Large Loft - Conversion

Engleberht · 5 Jun 2008

Hi.

I have a new house (five years old) with a large loft. Its a massive space and I'd like to convert it.

Internal walls measure 8,620 (wall plate to wall plate) x 10,350, gable to gable. Pitch is 35 degrees. There are no load bearing walls upstairs!

(steel stud & plasterboard).

My main question is....with the shortest span of 8,620, is it possible? if so...how?

Many thanks
Dean

JerryM · 5 Jun 2008

no expert on this but have been looking at the idea of loft conversion for a while.

i would have thought that your "new" house would have a truss rafter roof - lots of small struts held together by nailed plates all effectively blocking up the usable roof void.

my understanding if this is the case then it's no easy job and probably better to take the roof off and start again.

alternatively it would need large steels spanning the gables to support the new floor. all the existing rafters would need beefing up before the trusses could be taken apart to create the space - i think u would be best to get plans drawn and a Structural Engineer on board.

Other than that sounds very exciting.

chessspy · 5 Jun 2008

Wow! Englebert,
Quite how you are going to span 8.6 mts is quite beyond me, with no internal load bearing walls.
I seem to remember that a good rule of thumb for spans is 1" sq for each Ft spanned, so 27 ft, = 3x9s at 9mts long, oh boy, that is going to be expensive for 10 odd mts at 400mm centers

Shytalkz · 5 Jun 2008

With great difficulty is the answer.

Option 1: plybox beams spanning from gable to gable, forming the dwarf walls to the loft accommodation. Thes ply beams would need to be at least 1200 deep, would have numerous flange timbers top and bottom and a very intensive nailing pattern per flange. The new floor joists would span between the ply beams, so these would be supporting both the roof and the floor elements.

Option 2: run a steel beam or beams across the short span, to sub-divide the large one and give support to beams running from gable to gable, to support the side walls, roof and new floor.

The principal beam would be mahoosiv and very, very heavy. Depending on the headroom availablity in the loft, this/these beam/s may have to downstand from the existing ceiling, which would look pants.

Option 3: if there are any internal load-bearing walls at ground floor level, install support posts to the beams at first floor level. This would provide addtional support points to the loft structure, but is entirely dependent on how the first floor layout relates to the ground floor.

This is a trussed roof, so whichever option was used, it would be necessary to construct the new supporting structure, before cutting any diagonals out.

Shytalkz · 5 Jun 2008

chessspy said:
Wow! Englebert,
Quite how you are going to span 8.6 mts is quite beyond me, with no internal load bearing walls.
I seem to remember that a good rule of thumb for spans is 1" sq for each Ft spanned, so 27 ft, = 3x9s at 9mts long, oh boy, that is going to be expensive for 10 odd mts at 400mm centers

That might work for general floor joists, but most def not for ones supporting the roof structure!

chessspy · 5 Jun 2008

Well, I think it would work.
Expensive though,
I don't like steel in a structure as it is lethal if there is a fire due to the expansion factors involved, first it expands and pushes the walls out, then it gets soft and deforms.
I once worked on no6 wharehouse in Hull, which burnt down, it was a Victorian structure with immense spans and 9 ft ceilings, the timbers were very big.
It burnt all night one of the biggest fires in Hull in a non chemical environment.
3 days later when the fire had been put out, the timbers were still sound, burnt charred but basicaly sound.
If it had been a modern metal framed building it would have been burnt down to the ground as the steels would have pushed the walls over before collapsing.

Shytalkz · 6 Jun 2008

I can guarantee that you would not be able to sensibly size a timber section to span 8-odd metres, supporting a floor and roof. At a push, you could possibly get hold of 300 deep sections and they would not work - plus if some combo of timbers did work, you'd have to source 8m-odd long sections; either that or have them finger-jointed.

Steel's fine, you are extremely unlikely to get the extremes of temperature in a house fire that would cause both yielding and expansion of the order sufficient to push walls over. Don't forget that everything is tied in at the floor and roof levels as well these days, so there's far more robustness in structures than in the old days.

JerryM · 6 Jun 2008

Shytalz,

thanks for putting the colour so eloquently on the picture.

i'd had nightmares overnight on how the crane would lift the steels in.

i think this one (and i never like to say it) on a financial basis is probably - find a new house.

Shytalkz · 6 Jun 2008

The steels could be split into sections with designed bolted connections to limit the weight of the individual pieces, but each would still be kin heavy and the moments and forces would mean a lot of bolts at each connection point.

Think I'd be going for a plybox solution on this, but it would be labour-heavy, with literally thousands of nails to put in to designed arrangement, but could be constructed in situ up in the loft space, making material humping slightly easier.

There is always a way to solve a problem; whether it is financially viable or not is another matter, though.

Engleberht · 6 Jun 2008

Thanks Guys.

Shytalkz said:
There is always a way to solve a problem; whether it is financially viable or not is another matter, though.

This is the trouble. Such a shame really as it would make a very large room for us. It annoys me every time I go up there for something!

Shytalkz · 6 Jun 2008

You must by definition have some load-bearing walls at ground floor level that could be used to support some posts holding up the loft structure though, as the first floor structure isn't likely to span 8m either! Even if it meant messing around with the first floor layout, would that be a possibility?

Engleberht · 6 Jun 2008

Its probably the easiest and cheapest possibility. I'm just not sure everything is worth the aggro.
I guess I was hoping someone here might know a miracle cure for the problem !
Seriously, I'm just exploring every avenue before a decision is made..

thanks again,
Dean

Shytalkz · 6 Jun 2008

If you don't want to use anything below loft level (apart from the perimeter walls, obviously!), imo plybox beams are the best answer.

No method will be cheap as such, but then look at the floor area you'll be gaining: something in the order of 4.5m x 10.35m, which I reckon you could do for somewhere in the region of £30k depending on what you're putting up there, equates to around £650/m2 for useable area. Cheaper than an external extension.

JerryM · 6 Jun 2008

don't necessarily get down beat just yet.

if you’re planning to stay in the property for some time then the agro is worth it if the benefit to u is important.

u just need to think it through a little "laterally" first to establish if it is that important given everything else in life.

in terms of cost - providing you're not looking to sell in the short term then its going to be a good investment based on the use you will get out of it and the selling advantage when you ultimately come to move on (just make sure it's what u want - that's all as there is no simple fix).

If I was doing it I feel I would lean toward the option 3 ie get some support from ground level – u may even get to the point that a wood solution may be feasible.

You need to find a good structural engineer to have a look. I’ll have to add like Shytalkz but the trouble is I then have to add they are not that easy to find.

Shytalkz · 6 Jun 2008

Thanks for the vote of confidence, Jez, redresses the Joe accusations

!

I can do design off plans, if there's a decent set of them plus a set of photos. I do about two or three conversions a week for some London-based companies, never go to site, no need.

Use my email if you want to chat further off-forum.

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Large Loft - Conversion

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