Joists

[ianfish]

Getting my head round joists and I like the look of the metal web joists with timber which seem to come in a variety of trade used names.

 

Is there much variance in price for these?

 

How much energy is it worth spending to get the best price?

 

As it stands at 400mm centres i reckon i need approximately 7 @6m amd 19 @4.4m as longitudinal joists

 

 

 

 

 

Edited July 30, 2020 by ianfish

[ProDave]

The 6M ones might be quite large.  Are these just for a ceiling?  If so it won't matter if the shorter ones are smaller.  If for a foor, make them all the size dictated by the larger span.

[ianfish]

32 minutes ago, ProDave said:

The 6M ones might be quite large.  Are these just for a ceiling?  If so it won't matter if the shorter ones are smaller.  If for a foor, make them all the size dictated by the larger span.

 

I had it in my head to run them all front to back and yes they are fora single story roof joists.

[Russell griffiths]

You generally use metal web joists when you need to run lots of services, they are normally dearer than I joists, and a lot dearer than solid timber, if it’s for a roof you probably won’t have a lot of services. 

Also how are you insulating it, warm roof no problem all on top, hybrid- cold roof they are a pain to insulate because of the metal strutting. 

[CC45]

the metal web ones we had put in at 600mm centres I wish we had put them at 400mm centres......  I like a nice solid floor.

[ianfish]

2 hours ago, CC45 said:

the metal web ones we had put in at 600mm centres I wish we had put them at 400mm centres......  I like a nice solid floor.

Not good?

 

Im after a roof only - single story

[Russell griffiths]

What’s those dotted areas on your plan are they roof lights, if they are you joists will need to be doubled up on the sides, probably need a bit of design work doing. 

[CC45]

2 hours ago, ianfish said:

Not good?

 

Im after a roof only - single story

OK, ignore my comments.

[ianfish]

22 hours ago, CC45 said:

OK, ignore my comments.

Why?

 

Thanks for your contribution 

[Gus Potter]

On 30/07/2020 at 11:18, ianfish said:

Getting my head round joists and I like the look of the metal web joists with timber which seem to come in a variety of trade used names.

 

Is there much variance in price for these?

 

How much energy is it worth spending to get the best price?

 

As it stands at 400mm centres i reckon i need approximately 7 @6m amd 19 @4.4m as longitudinal joists

 

 

 

 

 

 

 

 

For all. Here is an old rule of thumb which I have applied to Ian's case. This can be used just to get you a feel for how deep a flat roof joist needs to be. Appolgies for mixing units.

 

Ian.. what about this? if you have enough support and it's a warm roof (insulation on top roughly and thus no ventilation required to the joist void) then run a beam within the roof depth to bridge the bit of the kitchen that juts out. Now you have 26 joists @ 4.4m and 7 @ 1.6m. The beam needs to span roughly 7 x 400 mm = 2.8m so you should manage to get something within the roof joist depth that does not result in the beam downstanding from the ceiling.

 

Based on an actual joist metric thickness of 47mm and a spacing of 600mm (2 feet ) then..

 

Ian has a 4.4m span domestic roof  ~ 14.4 feet. Take the span in feet and divide by two.. 14.4 / 2 = 7.2 inches. Add one inch = 7.2 + 1 = 8.2 inches. Now convert to metric 8.2 x 25.4 = 208mm. Take the next metric size up for a C graded timber = 220mm This is the ball park depth you need for the joists. A common length for a bit of structural timber is 4.8m so it's off the shelf = cheeper than say a longer offered length of 6.1m as they generally need to be cut from bigger trees and so on.

 

You may be able to form the transfer beam from say three 220 x 47 timbers, if that is not enough then you can introduce what is called a flitch beam (two bits of timber with a steel plate between & all bolted together) or if that is still not enough! then a small steel beam.

 

The attraction of this is that all the 4.4m joists will bend and sag over time by roughly the same amount. You are working with solid timbers which a lot of local builders / diy folk are more comfortable with. If you cut a joist too short, nip to the mechants and buy another off the shelf. The connections between the timbers and transfer beam can be done with off the shelf timber hangers too. If a steel beam.. you bolt timbers to the steel I beam web and fix your hangers to these, this avoids trying to fix the joists to the steel directly.

 

Now you have a rough joist size and the concept you can start fine tuning. Watch out for other "non standard" types of load such as snow drifting off a higher roof and so on. You can fine tune stuff by changing the timber grade, closing up the spacing of the joists and so on.

 

Lastly the same old rule of thumb applies to solid joist floors with normal domestic loading at concept design stage but generally you add 2 inches to the depth instead of one. This is an old rule so now we also check explicitly for floor vibration and so on.

 

Once you weigh all this up, the skills of the bulder, practicality, material procurement and so on you may find that solid timbers are the most economic / least risk option?

 

All the best

 

Gus

 

 

 

 

 

 

 

 

 

 

[Nickfromwales]

38 minutes ago, Gus Potter said:

run a beam within the roof depth to bridge the bit of the kitchen that juts out

+1. Just about to say the same thing. Will make the detailing around the furthest skylight less of a ball-ache too. Flitch beam should be ample, and easy to bring in and man-handle into place piece by piece.

[Gus Potter]

4 minutes ago, Nickfromwales said:

+1. Just about to say the same thing. Will make the detailing around the furthest skylight less of a ball-ache too. Flitch beam should be ample, and easy to bring in and man-handle into place piece by piece.

Bang on Nick about the trim out for the roof lights. As you say, it makes the detailing / buildability of the roof lights easier.

 

Gus

[Nickfromwales]

2 minutes ago, Gus Potter said:

Bang on Nick about the trim out for the roof lights. As you say, it makes the detailing / buildability of the roof lights easier.

 

Gus

And a flitch + regular solid timber joists is a cheap as chips solution.

Tres bien, Rodney.

[Gus Potter]

Absolutely agree Nick.

Flitch beams seemed to be falling out of fashion for a bit... like flares (the trousers) but they are back in fashion now!

This is a great friendly site for passing on / picking up tacit knowledge and so on.

[Nickfromwales]

23 hours ago, Gus Potter said:

This is a great friendly site for passing on / picking up tacit knowledge and so on.

...and finally, the last three digits on the back of your card please.

[ianfish]

On 01/08/2020 at 22:41, Nickfromwales said:

...and finally, the last three digits on the back of your card please.

007....