I'm trying to design a Mansard Roof in the UK. Some timber data is given here that local building control allows:


As you can see for a 11° as in diagram below a clear span of 4.59m with C24 and 47x197mm timber is permisable is that correct Q1 (amended) did I get that right? and Q2 (amended) what would the other internal structure of it be?

Q3 (amended). I want to get a feel of the maximum roof size using the tables above and the sizes shown on the diagram to design; an answer using any non product based reference (including books, software, internet references) or design sugestions that would suite the diagram shown would be helpful.

mansard schematic


1 Answer 1


We design for VERTICAL loads (holding floor loads and roof loads up as well as holding things down from wind uplift) and we design for LATERAL loads (wind and earthquake).

With a mansard roof, you’ll find the key element in the design is going to be the connections. The connections will need to keep the space from “racking” or you’ll have creeks throughout your house.

Here’s a website for “rough-sizing” your joists and beams: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Don’t forget the code requirement of minimum head height, etc.

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