max beam span for flat roof?

[mattysupra]

Hi, having a large garage being built at the moment. The engineer and architect keep cocking things up. They have said a max span of beams (175mm x 50mm) @ 450 centers is 3 meters.

Well i have just measured the span from the RSJ in the center of the roof and its 4.2 meters. Can the beams span this far without an extra RSJ? Or can i simply put larger timber beams in?

What size would be needed.

I will go and see the arctitect 'again' but would like some info as it seems they are clueless and now seem to be going off my suggestions.

Thanks.

 

[Static]

50x175 C16 joists at 450c/c can do about 3.5m on flat roof with standard dead loadings (assuming no plant loads).. your looking at about 50x200 C16s @400c/c for the 4.2m span

If they are saying the span of the 50x175s is 3m max.. do you have some abnormal loads on the flat roof?

 

[mattysupra]

sorry i was wrong, max is 3.75.

I have been to the engineer and he has changed to either -

50x200 @ 400c/c like you have suggested or
50x175 @ 350c/c,

Thanks for your help.

 

[Static]

Be warned that the 50x175 at 350 are fine so long as you get regularised 50x175s and not the normal 47x170s.. might be better off with the 50x200 (which work as 47x195) at 400c/c (which is also easier for fixing plasterboard etc to)..
The normal 47x170s are a little off on deflection so in heavy snow may cause cracking to finishes.. course if this is a garage with no plasterboard on underside then not really an issue..

ps.. make sure the roofing boards are well screwed into the joists to ensure load spread over the whole roof..

 

[mattysupra]

i have ordered some 50x200 and will have 400c/c.


I would like to isulate the roof for the rare days i may be working in there in the winter.

Whats the best method? can i use loft insulation and vent the roof through facia board? Do i need a vent either end of the roof or will one vent running the length of one side be o.k?

I have looked into kingspan i think it was that sits ontop of the joists that does not need venting but it seems a expensive method for a garage that i may heat only a few days a year.

Thanks for help so far

 

[noseall]

I would like to isulate the roof for the rare days i may be working in there in the winter.........Whats the best method?.........I have looked into kingspan

Kingspan insulated ply deck, fixed to the tops of the joists (warm deck) makes the most practical and financial sense.

If you work in there just for a few days during a cold winter you will benefit from it believe me.

 

[mattysupra]

Cheers noseall,

So i fasten through the built in top layer of plywood all the way through the insulation into the joists?

Then i can torch the roofing felt straight onto the top of the built in plywood?

No need for OSB or simular, just straight onto the joists and im ready for felt?

 

[noseall]

No need for OSB or simular, just straight onto the joists and im ready for felt?

Yup.

Unless of course the construction of your roof requires a sheathing layer to bind the joists laterally.

 

[mattysupra]

so then- I will have 400 c/c joists
then - Kingspan Thermaroof™ TR31


Next question! what is -

3 layer built–up felt roofing
utilising 3G felt base layer
partially bonded ????

Does that mean i have to have 3 layers of felt?

Or can i use any roofing felt i please to use?
P.s, any idea of how much this kingspan stuff costs?

 

[noseall]

The three layer felt system is usually fully bonded to the substrate.

What Kingspan are suggesting is that if you use their product AND decide to go with hot-pour-felt then you need to apply a partially bonded layer, i.e. a layer of felt full of holes.

I would imagine you are free to use whatever finish you wish, but i would still consult with Kingspan.

 

[mattysupra]

thanks mate, you have been a great help.

 

[Zeberdee]

Any idea of how much this kingspan stuff costs?

The kingspan TR31 Boards differ greatly I've found. You'll be looking at AROUND about £40 a board as an estimate, Depends where you get them from.

Celotex also do a similar Insulation & decking in one called the TD4000 (i think).

That maybe worth a look as an alternative.

 

[flatroofer666]

warm roof void
plywood
vapour barrier fully bonded
TR21 kingspan bonded with hot bitumen
vapour escape layer (perforated) laid loose
1 layer ht type 180 fully bonded
cap sheet ht type 250 fully bonded
polyester type green mineral surfaced for all drip edges ..flashings.
lay 3/8 white limestone fully bonded with hot gritting solution.
this is what myself and my company use and your looking at a roof to last at least 25 years.

 

[Cumbrianroofer30yrsexp]

I used to do alot of pour and roll back in the day

125,180,350 the old coolag insulation,cork insulation,fibre board(ivory).I use t/o these days but funny enough about 3 weeks ago i used hot stuff,first time in ages,didnt even have a pouring can,all done with pan

350 sq m job(section of factory roof).95/25 bit ,Vapour bar,kingspan,per glass, sanded underlay (all anderson icopol)and icopol technotorch with groves sbs torch on cap sheet(he wanted pour and roll cap sheet but convinced him other wise)

For some unkown reason the customer INSISTED on aluminium solar bar finish,it was a sunny day lol and we had to wear sunglasses to apply it.

 

[flatroofer666]

it was a sunny day and we had to wear sunglasses to apply it.

yea been doin that the other week...LOL