RSJ Vs engineering joists for flat roof

[romario]

Hi,

 

I would like to share my thoughts and ask for other people opinions.

 

I need to make 6m by 6m flat roof with 4 skylights.

 

I've been a fun of engineering joists because of the light weight and how easy they are to install as well as how easy it is too put any ducting or pipes etc.

However, with the timber price increases the quote that I am getting for engineering joists works out nearly double the price comparing to putting RSJ in the middle to half the span from wall to wall and use timber joists from RSJ to the walls.

 

Also there is 7 weeks lead time on engineering joists while the RSJ can be bought of the shelf.

The downside for RSJ is that will need to get structural engineer to confirm the spec and few people to lift it up :)

 

I have a dilemma what to go for.

Please see attached drawings.

 

Thanks in advance for any comments.

 

[nod]

No brainer 

 

Whilst Lam joists are good They are not as good as steel 

Steel has also rocketed in price 

But it’s on its way back down

 

[markc]

Take cost out of the equation and steel still wins hands down for this application.

the skylights would require SE input for timber as well because the application / load tables do not allow for your large and presumably very heavy skylights

[PeterW]

Steel H frame between the roof lights and then build the remainder in with timber. You won’t get that in engineered joists at a 6m span. 
 

Your issue will be one of ceiling depth - a quick fag packet would indicate you need a 305x127 section that would be near 500mm deep when you’ve created a warm roof - add in your upstands for the roof lights and it wil be very thick. 
 

Cold bridging through the steels will be a problem so you will need to either use a cold roof construction - and ventilate it - with at least 30mm of PIR on the ceiling below the steels or work out how to lose the insulation height in fascias etc. as you’ll need a lot of insulation to reach a reasonable uValue due to the area of glazing. 

 

 

[romario]

Thank you for all your replies they were very helpful.

 

Let me share my decision and design with you who replied and  anyone else who may benefit from it in the future.

 

At the end I decided to go with the web joists because:

1. Easier to handle and put together comparing to steel specially when installing with one helper and on 6 metre height.

2. Web joist supplier done all the calculation so no need to get separate structural calculations.

3. Web joists supplier manged to get the roof working with 219mm depth web joists.

4. With those wide web joist (width between 100mm and 150mm)  it is so easy to attach ceiling plasterboards

5. Roof insulation. I need to educate myself a bit more but I came across what is called hybrid roof ( warm roof insulation on top more than 50% of total depth and then remaining insulation between the joists) As I said I need to educate myself in that topic to make sure there are no condensation problems.

6. Easy to run any ducting in the ceiling.

 

On the other side the cost of the below design is £2500 inc vat so probably significantly more than a steel and infill timber but looking as a whole I thought the web joists was a better option.

 

 

 

 

 

Thanks for your comments and I hope this might be helpful to someone else in similar position.

[Mr Punter]

Is this for the round house?  I was trying to work out where this roof is going.  Unless you don't have the room you are better to put all the insulation on the top.

[romario]

2 hours ago, Mr Punter said:

Is this for the round house?  I was trying to work out where this roof is going.  Unless you don't have the room you are better to put all the insulation on the top.

Hi Mr Punter,

Yes, it is for the Roundhouse to close a  big hole

Let me share some photos from the roof

[Mr Punter]

Very nice!  Did you have to soak the top battens in water to persuade them round the roof?

[Makeitstop]

31 minutes ago, Mr Punter said:

Very nice!  Did you have to soak the top battens in water to persuade them round the roof?

 

Blimey, I was wondering that too, and thought he must have one hell of a large steam box.

 

Super interesting design

 

[markc]

That is a cool build,  ?? for not building a box.

[romario]

9 hours ago, Mr Punter said:

Very nice!  Did you have to soak the top battens in water to persuade them round the roof?

Thank you.

There was a bit of learning to be bending the 25mm x 50mm battens.

 

Going at the bottom of the roof on 10m radius was relatively easy. However,  going higher and higher was challenging.

 

At the end to be able to bend those on the top it was a combination of:

- selecting battens without any knots

- soaking in the water

- then none of the battens were perfect straight anyway so then was  trying to lay them in the direction that they are already bend.

 

Fortunately I didn't need to go as far as steaming boxes

 

Below photo showing precaution to avoid cracking on the knot

 

 

 

 

 

 

 

 

 

 

Top rows shows the slates coming on larger angle but still the corners don't stick out too much that was mine worry.

Still not sure how to finish off the top raw

Possibly with lead?

 

 

 

[Mr Punter]

I am not sure what you had in mind for the tops of the rafters either.  Maybe leadwork or have an aluminium coping fabricated that you can install in sections.

 

It looks a tad like a James Bond baddie lair.