General: Fixing a wall/ledger plate to a granite stone wall

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- CommentAuthorLehobbit
- CommentTimeJun 15th 2022 edited
Hello all

I have recently removed all the parquet flooring and joists from the second floor of our renovation project in France, an old granite built house in Haute Correze in the Massif Central.

Every joist had either rotted out in the wall or been attacked by Carpenter Ants, (probably due to moisture from a leaky leading edge on the slate tiled roof- now temporarily fixed until our new roof).

I have a load of new but s/h red cedar and some oak joists which are 200mm x 80mm. These will sit on an existing main central support beam in oak which runs parallel to the front and rear walls of the house.

The old joists were irregular sizes and spacing so the wall sockets are also uneven.

As the new joists are all regular I want to space them evenly at 400mm centres so it looks neat. I also want to keep them out of the walls.

I plan to fill in all the sockets with granite stone and a NHL 3.5 lime and sand mix. I then plan to use some of the joists I have (probably the oak ones) to fix wall/ledger plates to the front and rear walls in 4 metre sections. To these I will use either joist hangers or ledger strips with notches to the end of the beams to sit on the strips.

Overall I have just under 12 metres length of wall front and rear to cover in three seperate wall/ledger plates

I want advice on fixing with threaded rods and chemical resin? Anyone done this..how hard is it?

The total second floor area is around 85m2.

For each 4 metre long ledger plate how many threaded fixings would be needed. I have read somewhere that I should use M16 rod at 40cm length into the wall? This seems a lot to me? If my joists are spaced at 400mm centres along the ledger plate how should I space my threaded rods...one every metre or one every 500mm or another distance?

On a 12 metre run I am going to be drilling many holes into granite!!

The walls are about 600mm thick.

Advice appreciated

Regards Paul in Haute Correze
- CommentAuthorjfb
- CommentTimeJun 15th 2022
I'm no expert on this but I have done it a few times.
Is this for joists that will be walked on? (as in not the floor of the attic)
Assuming it is - definitely M16. 40mm into the wall seems much too short to me.
I would have thought 80mm minimum.
I would have thought every 500mm not metre but someone else might know better.

You have to make a hole bigger than the rod and enough to get sufficient chemical resin in. For M16 I would have thought minimum 20mm hole
Make sure you blow all dust out before filling with resin.
- CommentAuthorMike1
- CommentTimeJun 15th 2022 edited
From having seen it done several times, yes, it works well & isn't too difficult.

As for the sizing, this is another case for a structural engineer, or at least the technical department of the anchor manufacturer (even if you're not worried about your 10-year legal defects liability & assurance dommage ouvrage).
- CommentAuthorowlman
- CommentTimeJun 15th 2022 edited
I think your main problem is going to be accurately drilling lots of holes into solid granite, your average SDS drill is unlikely to be up to it. Plus an SDS drill going up through the sizes may shatter the rock.
Maybe another approach could be to affix purpose made steel shoes/brackets at either end of and in-between the ledger plates. and then sitting suitable section leger plates into these steel shoes so the plates are actually slightly off the wall with foam/insulation filling behind. Doing it this way you could have the steel shoes drilled and fixed professionally so they all line through. Yes it may be more expensive but less tears and pain than drilling deep holes into hard rock not to mention perhaps lots of expensive diamond drill bits.
- CommentAuthordjh
- CommentTimeJun 15th 2022
Posted By: LehobbitI want advice on fixing with threaded rods and chemical resin? Anyone done this..how hard is it?
I've done it a few times, just vertically into concrete outside to fasten sockets in place. It's very easy using the kits you can buy in many places. I've secured joists using ledger beams but into timber rather than stone. The main point is to ensure that whatever fixing you use is rated for shear load. My engineer specified the coach screws we used.

You will probably need to find some way to support the ledger beam whilst the resin cures. You drill the holes, put the resin in and then the threaded rod. But you'll need to make sure the rods are held exactly in line with the holes in the ledger beam and the easiest (only?) way to do that will be to feed the ledger beam onto them before the resin sets. And you don't want the weight of the ledger beam being supported by the rods until after the resin has set.

For each 4 metre long ledger plate how many threaded fixings would be needed. I have read somewhere that I should use M16 rod at 40mm length into the wall? This seems a lot to me? If my joists are spaced at 400mm along the ledger plate how should I space my threaded rods...one every metre or one every 500mm or another distance?
You'll need to ask an engineer how many bolts are needed and all the other details. Since it's supporting a floor it's structural and I expect there'll be a legal requirement. I was surprised at how many coach screws we needed.

On a 12 metre run I am going to be drilling many holes into granite!!
Yes, it'll be worth hiring a serious drill if you don't have one. :)
- CommentAuthorjfb
- CommentTimeJun 15th 2022
Posted By: djhYou will probably need to find some way to support the ledger beam whilst the resin cures. You drill the holes, put the resin in and then the threaded rod. But you'll need to make sure the rods are held exactly in line with the holes in the ledger beam and the easiest (only?) way to do that will be to feed the ledger beam onto them before the resin sets. And you don't want the weight of the ledger beam being supported by the rods until after the resin has set.

I would just fix the rods first and then cut holes in timber after. Should be able to get the right location by holding the timber up to the fixed rods and marking from behind.
- CommentAuthordjh
- CommentTimeJun 15th 2022
Posted By: jfbI would just fix the rods first and then cut holes in timber after. Should be able to get the right location by holding the timber up to the fixed rods and marking from behind.
Getting the holes in exactly the right places and at exactly the right angles without making them very oversized would be way beyond my skill level. I would drill the ledger beam first, use that to mark out where the holes in the wall are to be drilled and then proceed as I described. I expect a few bits of spare timber could be used to hold the ledger beam in place whilst the resin goes off. Another variant might be to do the holes at either end first and then they could be used to support the beam whilst the rest of the resin goes off.
- CommentAuthorcjard
- CommentTimeJun 16th 2022
If there are already holes in the wall where the old joists were, would it not make life easier to drive fixings through the ledger where the holes are, support the ledger in place and then fill the hole with concrete that will cure around the fixings?
- CommentAuthorrevor
- CommentTimeJun 16th 2022
I have done something similar in stone wall of varying hardness of stone. My wall was random stone so had to pick particular stones to anchor. That dictated where the holes went in the ledger beam and drilled holes in the beam first. I offered the beam in place and secured with props and used the holes to guide the masonry drill into the wall. This way I was sure to get the correct angle of the rod. I think I drilled in about 70 to 80 mm deep used I think 16 mm s/s threaded rod and made sure that the holes were clear of all dust. The spacing was at 400 to 600m or thereabouts depending on where the best stones were. The BCO was helpful in making the recommendations for the fixings. Hilti do and excellent range of drill bits I would contact them for advice they also do a range of fixings and specialise in chemical anchors so will be able to make recommendations for drill sizes etc. Their drills are about the best you can get and various hire shops hire them out
- CommentAuthorLehobbit
- CommentTimeJun 16th 2022
Posted By: jfbI'm no expert on this but I have done it a few times.
Is this for joists that will be walked on? (as in not the floor of the attic)
Assuming it is - definitely M16. 40mm into the wall seems much too short to me.
I would have thought 80mm minimum.
I would have thought every 500mm not metre but someone else might know better.

You have to make a hole bigger than the rod and enough to get sufficient chemical resin in. For M16 I would have thought minimum 20mm hole
Make sure you blow all dust out before filling with resin.

Sorry I made a mistake in my post I mean't the threaded rods would be 40cm into the wall!!!
- CommentAuthorLehobbit
- CommentTimeJun 16th 2022
Thank you for all the replies. I made an error in my original post with the advised length of threaded rod into the wall. I put 40mm but mean't 40cm.That seems quite deep to me?

Can anyone advise?

I have a decent Dewalt SDS hammer drill and also a Bosch Professional SDS plus drill. I plan to purchase a decent set of SDS drills for the job.

My main concern is the spacing of the threaded rods and how far they should go into the wall?
- CommentAuthorLehobbit
- CommentTimeJun 16th 2022
Posted By: cjardIf there are already holes in the wall where the old joists were, would it not make life easier to drive fixings through the ledger where the holes are, support the ledger in place and then fill the hole with concrete that will cure around the fixings?

We are using hemp lime on the walls applied with shuttering as an 11cm layer over the 60cm thick granite walls for natural insulation. We want to void using any cement based products, but I understand your point about ease.
- CommentAuthorfostertom
- CommentTimeJun 16th 2022
Posted By: cjardIf there are already holes in the wall where the old joists were, would it not make life easier to drive fixings through the ledger where the holes are, support the ledger in place and then fill the hole with concrete that will cure around the fixings?
Good idea, but
a) How do you fill the hole while the ledger's covering it?
b) Can you be sure that the conc will be so well rammed in and keyed to surrounding stones that it won't rotate (along with the ledger plate) when downward load is applied to the free end of the fixings?
c) If that rotation is resisted by the ledger plate's connections to the joisting, then all that the fixings need to do is resist shear at the wall face - the fixings could even go into the mortar joints, no need to resin them in!
- CommentAuthorLehobbit
- CommentTimeJun 16th 2022 edited
This is the drill bit I was thinking of purchasing for the job.

https://www.amazon.fr/gp/product/B08BZGZCTJ/ref=ox_sc_act_title_1?smid=APMPD8F9B7SBR&psc=1
- CommentAuthorWillInAberdeen
- CommentTimeJun 16th 2022
Tom I don't think joist hangers would reliably resist rotation? Only shear.

Sounds like the fixings need to project out through a layer of IWI so need to be stiff against sagging and rotating, as well as shear?
- CommentAuthorfostertom
- CommentTimeJun 16th 2022 edited
Posted By: WillInAberdeenTom I don't think joist hangers would reliably resist rotation? Only shear.
Gd question - I've assumed they would.
So skin the floor top and bottom with ply, glued - a monocoque 'casette' made up in situ.
Posted By: WillInAberdeenSounds like the fixings need to project out through a layer of IWI
Do they say that? would make it a real challenge on the stiffness, as well as fixity, of the fixings. There are thermally broken systems for hanging balconies outboard of EWI - treat the floor casette as a balcony but with the advantage of being supported on all edges, not cantilevered!
- CommentAuthorcjard
- CommentTimeJun 17th 2022 edited
@tom

a) well, we don't know it's covering it, and a ledger doesn't have to be the same height as a joist because it doesn't span the same distance between its bearing points but you only need a small gap at the top (which can be done with a notch) to fill the bit of the hole that isn't filled with concrete already; most of the hole can be filled by a low slump mix before the ledger is in place. Ledgers that are shorter in height can create a useful service void too
b) yes, per above, but also I wasn't planning on having the concrete rotate, by it's fit and by virtue of the ensuing construction; it's bearing a shear because the floor joist inhibits rotation by its presence (they aren't loose?) but one could fix too if worried/if the other end of the joist doesn't butt up against anything - bridging the gap at the bottom/cross fixing at an angle. Vibrating the wall and the ledger with an SDS drill with a blunted/mushroom shaped bit will help settle the concrete too, and uneven sections of wall where the ledger is proud can be coped with by expanding foam or some similar to stop it running out
c) yes; you can imagine the joist holes as large mortar joints into which multiple fixings will fit

I wasn't getting too deep into the design with the initial comment, just floating the idea - having pictures would help visualize
- CommentAuthorowlman
- CommentTimeJun 17th 2022
Posted By: LehobbitThank you for all the replies. I made an error in my original post with the advised length of threaded rod into the wall. I put 40mm but mean't 40cm.That seems quite deep to me?

Can anyone advise?

I have a decent Dewalt SDS hammer drill and also a Bosch Professional SDS plus drill. I plan to purchase a decent set of SDS drills for the job.

My main concern is the spacing of the threaded rods and how far they should go into the wall?

Good luck with drilling deep 20mm holes into granite with any standard drill bits in a standard SDS drill. Even old concrete is difficult enough to drill let alone Granite ( 7 on the Moh scale with diamond @ 10 )
You need rotary drilling action not hammer action and you really need diamond core bits ideally water cooled and a drill something like a Hilti. Some of the holes may end up in mortar joints if they are spaced equidistant and you could get away with the standard stuff in those cases but the friable nature of the mortar won't give the best fix. Your project is for a load bearing second floor, so good fixing is essential IMO. Additionally, the wall is unlikely to be flat, so spacers will end up being used if you want the leger plate to remain true.
- CommentAuthorfostertom
- CommentTimeJun 17th 2022
Living and working on Dartmoor, the very home of granite, I never appreciated these special difficulties with it. Thanks for enlightenment.

So I'm thinking that solutions must use the mortar joints somehow - not to get pull-out resistance or rotational fixity, but to make a non-rotatable bar rest directly on top of a stone. So the item being fixed must have a way to not rotate, and to not pull away.

Alternatively, the principle of removing and replacing a whole stone, of sufficient horizontal depth, with very well filled and compacted concrete or similar (or filling a joist pocket), and bedding the fixing into that, could create pull-out resistance and/or rotational fixity.

Or the traditional method, before even fibre Rawlplugs - drive in a well chosen oak wedge to pretty much fill a vertical mortar joint and screw into that. Surprisingly effective and durable - as long as wall movement doesn't relax the joints.
- CommentAuthorowlman
- CommentTimeJun 17th 2022
@ tom foster
Yes it's not easy to drill cleanly and hammering can cause it to shatter.
You could possibly get away with using shield/stud anchors into mortar joints if it's not too crumbly, but they still need a clean hole say M16, and not cheap @ about Â£100 a pop. possibly by using a belt and braces approach and injecting a resin compound around them, although I've never tried that. Heavy duty expansion stud fixings can be 250mm long +, but you'd need to be careful as to the projection of the stud when tightened especially if you need to accommodate spacers/packers behind the thick timber ledger plate.
An on site constructed cassette floor would be my preferred system with bracketed fixings to the granite, and as few as structurally sound. No direct timber contact to the wall.
- CommentAuthorlineweight
- CommentTimeJul 7th 2022
Why not just fill in the joist pockets (or at least the portions of them that will be hidden behind the ledger beams) with conventional concrete blocks instead of pieces of granite. Then drill the resin fixings into these, instead of into granite.

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