Joist queries

[Myatta]

Hi people, I'm new to the site and it seems to be very helpful in most situations so I thought I'd give it a try. I'm a roofer by trade so haven't done much carpentry/joinery and I've been asked by a client to put a roof starting from the joists.

 

It's a new build (family room in the back of the garden) that's roughly 5.8m x 5.6m and the client is thinking about putting an RSJ in to stop the roof from sagging. Could this be avoided by tripling up 8" x 2" C24 timbers in the centre and going out from that at 400 centres? With an overhang the joist lengths will be roughly 6m long. The actual roof will be a warm deck Fleeceback Ruberbond EPDM and it may have a small (1m x1m) velux dome installed as well.

 

Any help or ideas will be appreciated as I havent don't much carpentry before. I'd like to tackle it myself (with a friend) as I'm always wanting to learn more and it seems like a good straight forward one to do once I get a good idea of it. 

[markc]

If its going to be a vaulted ceiling then i reckon you will get sag with a timber ridge over that span unless the walls are strong enough to withstand the spread load from the rafters.

With a bottom chord/tie in the middle you would have no problem.

 

[Declan52]

First of all im a bit puzzled. Your a roofer by trade but need help to design how you would go about this job.

I take it that it's a flat roof construction.

What did the building drawings say with regards the size of timbers needed??

What is the brown thing on top of the concrete lintel??

 

[Myatta]

Never really done new joists for a roof as most of the work I do is for a company and is mainly re-roofs and repairs. I'm trying to startup a company with a friend and this is our first proper job (done loads of repairs and small re-roofs). The roof itself is no problem once I get the rafters in, I just want to be sure that if I go without the RSJ it won't sag.

 

I haven't seen any drawings for the job just yet as I only popped in to have a look, I'm going back tomorrow to have a proper chat about it. I think its a timber ontop I'm not to sure as I was in a rush when I first had a look.

[markc]

I had not considered the possibility of this being a flat roof as the walls dont appear to have any fall from front to rear, or side to side so i assumed it would be a pitched roof.

If its going to be flat then i would definitely go with a steel center support or the timbers will sag under snow load, if not under their own weight.

more info on intended roof design would be handy.

[Mr Punter]

You should have some drawings for this.  If you are new to the business it is a mistake to take on design responsibility.  You then just price and build something designed and specified by others.

[Myatta]

Thanks for the advice guys. I fitted a velux for the client before and he's spot on so I want to help out as much as I can. I think the guys that built the structure were a pain to deal with so he doesn't want to go with them anymore. 

After I've been tomorrow and had a proper chat I'll come back and give you an update.

[PeterW]

Irrespective of what you may think will work, that building exceeds the size required for Building Regulations approval so will need to be designed in accordance with the correct regulations. 
 

In terms of it being a flat roof, you are on the design limitation (5.66m clear span) of a 47x220 joist at 400 centres so this would need to be very carefully designed. What is the use of the building afterward out of interest ..? It may make a difference to how it is built up. 

[ProDave]

t have building control said so far?  At the moment all i see is a garage (no insulation, no windows just a big door opening)  and how are you going to turn that into a "family room"?

[Myatta]

@PeterW it's going to be sort of a family room and if he has anyone round a spare room, I think it's going to have a shower toilet and basin in it as well. The fall will go to the rear of the building and he's wanting an overhang (400 - 500mm) at the front and right hand side where the fence is. 

[Myatta]

@ProDave As I said before I haven't spoke in great detail with him about it I've just popped in to have a quick look so I can think about it. I'm going there tomorrow to have a proper chat. There's 3 window openings down the right hand side where the fence is which is south facing and will get the sun up to early afternoon in the summer. I think it will be more of a summerhouse and spare room in the winter. 

[Big Jimbo]

I would do the roof in wood i beams if it were me. They would span that width no problem. I have used them on extensions for years. So much better than the crappy timber available to buy these days.