70s bungalow - removing / redesigning a roof truss

[chris010569]

Hi all. I want to install a loft ladder to my 70's bungalow loft hatch. Assuming it pivots at the edge of the loft opening, the ladder has a 112 cm pivot radius (ie - the amount of space it needs, in the loft, to swing up and then down). As things stand, with the loft hatch where it is, the roof truss dotted red, in the attached picture, will be in the way. What are your thoughts, please, on getting rid of it and replacing it with a brace / truss approximately as per the green line in the diagram ? I'm not a structural engineer, but I don't see how this would seriously compromise the integrity of the roof. BUT - what would a Surveyor say, if he/she ever popped their head up there, maybe in a few years if I was to sell ? Many thanks to all in advance

 

[bernardgreen]

The diagonal ( red ) member is exerting both vertical and horizonal forces on the joint.

The proposed vertical member ( green ) can only exert a vertical force.

Without the horizontal ( red ) force that section of the roof could move in the direction shown ( blue )

 

[JP_]

Could you put some sort of steel support be put in (yellow) and not remove all ove the truss - my guess is that one bit won't bring the house down?

 

[chris010569]

View attachment 171847

The diagonal ( red ) member is exerting both vertical and horizonal forces on the joint.

The proposed vertical member ( green ) can only exert a vertical force.

Without the horizontal ( red ) force that section of the roof could move in the direction shown ( blue )

Many thanks Bernard. Yes, I agree.

But surely - the lateral force against your "Blue", is being worked against by "Yellow", in my newly attached pic ? I don't propose to get rid of that rafter. That action would tend to produce a "lifting" force at the apex, which would be counteracted by Green, I think ?

Kind Regards.

 

[chris010569]

Could you put some sort of steel support be put in (yellow) and not remove all ove the truss - my guess is that one bit won't bring the house down?
View attachment 171848

Thank you Jon. That is not a bad idea !. I hadn't thought of that TBH. BUT - apart from the structural questions - I suppose my other main concern is - what might a Surveyor say, if they poke their head in there, a few years hence, if I'm selling ? Is it actually "OK" to mess with the structure of a roof - be it mechanically sound and balanced force-wise or not ?

 

[bernardgreen]

The lower ends of rafters should exert only a vertical force onto the wall.

If there is a force along the rafter then at the wall the rafter will exert a horizontal force exerted on the wall.

 

[chris010569]

The lower ends of rafters should exert only a vertical force onto the wall.

If there is a force along the rafter then at the wall the rafter will exert a horizontal force exerted on the wall.

Ok thanks Bernard.

But - how can "Yellow" EVER exert anything other than a horizontal force on the wall, in the absence of red ? It is pushing out sideways. Unless tied-in with some kind of horizontal bar to its counterpart on the other side - but even then surely the force would be a downward and sideward vector ? Nominally it is counteracted by 'Red'. So yes - otherwise, a horizontal tie-bar to the other side, would be apt ?

Many thanks for your input, by the way. It is nice to discuss these things with someone who knows what he's talking about

#EDIT - on closer inspection, I fail to see how "red" is counteracting the horizontal force on "yellow" .... given the feeble and non-mechanical joint provided between them by that nasty Zinc plate ?

 

[JP_]

Thank you Jon. That is not a bad idea !. I hadn't thought of that TBH. BUT - apart from the structural questions - I suppose my other main concern is - what might a Surveyor say, if they poke their head in there, a few years hence, if I'm selling ? Is it actually "OK" to mess with the structure of a roof - be it mechanically sound and balanced force-wise or not ?

Not OK at all! Probably best not to follow my advice unless you are sure you know what you are doing and don't plan to sell!

 

[chris010569]

Well, I understand the laws of Physics, I don't plan to sell - and even if I do, my comment will be "It was like that when I bought it !!. Do you say "Not OK" from any professional basis - ie - are you (for example) a surveyor ?

 

[bernardgreen]

With the reds supporting the ridge then the rafters "hinge" from the ridge.

 

[chris010569]

With the reds supporting the ridge then the rafters "hinge" from the ridge.

Does "Yellow" not hinge the ridge too, in the absence of "Red" ?

 

[JP_]

Well, I understand the laws of Physics, I don't plan to sell - and even if I do, my comment will be "It was like that when I bought it !!. Do you say "Not OK" from any professional basis - ie - are you (for example) a surveyor ?

No, I say "not OK" from a cover my arse point of view! I am a keen DIYer who is very new to building, having been forced to learn through dodgy builders doing extensions and buying a 1930s bungalows that is rotting from the ground up!

 

[Leofric]

You should not cut or alter prefabricated gang nailed roof trusses in any way unless you have the proposed structural alterations designed by a structural engineer and approved under building regulations. No diy bodges or guesses allowed , full stop

 

[chris010569]

You should not cut or alter prefabricated gang nailed roof trusses in any way unless you have the proposed structural alterations designed by a structural engineer and approved under building regulations. No diy bodges or guesses allowed , full stop

Thanks Leo. That sounds like an extract from some text somewhere - so, as far as we're concerned is this an "official" point of view ? Or just common sense ?

 

[Leofric]

Thanks Leo. That sounds like an extract from some text somewhere - so, as far as we're concerned is this an "official" point of view ? Or just common sense ?

It isn't an extract from some text somewhere repeated parrot fashion if that's what you mean , it is my wording for the situation regarding roof trusses as provided by all truss manufacturers. I can't remember any truss supplier's exact wording but the gist is as I have stated. Truss suppliers design their trusses including timber components, connections and bracing for specific jobs and don't allow any modifications to their design. Can't really put it any clearer than that.