Right on brother! The proletariat are behind you all the way.
Loft conversion - do I need structural engineer?
- Thread starter pilsbury
- Start date
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“To accomplish much, you must first loose everything”
Some on the internet miss the irony and take things literally. I did not miss this, but Woody’s quip has got me thinking. Dangerous I know, but after a fruitless afternoon of trawling the internet for a simple loft structure designer, I’m drawing a blank and have lost my mind. That’s the “loosing everything” bit out of the way. I’ve found a million fancy websites who will take the whole thing from start to finish out of my hands, plenty of architects and structural engineers who can charge me £1k plus for drawings and layouts I don’t want, but no one, who will either stick their head in my loft or look at my measurements and say:
“you want a x size steel going from there to there and x size joists going from there to there. For the dormer, bang in a x size timber here and you’re good to go”.
Far from it for me to say I don’t want to pay for that experience or liability, I simply don’t want to pay for the superfluous nonsense. No one I can find seems to offer a basic service for a self build. They are for complete solutions to a finished loft.
Anyhow - Floor. I’ve done my own poking around. I clearly have external walls, but on looking I have several other walls roof to ground that are nearly mid span. There or there about. 4.5m would be a max clear span and looking at the span tables for a typical upper floor with typical loads, C24 50x225 would be plenty at 400-450 centres (link below). However, the table does show an asterisk saying 2 additional joists required. What does this mean? First thoughts were sistering….. then I thought perhaps it’s talking about extra joists needed over a given distance as compared to 600 spacing. Can anyone clear this up. To cut my rambling short, I think the floor is in my remit without any input (plenty asked on here though) No steel needed, just wood at the correct size and spacing.
Which leads to the dormer question. Just so I can hone my research, how usually is the back side of a roof cut out of a detached property with a traditional purlin roof? Is the cut purlin just supported? Are the front side rafters happy resting on the ridge beam without the back rafters opposing them? If anyone has even a direction or clue it would be appreciated. In an ideal world, timber would be the solution.
https://nhbc-standards.co.uk/6-supe...ncrete-upper-floors/6-4-8-timber-joist-spans/
Some on the internet miss the irony and take things literally. I did not miss this, but Woody’s quip has got me thinking. Dangerous I know, but after a fruitless afternoon of trawling the internet for a simple loft structure designer, I’m drawing a blank and have lost my mind. That’s the “loosing everything” bit out of the way. I’ve found a million fancy websites who will take the whole thing from start to finish out of my hands, plenty of architects and structural engineers who can charge me £1k plus for drawings and layouts I don’t want, but no one, who will either stick their head in my loft or look at my measurements and say:
“you want a x size steel going from there to there and x size joists going from there to there. For the dormer, bang in a x size timber here and you’re good to go”.
Far from it for me to say I don’t want to pay for that experience or liability, I simply don’t want to pay for the superfluous nonsense. No one I can find seems to offer a basic service for a self build. They are for complete solutions to a finished loft.
Anyhow - Floor. I’ve done my own poking around. I clearly have external walls, but on looking I have several other walls roof to ground that are nearly mid span. There or there about. 4.5m would be a max clear span and looking at the span tables for a typical upper floor with typical loads, C24 50x225 would be plenty at 400-450 centres (link below). However, the table does show an asterisk saying 2 additional joists required. What does this mean? First thoughts were sistering….. then I thought perhaps it’s talking about extra joists needed over a given distance as compared to 600 spacing. Can anyone clear this up. To cut my rambling short, I think the floor is in my remit without any input (plenty asked on here though) No steel needed, just wood at the correct size and spacing.
Which leads to the dormer question. Just so I can hone my research, how usually is the back side of a roof cut out of a detached property with a traditional purlin roof? Is the cut purlin just supported? Are the front side rafters happy resting on the ridge beam without the back rafters opposing them? If anyone has even a direction or clue it would be appreciated. In an ideal world, timber would be the solution.
https://nhbc-standards.co.uk/6-supe...ncrete-upper-floors/6-4-8-timber-joist-spans/
The extra is just doubling/tripling up to support parallel stud walls. You probably have a ridge board not a beam. I got rid of purlins and fitted much deeper rafters - makes the insulation easier to deal with and my purlins/rafters had nearly 2" deflection mid span so didn't have to worry about leveling them up. I fitted everything from inside. As I made mine up as I went along, in lieu of a ridge beam I fitted 2 high level "purlins" (8x2 timbers) near the apex of the roof to pick up the top end of the new rafters and the ceiling joists/incoming dormer roof joists (when I applied for a regularisation my engineer couldn't prove the purlins so I had to strengthen by bolting steel plates to them):
I had two steels at floor level, to hang floor from and support front wall of dormer (set back from eaves) and dwarf wall with rafters on at non dormer end. Steels were sized by looking through the party wall to see what my neighbour had in their loft conversion and fitting "one size bigger" to be on the safe side (in fact they were £35 each from my local second hand steel place and were so old the guy who did the calcs said he couldn't find them in his tables, but they did the job).
Personally I think the 2 steels were a good idea because it meant I didn't have to run the new floor or rafters into the eaves (and the floor joist span could be reduced) but may not be relevant to you if you're building the dormer off the eaves.
The other thing to start thinking about is fire protection - I did mine 10 years ago and it was fire fire doors with seals and closers on the escape route, protection of new floor - 2 layers of plasterboard under or some arrangement of loft insulation and chicken wire, escape windows, and mains interlinked smoke alarms, I believed it's changed a bit now.
And if you're having a bathroom up there think about a route for the soil pipe.
If you really can't find anyone, there are people on the internet who'll do just calcs for you - guy did mine for my double chimney breast removal - I sent him a £95 and a child's picture with the sizes on:
He sent me a page of sums, 3 padstone sizes, 2 steel beam sizes and a connection detail to bolt them together over the spine wall. I didn't have a clue but Council were happy with it all.
I had two steels at floor level, to hang floor from and support front wall of dormer (set back from eaves) and dwarf wall with rafters on at non dormer end. Steels were sized by looking through the party wall to see what my neighbour had in their loft conversion and fitting "one size bigger" to be on the safe side (in fact they were £35 each from my local second hand steel place and were so old the guy who did the calcs said he couldn't find them in his tables, but they did the job).
Personally I think the 2 steels were a good idea because it meant I didn't have to run the new floor or rafters into the eaves (and the floor joist span could be reduced) but may not be relevant to you if you're building the dormer off the eaves.
The other thing to start thinking about is fire protection - I did mine 10 years ago and it was fire fire doors with seals and closers on the escape route, protection of new floor - 2 layers of plasterboard under or some arrangement of loft insulation and chicken wire, escape windows, and mains interlinked smoke alarms, I believed it's changed a bit now.
And if you're having a bathroom up there think about a route for the soil pipe.
If you really can't find anyone, there are people on the internet who'll do just calcs for you - guy did mine for my double chimney breast removal - I sent him a £95 and a child's picture with the sizes on:
He sent me a page of sums, 3 padstone sizes, 2 steel beam sizes and a connection detail to bolt them together over the spine wall. I didn't have a clue but Council were happy with it all.
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@cdbe - a really great post which is very helpful in getting me thinking.
I like the idea of the high level purlins. Also a good idea of sistering the existing rafters to help with support and insulation. Not sure how I’d implement if I were to be keeping the existing purlins in the rest of the roof. Something to consider
I have to investigate further as to whether I can build off the external wall. The 200mm set back reg will dictate that. Is going to be close and will dictate whether I need to put steel in or similar.
I will also very soon check out the fire regs. From my limited info already, I think mains interlinked alarms, a fire door on an 800mm landing are necessary. As for escape windows I’m unsure but was thinking of a set of french doors and Juliet balcony on the dormer. Can’t get much better escape than that…… with a couple of broken legs!
I like the idea of the high level purlins. Also a good idea of sistering the existing rafters to help with support and insulation. Not sure how I’d implement if I were to be keeping the existing purlins in the rest of the roof. Something to consider
I have to investigate further as to whether I can build off the external wall. The 200mm set back reg will dictate that. Is going to be close and will dictate whether I need to put steel in or similar.
I will also very soon check out the fire regs. From my limited info already, I think mains interlinked alarms, a fire door on an 800mm landing are necessary. As for escape windows I’m unsure but was thinking of a set of french doors and Juliet balcony on the dormer. Can’t get much better escape than that…… with a couple of broken legs!
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