A not-quite loft conversion

[Chris Styles]

All,

I've decided that getting my plans past the BCO would be easy compare to getting them approved by the missus, but I still have a big loft space that is being wasted, and I do need the storage.

So, the new plan is to fit proper load bearing floor into the loft (2"x10" for the 16' span, raise them an extra 1" off the wallplate to stop them interfering with the plasterboard ceiling below when they deflect). I plan to leave access via the loft hatch exactly as before, so that is not extra "livingspace", just a loft with a decent load bearing floor. At a later date, I'll come back and put the staircase in, insulate the roof, fire alarms, electrics, plumbing etc and make it a proper conversion.

Can anyone see any snags with this? Obviously I want to do the load bearing floor such that when I do the loft conversion for real, it is suitable, so any suggestions appreciated.

Cheers

Chris

[dg]

A 250mm deep joist 25mm up off the wallplate is [potentially] going to need a significant chamfer at the ends to stop them protruding throught the roof. The BCO may well not approve this unless the joist is strengthend at the bearing or calculated to prove it is OK.

Any purlins to be removed?

dg

[John Rumm]

Sounds reasonable enough... you may have to do something slightly "non final" at the loft hatch otherwise you may find yourself with a joist flying right over the opening making access difficult.

Depending on your loft hatch you may well find getting long beams into the loft space impossible without making some form of access hole through the side of the roof. Having at a minimum some scaffolding tower in front of this would be a good idea. Next time I do one I would also put a jib on the top of the scaffold and add an electric winch!

Think about where you may need dwarf walls, or other supporting structures since you will need to size the joists to carry these point loads as well and not just the uniform distributed floor loads.

See if there is anything else on my floor construction page that helps:

[John Rumm]

I had 200mm deep joists, and as you suggest needed to chamfer them at the front of the house where the original pitched roof was to remain. The BCO seemed ok with that though.

One downside of chamfering though is if you want to add a full depth dormer later - you will no longer have an easy way to fix the rear wall of it to the ends of the floor joists without having to fix little extensions onto each of the joists

[Chris Styles]

I had a good look at your site before (after my last loft-conversion posting), and it is an inspiration and damn scary all at the same time :-)

Cheers, Chris

[Chris Styles]

How would I go about strengthening them? Do you mean like bolting an extra bit of 250mm joint (say) 1m long with the same profile, so that it is thicker for a little bit leading up to the chamfer

Nope, the house is 3 years old, and the roof comprises of the finest 2"x3" trusses you can buy (for ~£10 each)

The main reason I dont want to board the loft as I did at my last house is that the trusses have a 16' span, about 10' at the apex, but the only internal tie goes from the apex to the mid point of the joist. The trusses at the last house had a larger span, but had internal ties in a "W" formation, so it look quite substantial.

I'm going to have to get a structural engineer in to work out how to remove these when I come to do a conversion. I had wondered if I'd be able to put a

16' steel parallel to the joists, put a vertical steel off of that at the mid point, then run a third steel from the gable to the top of the vertical steel, running along under the apex to take the weight. Given the the only tie inside the joist is a direct vrtical to the midpoint of the bottom of the truss, it would seem that it is only a vertical load that needs to be taken.

Maybe I should take a picture, so you can all see what I mean (and then all breathe in sharply through your teeth... "Oooohhh, tricky...." :-)

Cheers

Chris