structural ridge size?

[sunroomdiy]

Looking at what size of ridge beam i require for my sunroom extension? Want to vault the ceiling.

4m out x3m wide extension, walls standard 100mm cavity 2.4m high

span 2.4m betwewen wallplates internal, 2.6m span from outside of wall plates

lenght of ext 3.75m int, so ridge will be 3.95m length
rafters 100x47mm @ 400mm centres
angle 35deg , 9/12 pitch approx, peak around 1m above w/plate
area of roof approx 18m2
concrete roof tiles 50kg/m2

plasterboard with 25mm insulation under rafters, 100mm insulation between, breather paper taut, counter battons, battons and tiles

live load been told 0.75 - 1 kN/m2

location 10miles from Belfast, do get light snow falls


Length of rafters is approx 1.6m to birds mouth. 2.2m total length



any help appreciated

Hoping a 225x75mm would be enough?

 

[Jase750]

200x50 would do it, big enough to accept the cut lengths of your rafters, which is where the load is distributed through.

 

[Static]

200x50 would do it, big enough to accept the cut lengths of your rafters, which is where the load is distributed through.

Urm.. ok so if the load is going down the rafters how do you intend to laterally support them? or just wait for the walls to fall over? as without tie members what you suggest is dangerous..

Its more like 2No 47x225 C24 members bolted together at staggered centres..

 

[RonnyRaygun]

Static is right. A 200x50 isn't strong enough to be a structural ridge. You would need rafter ties and then the ceiling isn't vaulted...

A structural ridge needs to be a pair of 225x50s C24, as Static said, or alternitively, you could use a 250x75 C24, but might be harder to get hold of.

 

[Jase750]

I used a 200x50 6m long for the same purpose, vaulted insulated roof. This was what building control and se recommended. Had no problems with that. The ridge beam in my house isn't that big and it's lasted 50 years so far. Whatever extra support is needed would still be needed whatever size the beam is. Op asked for beam size, how does an extra 25mm thickness on a ridge beam prevent the walls pushing out?

 

[Static]

Either you triangulate the vaulted roof with tie members or you design the ridge beam to take the roof loading..

I would guess your 6m vaulted roof would have had tie members to stop the rafters kicking out.. and your 50yr old roof would probably have a ridgeboard which is not the same as a ridge beam and the roof is a standard cut one with ceiling joists etc..

 

[RonnyRaygun]

I used a 200x50 6m long for the same purpose, vaulted insulated roof. This was what building control and se recommended. Had no problems with that. The ridge beam in my house isn't that big and it's lasted 50 years so far. Whatever extra support is needed would still be needed whatever size the beam is. Op asked for beam size, how does an extra 25mm thickness on a ridge beam prevent the walls pushing out?

It worries me that an SE and Building Control would respectively specify, and approve, an undersized section.
You can design to a British Standard (as Static and I have done) or you can stick your finger in the air and have a guess. I know what I'd rather do.

In answer to your second question, a 200x50 supporting the loads given, over 6m is calculated to deflect 146mm!!
The only way it isn't doing that is by putting serious stress into the masonry. Something will give eventually and the roof will start spreading and crack the masonry.

Or you might be lucky and it might not happen...but why take the risk?

 

[Jase750]

Thanks for the info ronnie and static, I was only commenting on the beam size itself, op would obviously have to construct the rest according to se specs (not my se apparently lol). I put my faith in their knowledge and experience and constructed accordingly. I wouldn't expect anyone to blindly follow any advice given on here as it is exactly that, advice. I think my new extension will look nice with the new gothic buttresses I am going to add anyway lol.

 

[sunroomdiy]

Going to go for 3.9m 222x73 built into the 2No. 100mm internal walls at each end.

 

[RonnyRaygun]

Going to go for 3.9m 222x73 built into the 2No. 100mm internal walls at each end.

You did read above didn't you? 222x73 isn't big enough...

Although the roof load should be able to be reduced from the 1 kN/m2 I worked it out on, so you might be OK with that size.

It was also worked out on grade C24. C16 will be significantly weaker.

 

[sunroomdiy]

Will use c24. So needs to be a pair of 225x50, or 250x75

 

[RonnyRaygun]

It did, but it also said this:

A structural ridge needs to be a pair of 225x50s C24, as Static said, or alternitively, you could use a 250x75 C24, but might be harder to get hold of.

Just checked and taking 0.75 as the imposed load you will get away with a 222x73 C24

 

[sunroomdiy]

Excellent, thanks as 222x73 c24 arrived today, full steam ahead this weekend!

 

[sunroomdiy]

Excellent, thanks as 222x73 c24 arrived today, full steam ahead this weekend!