DIY beam load calculation

[Niel A. Farrow]

Hi, I'm doing a DIY loft conversion which involves building a dormer on one side. I have a simple pitched roof supported on each side by 2 purlins (11"x3"), 1/3rd and 2/3rds up the roof slope.

On the dormer side both purlins are to be removed. The top purlin will be replaced by either a metal or glulam beam. From this beam a flat roof will extend to the wall plate, where the eaves used to be.

So, on one side of the pitched roof, 1/3rd of the roof immediately below the ridge will remain, the rest is replaced by a full width dormer. The other side of the roof will have a couple of Velux windows.

I need to calculate the loads for the new beam which are: the remaining roof dormer flat roof joists snow etc. ?

Does anyone know of online resources or a good library book, or a structural engineer who wont charge an arm and both legs.

What dimensions (approx.) of I-beam will replace my 11"x3" purlin?

Thank you,

Neil

[Peter Crosland]

A structural engineer is not going to cost a fortune and in any case you will need to convince the building control officer that the calcualtions are sound.

[John Rumm]

Wander over to:-

get the demo of superbeam. This seems to be the program most of the structural engineers use to do the sums for them. The printed results are also acceptable to the BCOs. (Note that the demo version is fully functioning with the limitation that it can't print!).

The above will handle the calculations, however you will still need to put some sensible values in for the loadings to actually get a meaningfull result out at the end.

With the flat roof on my loft conversion the architect used a loading of

0.6 kN/m distributed over the length of the beam. You will also have a point load at one end of the beam from the pitched roof. In my case this was stated at 0.48 kN - but there was no residual part of the pitched roof above the dormer - i.e. it went right to the apex of the roof, so I presume the load represents that due to the oposite side of the roof. What book/basis/finger in the air the architect used to arrive at these figures however I do ot know. I expect there is a standard manual somewhere with lots of these data in.

[Tony Bryer]

Thanks for the plug! But ....

This is the key thing and the reason why we don't give much help when it comes to assessing loads etc. If you need that sort of help then you shouldn't be using the program. I had a letter published in the Structural Engineer a couple of weeks back which said, inter alia, "like the woodworking machinery shop yards from our office we sell tools that can be dangerous in the wrong hands". IMO the OP is better off talking to a structural engineer or someone who is experienced in loft conversion design.

[John Rumm]

I would tend to agree with your recommendation. However, I was highlighting the possibility that if you are determined to DIY, then you can so long as you are prepared do the necessary research to use the available tools safely (the "tools" in this case being software rather than power).

In my situation I had an architect produce the original plans and calcs (which he obviously produced in superbeam). Having access to your program was very useful as a way to check the implications of slight alterations to the plan as they were required during the build. Having access to the original plans however gave me all the loadings as a starting point, something the OP does not have.

In the OPs case, if he has suitable loadings available, and only wants to design one beam, then he may be OK. Especially if the design is then submitted to the BCO as a "full plans submission" where (one hopes) their engineer will verify the assumptions and source data as a part of the approval process.