Loft conversion - do I need structural engineer?

Some of my novice previous questions on this subject may have you thinking I need more than a structural engineer, but hear me out!

It’s been a couple of years since I finished the extension which is still standing so I’m itching for a new monster DIY project. I’ve built out, so now it’s time to build up.

I’ve a good sized loft, 1925 detached house with a typical roof of the time….. a gable one end, hip the other and a couple of gables sticking out of the face and rear for good measure. There is a massive original 12” steel already in the loft mid way spanning front to back where brick piers have been built off to support the purlins mid way.

My initial thought is to leave the roof as is, plain tiles with friable mortar fillets behind, and just bang a dormer on the back. Then insulate between rafters leaving an air gap the insulate over the top. Throw a floor down, cut a hole in the landing ceiling and Bob’s your uncle.

At the moment I’m unsure on the structural side of things. Firstly the roof. Cutting out the back is sure to throw the roof off balance. I’d be cutting a purlin out alongside a decent proportion of rafters to accommodate the dormer. So that’s going to need some support. Secondly the floor. Clearly new floor joists need adding. The 4x2s currently support the Christmas decorations. But do I need steels? What size? I’m not expecting you lot to answer my how long is a piece of string question just yet. Just thinking out loud.

I suppose the advice I’m after is whether a structural engineer is required for building regs. Also, from my usual YouTube research, every loft conversion I’ve seen has enough steel in to build the Eiffel Tower. I’m wondering if I even need steel with the current big steel in there and some nice mid span supporting walls.

Thoughts and reality check please gents. Remember, tight as a gnats chuff and bloody minded gung ho full steam ahead attitude.

Never let an engineer design a loft conversion.

Find an experienced designer - who should have an understanding of engineering principles, and let him design the conversion and then only instruct an engineer to design the beams and supports that the designer requires.

Some sort of beam/s are normally required, but they don't necessarily need to be steel.

You got any recommendations Woody? You’re West Mids based too. I think I’m only after the structural supporting side of things. Remember, I’m pretty much expert on this at present…. I’ve watched 4 YouTube videos.

In my limited DIY experience - (and I probably rival you in the tightness stakes) - a loft conversion with 2 steels and 2 steel/timber flitch beams in my last house and a large rear extension/knock through with 8 steels in the current one, the expense is not so much in the steels themselves, especially if just laid horizontally on two lumps of concrete, it's more all the extra stuff - fancy connectors, welded frames, plates, posts, cranes etc - this is what turns hundreds of pounds worth of steel into thousands of pounds worth of steel work.

Well worth stressing to the designer/engineer that it's a DIY job and that, in contrast to your average builder, you'd much rather spend 2 days building a blockwork supporting wall than spending £1,200 on some welded/spliced/cranked A-frame etc.

Edit - I didn't attempt to design/calculate anything, way beyond me but for my extension I was lucky to find a great SE who produced a very economical design and dealt with things like lateral stability which I don't think those online calculators do.

If you have 4x2" ceiling ties then you could use a length of the same as a pole plate and start your new floor from there, sistering them over the spine wall.

catlad said:

If you have 4x2" ceiling ties then you could use a length of the same as a pole plate and start your new floor from there, sistering them over the spine wall.

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I’m not going to be demonstrating my ability here to inspire confidence in me pulling this off….

ceiling ties?? Is this what your calling the loft floor joists? Because they stop the roof spreading?

pole plate?

Ceiling ties are what you! are calling your loft floor joists, Pole plate same as wall plate only sat on ceiling ties next to rafter feet. You could then chamfer your new floor joists to the pitch of your roof and sit them on the pole plate providing you can spare the height.

Ah, I see…. So by positioning a mere 4x2 (pole plate) right near the wall plate but only sitting on the ceiling ties, there will be virtually no deflection in the ceiling ties as I’m right at the edge of the span. Correct?

I suppose I could then add further pole plates on the ceiling ties mid span where they sit on a structural wall. - one that goes to ground.

That's the gist of it.

You can't just guess at using joists and not a beam. Or using a certain beam in a certain place. The layout and structural members need to be thought about and the loft designed, not cobbled together.

But where appropriate, new joists can just sit on the wall plate, I don't understand this pole plate malarkey.

Guessing and cobbling together is a forte of mine.

But I do take your point and think I will go down a basic route of getting a professional to design the floor and dormer. 2 main reasons… firstly I don’t know what I’m doing, but more importantly I think building control are going to want to see some calculations.

I do very much like the suggestion of clarifying my one man band diy approach and penny pinching tightness to the designer.

^woody^ said:

You can't just guess at using joists and not a beam. Or using a certain beam in a certain place. The layout and structural members need to be thought about and the loft designed, not cobbled together.

But where appropriate, new joists can just sit on the wall plate, I don't understand this pole plate malarkey.

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Guessing! Bloody cheek! I'm guessing your not really Che Guevara either!

catlad said:

Guessing! Bloody cheek! I'm guessing your not really Che Guevara either!

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I am too!

And to prove it, he's some of my inspirational words of wisdom for the OP

"To accomplish much, you must first lose everything"

^woody^ said:

"To accomplish much, you must first lose everything"

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From these inspirational words, I’m taking that I should say balls to the designer or engineer, go full bore ahead with my own crackpot plan, possibly loose the roof and back part of the house, but then rebuild the lot and then sit back in wonder at my accomplishment?

there or there about?

Bang on Pilsbury! I'm in.