Hi, my husband has asked me to seek a bit of advice here, so this is the situation:

We are renovating an old stone farmhouse. Part of the building is the old stable, with a loft above. The ground floor of this area will become our entrance hall and utility room on one side, with a workshop on the other side for my husband. The first floor loft will become my office and storeroom for my business. The exterior 50cm thick stone walls reach a height of around 2.5m on the first floor, and the roof is very high, so we’d like to put in another floor just for loft space and storage. So, we’ve ordered and had delivered 12 x Steico i-joists (90 x 360mm) which we want to sit on top of the walls, and then lay OSB flooring on top. My husband has been scratching his head about how best to do this, and has come up with the following method:

Since the stone walls are not at all level, he is thinking of placing a piece of timber 50 x 150mm on top of the wall, and filling in underneath with mortar to make it level. He will then place the beams on top and fix them in with angle brackets and wood noggins to brace. We will then lay t&g OSB flooring on top, just for storage, light use, not habitable space.

Does this sound like a good way to do it? We prefer not to put joist hangers on the wall itself as this would mean lowering the ceiling on the first floor quite a bit, and since it is a big space it could feel disproportionately low. And it just seems much easier to place them on top of an existing wall. Would be glad for any opinions on this.

I might sit the top chord on top of his wall plate and use noggins to hold them in place, no angle brackets. Brace them up, bottom rails on top of bottom chords and perpendicular to beams. Diagonal bracing with builders band pulled tight against temporary battens.

Lay floor with t&g material.

Posted By: Yasminh14We prefer not to put joist hangers on the wall itself

Too right - If your stone walls are anything like mine it's the devils own job to get a firm anchorage where you want it in this type of wall.

50cm stone walls in my experience are stone - rubble - stone with lime mortar between the facing stones and earth/rubble mix in the middle. The ability to get a good fixing depends upon the type of stone used and some luck about the position of the fixing and the wall at that point. Also if you have to drill through the wall I have usually found that as you withdraw the drill the earth/rubble infill drops down into the hole you have just drilled or just jams up the drill

Otherwise your solution sounds fine.

The chords are the bits of timber at top and bottom of the I-beam, so Tony is referring to hanging the beam from the top, not sitting the bottom timber (chord) on the wall.

Your farmhouse sounds similar construction to mine. If it was mine I would replace the timber with through stones as the rubble core has a tendency to allow the inner and outer skin to separate under load. As for use of timber I have real concerns over woodworm and now replace lintels etc with stone reserving timber only for areas where I can easily treat on a regular basis.

Posted By: the souterJoist hangers would be very dull;

Never mind dull - I doubt you would be able to get joist hangers fixed to the (stone / rubble) wall with sufficient security and positioning accuracy to make them a viable option. (Opinion based on trying to fix stuff to my stone/rubble walls)

Sorry, have not been on the site for a few days again. To answer all questions:

The span is 6.3m, so yes it is a big one and we chose the beam size based on Steico's guidance charts.

Regarding stone walls - it would seem that construction of British stone houses is somewhat different to Breton ones. Our walls definitely do not have rubble in the middle, they are solid granite all the way through. Often very large and heavy stones. We ruled out joist hangers attached to the wall as previous experience has shown us that it is very difficult to fix thing directly to these walls and as the stones are not flat, it's virtually impossible to get anything level.

We have now based on previous comments and our own gut feeling about the right way to do it, placed a treated timber plate of 90mm x 150mm on the wall, well cemented in underneath and behind. We have started to put some of the beams in place, sitting on top, but less than half so far and they are not yet fixed. My husband has yet to decide exactly how to fix them, but has said he will also do a certain amount of bracing across the span too.

The wall plate will need straps to stop wind lifting the roof off. Have you check the slope of the roof? As you have 500 - 150 = 350mm free between the end of the I beam and the outside of the wall and a 360mm high beam, you either need to build up the outer skin to reduce the 360mm or have a roof slope bigger then 45 degrees. I would definitely use web stifffeners.
Frank

Posted By: Yasminh14We are not creating a roof, that is already there. The wall is 50cm thick and has plenty of depth under the eaves to sit a joist on top. So there is no danger of the wind lifting the roof off, we are using existing structure and space. You have to picture a massive stone barn with 2.7m height on the ground floor and then on the first floor 2.5m to top of the wall with a roof space going up to about 6m in total at the peak. We are laying joists on top of the wall to create storage space under the roof.

The numbers don't seem to add up. Even if we allow nothing for the thickness of the first floor, 2.5 + 2.7 = 5.2 which only allows 800 mm below the peak of the roof, from which we need to deduct the thickness of the roof and your 360 mm joists. Which doesn't leave a lot of space!

In the UK, everything has to be positively fastened to the ground in one way or another. You can't just lay a floor on top of a wall and have it stay there just through its own weight, for example. I presume there were cases where things got blown away by swirling winds. Our building inspectors are very picky about the straps, but I've no idea about France.

Posted By: Yasminh14Hi djh, sorry I'm obviously still not explaining this very well! The height from the floor level of the FIRST floor is 6m to the top of the roof ridge (interior measurement). From the ground floor level right to the top t is about 8.5m. So the height of the roof at the highest point in the loft will be around 3.5m.

Ah, my misunderstanding, sorry. That makes a lot more sense.

Posted By: the souterJoist hangers would be very dull; go with wall plate.
What is the span?- 360mm is a big I-joist for light storage...
Wouldn't bother with angle brackets, you can pin the chord ends onto your 150mm wall plate and the noggins will keep everything stable. How will this new wallplate relate to the rafters and what are they currently resting on?

renewablejohn: With respect, a treated wall plate on well bedded cement would be normal practice. Are you suggesting removing both inner and outer stones at say 600 centres all along the wall head then socketing in a stone that bridges inner and outer leafs?

Ed D: Web stiffeners? Not too obvious to me and we've used architect/ Structural Engineer approved I-joists a few times. Are you saying the noggin needs to be notched to fully clamp the web and the chord?

Cement and modern plasters are banned from our farmhouse and we have more substantial timbers at 2 mtr centers not 600 centers