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1. Sillyoldhector - Member

   ?hi, I need to provide structural calcs for a glass roof that we are fitting to an extension. The area is approx 4.5m2 and the weight of the glazing will be 31.2kg m2. It will a "lean to" style with the rafters supported by a 100mm cavity block wall at the eaves end and joist hung off a joist bolted to another 100m cavity block wall at the other.

   Does anyone know how to calculate what size timbers (@ 600 centres) would be needed to safely carry this load and what figures I can show to building control to satisfy their conditions.

   Dont really want to elist the services of a structural engineer as just had to pay them £250 for calcs for 3 steel beams.

   Thanks in advance

   Posted 2 years ago #
2. epicyclo - Member

   Ooh, the thought of that glass cascading down on me would be enough to persuade me to happily pay a structural engineer....

   Posted 2 years ago #
3. cdaimers - Member

   You should try load span tables published in the old building regulations, or by TRADA, the timber association, however I would recommend that you get an engineer to size it properly. £250 seems very expensive for 3 calcs which would only take 30-40 mins.

   Posted 2 years ago #
4. epicyclo - Member

   I think you're paying for his public liability which never goes away.

   It's always worth paying an expert - you don't know what you don't know even if you think you've got a fair idea

   Posted 2 years ago #
5. Stoner - Member

   TRADA tables give you span limits for joist specs given static and imposed loads. Calculate imposed loads from the glass mass you specify and then work it out from there...

   They wont specify wallplate fixings though. Use big f-off ones.

   Personally I would bother with a structural engineer unless it was complicated.

   Posted 2 years ago #
6. cdaimers - Member

   We have a new lean to conservatory up last year and the building control guy started to quote new design codes (eurocodes) which consider snow loads up to 500mm thick in this area of the UK (Cumbria), how I mocked him until 14th Jan when the panels were looking a little heavily loaded with ice.

   I am a structural engineer and generally professional indemnity insurance cover carries a £10000 excess on £1M cover so the £250 quid still seems excessive. I dont do domestic but for 1 hours work £55 an hour covers insurance as well as everything else (office, copying, load tables etc).

   Posted 2 years ago #
7. Stoner - Member

   Right. The TRADA tables give span limits by joist spec for dead loads of <0.5kN/m2, 0.5<0.75 and 0.75<1.0 and imposed load of 0.75kN/m2.

   The tables give you span limits for 400, 450 and 600 centres with C16 and C24 timber.

   Im looking at the flat roof (to 10deg) table.

   Posted 2 years ago #

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