Joist doubling equivalence

[John37]

We are making a flat roof ground floor extension 4.5m x 6.4m and I would like to use roof joists no deeper than 175mm because there are height issues.

Would doubled 47x175 joists connected together with double sided toothed connectors be suitable for the 4.5m span?

Would such doubled up joists have deflection strength equivalent to single joists of 94x175?

Grateful for any advice. John

 

[catlad]

You could use engineered joists like Glue lam or I beam for greater distance.

 

[Static]

If you went for grade C24 joists doubled up it should be ok but would be a little bouncy but within limits assuming you dont plan to put a room up there.. and no it doesn't have the same strength its slightly stronger doubling up..

 

[theoldun]

For access and maintenance only, to support a 0.75kN/m2 snow load and a 0.075kN/m2 dead load, C16 75x175 regularised to 170 at 400 centres will span 4500, or C24 ditto, ditto, ditto will span 4850.
You will get no bounce on a flat roof unless you keep a pet elephant up there, but put two runs of C16 47x150 solid bridging in to strengthen it up
You need to order 4800 lengths and they will normally be 7 day delivery.
Regards oldun

 

[Static]

You will get no bounce on a flat roof unless you keep a pet elephant up there

Elephants weigh 5-6tn the roof would fail in shear way before it got a chance to bounce..

C16's doubled would be close to 13mm deflection, why not go C24 and avoid cracks in ceiling plaster..

 

[John37]

Thank you for your helpful replies, sorry about the elephant though!

According to the TRADA span tables (looking at C24, access & maintenance only, 0.75kN/m2 imposed and dead loads, 400mm centres), 47x170 would be strong enough for 4.07m span and 75x170 would be good for 4.85m span.

I need 4.6m span but I can't get C24 75x170 locally. Therefore, following Static's advice about doubled up strength, if I double up all the C24 47x170 joists to make 94x170 it seems that they should be more than adequate. Does this seem correct, please?

I need to put two large Velux roof lights 1200 x 1500 in line with each other which means removing two joists and trimming around them. Would I need to quadruple the joists and trimmers around these window openings?

John

 

[theoldun]

Elephants weigh 5-6tn the roof would fail in shear way before it got a chance to bounce..

Never knew that, must make a note for the future.
Regards oldun

 

[Static]

Yep 2no. 47x170 C24's should be fine at 400centres..

The roof lights may be an issue.. i doubt you could get sensible timber to work between the lights, may need to put a steel in there..

 

[John37]

Thanks.
I don't fully understand what you mean about getting sensible timber to work between (around?) the roof lights. Please can you tell me more?

Since I'd be removing two of the "composite" joists for a distance of 1500 + 1500 to make the two 1200 openings, wouldn't the rule of thumb work, i.e. doubling up the composites around the opening? I know I'd have to offset the quadrupled joists (which would be 188 wide). Is this the problem, they would be at greater than 400 centres?
The roof lights would be separated from each other in the direction of the joist span by 500mm or more (tbd) so there would be space to fully trim them both with the very thick timber.

 

[tony1851]

Just a small point to add to the other posts.
Toothed connectors are fine for firmly fixing joists together. However, if fitting them to C24 timber, you may run ino difficulties unless you do it the correct way.
Once you've drilled your holes, you should tighten up the connectors with a high-tensile threaded bar and nuts. This is because the connectors are so hard to bite into the timber that you will strip normal threaded-steel bar. Once the timbers are tightened up together, you can then re-secure with normal threaded bar.
Why not save all this messing about and just screw, bolt or nail them together? The loading in your situation is not so great that particular attention has to be paid to the method of connection.

 

[John37]

Thank you. Just screwing them together would be much simpler and quicker - at 300 centres staggered maybe?. I wonder if the building inspector would be happy if they weren't bolted together?

 

[tony1851]

That would be fine. Just drill suitable pilot holes first; use something like 3" x 12s screws.

 

[Static]

Sorry thought you intended to sit the windows side by side with only one member between, quad'ing the joists may work (dont have time to run a calc sorry).. may be better to start walking to the BCO about what he would suggest, he may ask for calcs, so you may need to get an SE involved..

 

[John37]

Thanks, Static,sorry I didn't make it clear at the beginning.