We are planning a loft conversion and we have to span an area of 20 ft from front wall to back, the party wall is not strong enough to support joists so we are thinking of going front to back walls. Is is possible to span this distance, and would it be best to use steel or wood? Any ideas??? Thanks
Loft conversion
- Thread starter palmer1
- Start date
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Yes its quite possible to span 20ft with steel, you can also span it with timber but at a certain span it will become more economical and practical to use steel as the depth/size of the timber beam will become too big.
Its pretty normal to do loft conversions with a few steel beams.
Its pretty normal to do loft conversions with a few steel beams.
H'mm this thread needs a few answers really, firstly why do you think your wall won't be strong enough, is this a beam to take the ends of your floor joists? We'll assumes so unless you say otherwise.
A crude rule of thumb for beam depth is that the depth is around 1/20th of the span.
So typically if steel it would be a 254UC or 305UB or thereabouts. This can be made up of say 3 shorter beams spliced together in the loft.
If timber I would guess its gonna be say 2 65x250 joists bolted together maybe with a steel plate sandwiched between them (flitch beam) (but frankly that could be easily undersized as I have never done a lofty with timber beams only steels). Getting a 20ft long timber beam into your loft may also be easier said than done! Not to mention that even getting hold of 20ft long timber joists is not gonna be that easy in the first place given that its so long so that may restrict your purchasing power too!
All of this is here-say though really as we don't know if you have modern trusses or a traditional roof or what, both of which will need some meaty beams in order to open up the spaces.
A crude rule of thumb for beam depth is that the depth is around 1/20th of the span.
So typically if steel it would be a 254UC or 305UB or thereabouts. This can be made up of say 3 shorter beams spliced together in the loft.
If timber I would guess its gonna be say 2 65x250 joists bolted together maybe with a steel plate sandwiched between them (flitch beam) (but frankly that could be easily undersized as I have never done a lofty with timber beams only steels). Getting a 20ft long timber beam into your loft may also be easier said than done! Not to mention that even getting hold of 20ft long timber joists is not gonna be that easy in the first place given that its so long so that may restrict your purchasing power too!
All of this is here-say though really as we don't know if you have modern trusses or a traditional roof or what, both of which will need some meaty beams in order to open up the spaces.
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The party wall is of cob construction and it seems to finish below the existing loft floor level. It is not a very wide party wall anyway and it just seems to taper way as it goes up towards the loft. The party wall in the loft is just lathe and plaster on the lower section and modern plaster board at the top. We don't think we can rely on the cob wall party wall to support joists running from side to side, so we are thinking it might be best to use the front and back walls which are of stone construction.
The existing loft already has some timber joists and beams in place which look as if they are not very old so it looks as if someone in the recent past has already started the conversion but didn't continue with it. We are not sure if what is already there is sufficient or if not can any of it be re-used.
The whole loft area is 12.5 ft by 22ft. There is a double beam in situ which is just off centre running from gable end to party wall. It consists of two 6 x 2in's beams. One end of these is secured in the gable end wall but the other end is not attached to the party wall, we are not sure how it is supported really at that end! ( it just seems to be attached to uprights) This double beam which divides the space into two sections. Section 1 is at the front of the property measures 12.5ft by 12.5ft, and the other section 2 at the back is 12.5ft by 9.5ft. In section 1 there are 10 joists running from the double beam and into the front stone wall they are 6x2 in's. In the smaller section 2 there are 10 5x2 in's joists into the back stone wall. So all the existing timber is supported by three good stone walls it is just the party wall side that seems a little dodgy.
We were thinking of including a rear dormer window in the loft but it seems to complicated and costly and so now we are only wanting to make the room into a small double bedroom with roof windows probably on the front and back. So we would only be using the middle section of the loft where the head height allows. The lower sections into the eves would just be for storage.
Does the existing joists and beams sound anything like adequate for what we want to do, or do we have to introduce larger stronger beams? If so is it best to go from front to back as we thought. We are thinking that we might be able to just have one beam on the party wall side and from that we could secure the existing structure. Is that possible. Thank you for your thoughts and advice.
The existing loft already has some timber joists and beams in place which look as if they are not very old so it looks as if someone in the recent past has already started the conversion but didn't continue with it. We are not sure if what is already there is sufficient or if not can any of it be re-used.
The whole loft area is 12.5 ft by 22ft. There is a double beam in situ which is just off centre running from gable end to party wall. It consists of two 6 x 2in's beams. One end of these is secured in the gable end wall but the other end is not attached to the party wall, we are not sure how it is supported really at that end! ( it just seems to be attached to uprights) This double beam which divides the space into two sections. Section 1 is at the front of the property measures 12.5ft by 12.5ft, and the other section 2 at the back is 12.5ft by 9.5ft. In section 1 there are 10 joists running from the double beam and into the front stone wall they are 6x2 in's. In the smaller section 2 there are 10 5x2 in's joists into the back stone wall. So all the existing timber is supported by three good stone walls it is just the party wall side that seems a little dodgy.
We were thinking of including a rear dormer window in the loft but it seems to complicated and costly and so now we are only wanting to make the room into a small double bedroom with roof windows probably on the front and back. So we would only be using the middle section of the loft where the head height allows. The lower sections into the eves would just be for storage.
Does the existing joists and beams sound anything like adequate for what we want to do, or do we have to introduce larger stronger beams? If so is it best to go from front to back as we thought. We are thinking that we might be able to just have one beam on the party wall side and from that we could secure the existing structure. Is that possible. Thank you for your thoughts and advice.
I doubt what is there would be of any use structurally, non legit attempts at converting lofts are seldom done with any regard to Building Regs. Is there a supporting wall under the 2 6x2's? Your floor joists are not too bad but would struggle to comply with modern requirements.
Really you should assume at this stage your existing joists will likely need upgrading at the least. You will also need a few steels to support the roof to enable the existing ties/struts out. The rafters will likely need to be beefed up too.
To include a dormer does slightly complicate the required structure, often a structural ridge beam needs to be introduced and this normal sits on each wall at each end and if (at least one) of your walls is unsuitable, so support for this would need to come off a new steel running back to front too, increasing its size probably.
Really you need to get a professional involved as this will clarify matters and a pragmatic approach can be made. Do you have anyone in mind?
Really you should assume at this stage your existing joists will likely need upgrading at the least. You will also need a few steels to support the roof to enable the existing ties/struts out. The rafters will likely need to be beefed up too.
To include a dormer does slightly complicate the required structure, often a structural ridge beam needs to be introduced and this normal sits on each wall at each end and if (at least one) of your walls is unsuitable, so support for this would need to come off a new steel running back to front too, increasing its size probably.
Really you need to get a professional involved as this will clarify matters and a pragmatic approach can be made. Do you have anyone in mind?
We are definitely not going to go for a dormer window, just a roof window/skylight window. We are not touching the existing perlins. We have horizontal collars which we will move higher up the roof above what will be the new bedroom ceiling and also we will also use the eves to use vertical and diagional struts from the rafters to the joists to prevent the roof from spreading. We have remeasured the span and it is actually 23 ft from back to front wall. Is a 305UB 30cm's high? Would this be a suitable size of steel to support new floor? Thanks
Only a structural engineer or other competent person can determine what structural alterations and size of beams etc that must be utilised. Are you getting Building Regulations approval for this conversion?
Yes, we are getting building regs concent from council for the conversion. But we are getting conflicting advice from different builders about what is required. Just heard that the maximum span for floors must not exceed 6 meters, and ours is, so now we are not sure we can do it?
A builder is not qualified to say what is and is not possible. Any builders you have in mind for the works should be quoting off a good set of drawings not a look round the loft and a few sharp intakes of breath here and there, thereby then they are quoting on a like for like basis and you will actually end up with what you are expecting.
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