Garden Workshop lintels and load bearing

[Hia87]

Hi all,

We're building a workshop at the bottom of our garden. We were going to have an engineer help us with it all but, amidst Covid 19, our engineer stopped all communication with us and finding a new one was pretty much imposible, so having already started with the groundworks, we decided to go ahead without an engineer.
So far we've done all the groundworks which involved building three retaining walls at 1.5m high each and preparing the base for the workshop. Because our terrain is mainly clay soil, the foundations for our retaining walls are 500mm deep by 700mm wide reinforced with 10mm rebar cages. The workshop base has a footing ring at the same depth and width as the walls on the sides and an 800mm deep by 600mm wide at the front plus a 150mm deep raft with 4mm reinforced mesh on top of the ring.
We built the retaining walls with stepoc blocks reinforced with 10mm rebar horizontals on every course and vertically on every whole and infilled with concrete.
The back wall of our workshop will sit on the last retaining wall and, as mentioned before, the base is a 150mm raft sitting on 500mm d by 700mm w ring footing.
The workshop measures 7x5m externally with a flat roof which we hope to make into a green warm roof and an overhang of 1m at the front, hence our roof will be 7x6m.
We are using 4'x2' timbers for our stud walls and 9'x3' for our roof joists.

We've had some help from some friends who have built similar things and were feeling quite confident until we started trying to figure out the size of the timbers for our door lintels.
We want to have 3m bifold door and a single door next to it at the front of the building, but this obviously leaves very few studs taking the weight of the roof at the front. We planned for two 8'x2' bolted together, but are now unsure if that would be enough.
According to our calculations, our roof will weigh about 5400 kg; this includes joists, 2 layers of OSB, EPDM, insulation, 2 rooflights and an 80mm extensive green roof.

Your thoughts on the lintel above the doors would be much appreciated.

 

 

Read more: https://www.diynot.com/diy/threads/garden-workshop-door-lintels.548409/#ixzz6RIYiomvv

Edited July 5, 2020 by Hia87

[PeterW]

Welcome. 
 

£60 would get you access to the Timber Beam calculator for a month Here

 

Conversely I would suggest the quickest and simplest method would be to use steels and make a goalpost arrangement at the front. A 7m steel with a UC upright at both ends, and then one between the doors would work. Your issue is not the beam above the doors here btw it will be the compressive load on the timber frame. 
 

Also, who has calculated your roof loading ..? I make it 128kg/sqm which for that build up seems light. You’ll also need to use 72x220 joists at 400 centres to get your spans. 

[Hia87]

Good morning,

 

Thanks for your reply PeterW. We are using the Timber Beam calculator and are very pleased with the program as well as with the team behind it. We gave them a ring with a few questions and they were very helpful. 
With their advice, we've decided to go for a flitch beam, which would be more than sufficient. 

We've also checked the compressive load on the timber frame using the Timber Beam calculator and it's more than adequate. We will, nevertheless,  quadruple up the uprights either side of the doors just for our own peace of mind. 

We calculated the roof loading ourselves. We've checked again and keep getting the same.

We are using 72x220 joists at 400 centres and doubling up either side of the roof lights, which seems incredibly excessive. Any thoughts on that? Do we really need to double up the whole joist? Or could we get away with just doubling up the trimmers? 
Also, any idea on what's the best way of attaching the trimmers? I guess joist hangers?