Building a wood mezzanine in a cottage - advice & structural calcs?

[Nomad]

Hi, there. For an ongoing project of mine I'm looking at putting a wood mezzanine in the main room (partly as it's a tiny house with no storage, also for the possibility of putting a mattress up there as it's a good level to look out of the velux window and views will be rather special).

Ideas & suggestions welcome, especially dimensions for the main support beams.

The room & planned floor :


An idea of what I'm hoping for:



Requirements:
Length approx 8M / 24ft (I'll measure and confirm)
Spacing for beams 130cm (there's a gap on one wall that the beams will fit into, I can't alter this)
Width for flooring - maybe 140cm so I can put a double mattress up there?
Weight? Optimistically a lightweight bed + 1 large / 2 medium people at one end, bulky storage ( maybe some heavy tools??) at the other.
Minimum would be 1 person up there moving light bulky thing into storage.
No vertical supports to floor level (space is tight on the ground and having vertical beams supporting this will be a problem)
Weight limit?

For the future :
railings / fence to stop people & things falling off ! (legal railings are stage 3 and not needed yet, fitting the framework is stage 1, fitting the flooring is stage 2)
steps (I'm thinking a lightweight stepladder I can move onto position when needed)
Head height above will be limited but fine for storage or sitting up on a low bed.

I can do the building work myself but don't know what size support beams I need (I'm expecting to put a lintel into the end walls and rest the beams on that)
Tell me your thoughts

 

[gaz1]

your answer is no single bed only as the roof at the edge is too low for you to access it for a double matress up there

2 single matress can be fitted though and you can also create storage as well

 

[gaz1]

whats behind that wall all the way along it ?

 

[addjunkie]

I did very similar in one of my barn conversions. The timber accross the room at 8 m with no support will need to be a big steel. Its from memory roughly 6 m wide. I have the calcs somewhere for the beam. Joist sizes were just picked from the span tables for house upstairs flooring. The ledger was chem fixed into the back wall on 12mm studding. Joists are sculptured into the beam, every other one bolted to prevent movement. Beam is on concrete pad stones in the walls. Floor is tongue and grove ply glued and screwed rather than wayrock, as specified by the engineer who did the calcs.

Free UK Span Table for Domestic Floor Joists to BS 5268-7.1 (C24, 1.5 kN/m² load) - Timber Beam Calculator

www.timberbeamcalculator.co.uk

 

[Erie Fred]

The forum is called "MIG-Welding" and your asking about using wood.....
Structural steel would be my answer.

 

[Agroshield]

The 8m span is your biggest challenge. You need something that will meet the deflection criteria for the applied live and dead loads (which you should take from standard tables, not just make up numbers based on dieting for a week before you climb the ladder and only sleeping naked under a summer weight duvet).

You also need something shallow enough to maintain headroom below - it is not clear if the 2.27m is the underside of the structure or the mezzanine floor level. That is something to consider early on - the maximum structural depth of the new floor so neither above or below is too cramped.

If you know what you are doing, you can subscribe to this website for a month (and upgrade the subsciption to cover steel beams if a timber solution is not possible): https://www.timberbeamcalculator.co.uk/en-gb/ That will give you an idea of required section size.

A possibility for the 8m span beam using timber is what is called flitch beam - a steel plate sandwiched between two timbers.

There are a couple of practical issues: finding a piece of timber that is 8m long and dragging an 8m long piece of something into place from your driveway. If neither of these are an option, a structural engineer would need to design any joints/connections in the item to be used.

 

[gaz1]

if you take a look at this pic whilst its an apex point

it means you can fit single beds all the way down either side



the mattress can be on the floor itself

but i still need floor measurements of the room and whats on the outside of that wall

but like above that beam is going to be x2 12x4s but if the front to that wall is shorter its better to split the beam length as long as that wall is a supporting wall but you would still need a pillar

me personally id split the room into 2 depending on the windows and still have storage above the rooms as that room looks big enough for 2 double beds in a split room config then build a passageway to the other room external on the otherside of the wall

 

[Agroshield]

It is getting into the weeds and will be expensive, but if you absolutely could not have a column under the mezzanine floor, a complicated structural solution is to do something at roof level and have one or more tension supports (could be stainless steel rods) holding the mezzanine beam.

In other words, the mezzanine hangs from above.

Whatever is needed structurally might fit within the existing rafter depth in your initial picture and could span the same direction as the rafters. That also minimises the structural depth of the mezzanine so up and down headroom is maximised.

 

[Morrisman]

Looks to me that by the time the 24’ long crossbeam is in there, high enough to walk under comfortably while downstairs, there will hardly be room to crawl over it into the space behind it.

 

[Nomad]

Thanks everyone, answers in no particular order :
The left wall and the left 2/3 of the back wall are "old farmhouse" build (at least a foot thick, concrete both sides, infill a mix of rubble, cement, clay..), the right wall and right 1/3 of the back wall are new brickwork.
Good point about sourcing 24ft lengths of timber - I need to look at options, getting it on-site and into the room will be do-able with a helper.
Metal is my least preferred option for this (apologies - I know it's a welding forum ! )
The roof is quite new and was over-spec'd to allow for solar panels etc in the future, I had already thought about suspending some of the mez weight from the roof. It's an option and there are 10 roof beams so plenty of loadsharing.
A flitch beam (timbers & steel plate sandwich) is something i hadn't thought about, good idea.
I have furniture on the left wall that is 227cm high. Ideally the base of the new floor (ie top of the new beams) will be a fraction higher than this, this leaves head height of almost a meter above the front of the new mez, about 1.8m at the back. It's not ideal but at present it's wasted space in a house that has almost no floor space. Beams would be lower than this but it's an area where you don't walk around so some loss of head height is ok.

Keep the ideas coming, it's all appreciated

 

[addjunkie]

You can get various types of engineered joists, some with board in the middle others with like spring steel, posi joists.

Well worth investigating, but again not sure it will do the width.

Interactive Span Table | James Jones and Sons Limited

James Jones & Sons Ltd is one of the largest, most efficient and innovative timber processing groups in Europe, offering the widest range of sawn timber products including carcassing, fencing, palletwood and engineered timber products.

www.jamesjones.co.uk

The timbers in my barn floor are 8”x2” off the top of my head the beam is 203x133 something like that. The bolt fixings are 6mm steel welded both sides. And 10mm bolts.