Unbroken wall longer than 6m
- Thread starter L.Lam
- Start date
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Beams. We are doing internal modelling of existing house and loft conversion.
If you are re-modelling the interior and removing load-bearing and/or buttressing walls, Building Control will usually ask for structural calcs.
But the side extension itself doesn't need calcs. It's a straight up-and-over gable roof; the ceiling joists and rafters run parallel to the new side wall, and will be secured to it with steel straps in the usual way. Google 'Approved Document A' and it shows simple line diagrams of what your builder needs to do regarding the extension.
But the side wall itself does not need a steel windpost. Particularly as it's 300 thick, building control should just accept it without the post and beam. In the unlikely event an inspector did ask for justifying figures, any competent SE should easily be able to show the wall is fine without that nonsense.
But the side extension itself doesn't need calcs. It's a straight up-and-over gable roof; the ceiling joists and rafters run parallel to the new side wall, and will be secured to it with steel straps in the usual way. Google 'Approved Document A' and it shows simple line diagrams of what your builder needs to do regarding the extension.
But the side wall itself does not need a steel windpost. Particularly as it's 300 thick, building control should just accept it without the post and beam. In the unlikely event an inspector did ask for justifying figures, any competent SE should easily be able to show the wall is fine without that nonsense.
Thanks Tony for your advice. The wall is actually 215mm thick. So is this still ok without the windpost?
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Finally managed to get a reply from SE who has offered an alternative of 2 steel beams across the width of each floor (with joists running perpendicular to it). This seems to be more logical in terms of providing lateral restraint. Also the new side wall is over 6m so we do need a beam in the middle because I don't think you can get floor joists > 6m. Is it better/cheaper to use timber beam?
If the floor joists are running parallel to the wall, the builder still needs to add restraint straps between the 1st 3 joists and wall,to ensure that the wall is adequately restrained at floor level; ditto at ceiling- and rafter level.
For the cross-beam, timber might need to be excessively large to avoid too much deflection (depends on span). Steel might be more practical - the floor joists can be notched onto the beam to give a flush ceiling.
For the cross-beam, timber might need to be excessively large to avoid too much deflection (depends on span). Steel might be more practical - the floor joists can be notched onto the beam to give a flush ceiling.
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Sorry for harping on about this but just hate the thought of paying £600 for 2 beams that we may not actually need.
The SE was a packaged deal with the architect so I have never actually spoken to engineer. Architect now admits that he won’t use him again for future projects. I have also found that he contracts a lot of work overseas. The fact that his company gets audited quite a lot means he is often over cautious.
Now I need to decide whether to go ahead with the 2nd option or get another SE to justify an alternative design to BC.
To have a structure with steel means that the rafters and joists will be perpendicular to each other since the length is >6m. From what I have read, it’s when the rafters and joists that are parallel which gives the walls lateral support. So is it at all possible to have lateral stability on the long wall (without piers) with joists across the span which is 3.6m and is perpendicular to rafters?
The SE was a packaged deal with the architect so I have never actually spoken to engineer. Architect now admits that he won’t use him again for future projects. I have also found that he contracts a lot of work overseas. The fact that his company gets audited quite a lot means he is often over cautious.
Now I need to decide whether to go ahead with the 2nd option or get another SE to justify an alternative design to BC.
To have a structure with steel means that the rafters and joists will be perpendicular to each other since the length is >6m. From what I have read, it’s when the rafters and joists that are parallel which gives the walls lateral support. So is it at all possible to have lateral stability on the long wall (without piers) with joists across the span which is 3.6m and is perpendicular to rafters?
Someone - either architect or SE - is overthinking this design and making a meal out of what is really a simple structure.
1. Why a solid wall of lightweight block? There is a tendency for this block to shrink on drying out, leading to vertical cracking. A cavity wall of 7N dense block would be better.
2. Span the floor joists side-to-side off a pole-plate fixed to opposing walls secured with resin-fixed bolts. That will make a rigid plate restraining the wall.
3. The rafters and ceiling joists run parallel to the gable wall - the timber sizes used depends on the span. Rafters and ceiling joists are secured to the new gable and existing gable with steel straps - 4 each side should be OK. That will make a secure structure without faffing about with steel columns and beams.
1. Why a solid wall of lightweight block? There is a tendency for this block to shrink on drying out, leading to vertical cracking. A cavity wall of 7N dense block would be better.
2. Span the floor joists side-to-side off a pole-plate fixed to opposing walls secured with resin-fixed bolts. That will make a rigid plate restraining the wall.
3. The rafters and ceiling joists run parallel to the gable wall - the timber sizes used depends on the span. Rafters and ceiling joists are secured to the new gable and existing gable with steel straps - 4 each side should be OK. That will make a secure structure without faffing about with steel columns and beams.
Thanks for the drawing Tony.
1. We want to use EWI on the whole house so didn’t want to use cavity wall for the extension which gives some extra space inside.
2. So here you suggest floor joists for the first floor across the span from old gable to new gable wall.
3. Not sure I understand this because the length of the gable wall is 6.5m long so can’t get joists that long. Roof plan attached.
Building control won’t override the design with another unless we get new calculations for any alternatives. I spoke to another SE who won’t design just one part of the building as this complicate the situation if any problems occur in the future. Can’t really afford time and money to go to another SE for complete service of new drawings. I am trying to push SE for another alternative without steel but fear he may never respond. I have also since found out he doesn’t do the design and calculations himself but contracts the work out to a company overseas.
1. We want to use EWI on the whole house so didn’t want to use cavity wall for the extension which gives some extra space inside.
2. So here you suggest floor joists for the first floor across the span from old gable to new gable wall.
3. Not sure I understand this because the length of the gable wall is 6.5m long so can’t get joists that long. Roof plan attached.
Building control won’t override the design with another unless we get new calculations for any alternatives. I spoke to another SE who won’t design just one part of the building as this complicate the situation if any problems occur in the future. Can’t really afford time and money to go to another SE for complete service of new drawings. I am trying to push SE for another alternative without steel but fear he may never respond. I have also since found out he doesn’t do the design and calculations himself but contracts the work out to a company overseas.
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He's probably subbed it out to an SE in San Diego who has designed it for a 10 on the Richter scale.
Your extension is 3.8m wide, yes? So you only need to span the joists across the narrow part. This is shown on the first screenshot you put up, where the joists are going left-to-right, which is the logical, economic way.
8 x 2s @ 400 centres would be fine.
8 x 2s @ 400 centres would be fine.
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