey but handy when/where needs must. 

[eandg]

21 hours ago, PeterW said:

 

Do you mean JJI type with the OSB web ..? They can be drilled - manufacturers guidelines tell you where and by how much

Yes - sounds hopefully straightforward then!