Reinforce or replace ceiling joists?

[retrophit]

I'm renovating my 1930s semi and have taken down the old lath and plaster ceiling in one of the rooms, as it was crumbling to pieces. I want to put plasterboard up but the ceiling joists are warped and sagging slightly, so it's not a flat surface to work with. I also want to install 270mm of insulation in the loft and board over it for storage, so I'm not convinced that the joists are strong enough to take the added weight.

 

The existing joists measure about 40x105mm (2x4s?) and are inconsistently spaced between 350-450mm, but at around 400mm centres on average - the longest unsupported span is 3.6m. Based on these span tables for ceiling joists, if they're roughly equivalent to C16 at 400mm centres, even at light load the maximum recommended span is < 2m?

 

I'd like to either replace the existing joists, or install some new ones alongside them so that I have a strong and level surface for a plasterboard ceiling and for loft boarding (C24 2x6 or 2x8?). I don't need them to be up to the standard needed for loft conversion floor joists, but I'd like to be able to use the loft for more than just light Christmas decorations.

 

I'm not sure if the wooden lintel over the bay window can take much additional weight either - it's constructed from two 50x180mm timbers doubled up to make 100x180mm x 2.7m long (unsupported span is 2.25m), and is possibly already sagging slightly.

 

I've measured everything up and put together a rough model in SketchUp (front and top views attached) - I haven't bothered modelling the ground floor, or windows other than the bay opening. Colour coding as follows:

• White walls are made of brick and are load bearing
• Yellow walls are made of cinder blocks, seemingly built after the lath and plaster ceilings were put up, so not load bearing
• Red joists are "short" (~3.7m)
• Green joists are "long" (~4.3m)
• Pink joists just rest on the chimney breasts, so don't seem to be helping structurally
• Blue is the bay window lintel
• Brown is the wall plate

The joists are just nailed together in the middle where they overlap, and are connected to the wall plate with a single skew nail - they don't seem to be connected directly to the rafters, which possibly explains why they've warped away from them. The rafters are presumably connected to the wall plate, so indirectly supported by the joists?

 

Would it be best to pay a structural engineer to do some calculations? Any suggestions would be much appreciated!
 

[markc]

Sagging joists is pretty normal in older properties. If you want to stiffen and level up the underside add additional timbers alongside (sistering) and nail or screw to the existing

Edited November 7, 2022 by markc

[retrophit]

Thanks @markc - something like this?

 

I think 44x170mm C24 joists should be ok. What's the best way of attaching them to the wall plate - angle brackets?

 

Any recommendations on bolt sizes for sistering? Presumably I need to use square washers on either side, and a dog tooth washer in between timbers?

 

[markc]

No need for dog washers and the sistering can be staggered screws or bolts if you prefer. M8 will be plenty and circular repair washers will be more than enough. Drill a snug hole for the bolts and the washers and nuts will be doing very little.

connection to wall plate can be angle brackets (Screwfix) or skew screw/nail as this connection will not be loaded

 

Edited November 9, 2022 by markc

[Miek]

I like these for timber joining, strong as a bolt but quick as a screw, you can drive them in with a rattle gun with one hand. 

 

https://www.screwfix.com/p/turbocoach-hex-flange-coach-screws-m8-x-100mm-50-pack/78121

 

 

[markc]

5 hours ago, Miek said:

I like these for timber joining, strong as a bolt but quick as a screw, you can drive them in with a rattle gun with one hand. 

 

https://www.screwfix.com/p/turbocoach-hex-flange-coach-screws-m8-x-100mm-50-pack/78121

 

 

+1 on these

[George]

I don't think they're rated for use in shear. Have just been digging around for a declaration of performance and only tensile strength and pull out strength is listed.

 

Can use screws to hold things in place but I'd always rather a proper bolt and toothed timber connector.

[joe90]

As the OP is really looking for a flat surface and strengthening the existing a little I would use the above “screws every 600mm, no need to fit over the wall plate IMO.

[joe90]

On 07/11/2022 at 21:06, retrophit said:

The rafters are presumably connected to the wall plate, so indirectly supported by the joists?

No, the rafters are connected to the wall plate, the joists will give lateral support only (stop the roof spreading).

[SteamyTea]

 

 

[Gus Potter]

On 07/11/2022 at 21:06, retrophit said:

I'm renovating my 1930s semi and have taken down the old lath and plaster ceiling in one of the rooms

Well done you for posting and asking the questions you are and compliments on your model.

 

Funnily have just done a design for something very similar. I'll split the post into two parts: Your original post has three photos.. call them photos 1 ,2 & 3

 

Part One if you were not adding load to the attic floor:

 

Photo 2 shows the joists spliced in mid air. Often you find that there were back to back cupboards in these houses and someone has taken away the cupboard on one side that supported the end of the ceiling joists.

 

One way of fixing this is where the joist does not rest on the wall is to extend it. Say it is 500 mm short of the new support.. and the support is 100 mm wide. Get another bit of timber and cut it 1800m long.. nice size as you an get 4 bits out of a standard 4.8m length of timber. That gives you a 1200mm overlap. Now the old timber will be imperial size.. new ones are often metric so watch out for that.

 

Now get some Cascamite structural glue.. read instructions!

 

This is important. You need to look at the end grain of the existing timber and see which way the circular pattern of the grain runs. The side of the old joist will be "cupped" Turn your new timber so the end grain pattern runs the same way so that when it dries it tends to cup the same way as the old timber, if not it will pull the joint apart or split the timber.

 

Next offer it up and pilot drill the new timber. Clean the old timber and new and cover both sides in the glue. Now screw them together with 5 x 80mm screws staggered at 200mm horizontal centres. Keep the glue off you.. messy stuff. The screws do the clamping while the glue cures. The glue is stonger than the wood so job done.. no need to check the screws for strength.. other than to say the screws provide an alternative load path if something goes wrong with the glue.

 

When you look at the ceiling joist spans you'll see that by extending the now short ones they are spanning no further than the longer ones.

 

Lastly don't forget to put in the noggings / dwangs.

 

The above sorts out how you remove these back to back cupboards you find in these houses when you want to take them out an make straight wall. Provided you are only going to put around 25 kg / m^2 including the self weight of the flooring ceiling etc.. which leaves you with about 10kg  per sq metre load for xmas decorations and the point load from a plumber. In summary this about the safe working load you can put on these old ceilings. Sometimes you can put on extra load but this needs to be over the supporting walls below provided they are truly load bearing.

 

Part two: Adding further load: 

 

You need to start in the solum. On the ground floor you'll probably have brick walls on a found than can transition to thinner (3") cinder block on the first floor. But often the first floor cinder block is built off the floor joists and offset from the structural walls on the ground floor. You should consult an SE as there are quite a lot of unforseen implications. Don't mess with the roof at this stage until you get a better understanding of the issues.

 

For example I often see that folk have on the ground floor widened the opening between the front sitting room and the rear room to make it open plan.. it needs a bit of thought. What you do up top has an impact lower down.

 

That said if you want to post more drawings showing how the walls on the first and ground floor line up I and others will chip with potential solutions that you can mull over. Easy way is to do some floor plans with gridlines.

 

Once you nut this out at the lower levels then you turn back to the attic floor.

 

If you are going to do this then why not design it for full domestic loading to be on the safe side.

 

It may be that in the round you might be better off spending your money on a rear ground floor extension that will add real value to your house?

 

For all:

 

On the face of it you may think hey.. I'll strengthen the attic floor by putting in big timber running continuously from the front to rear wall and let them rest mid point on an internal wall.. On the face of it that looks good..it reduces deflections a good bit but there is no free lunch as what you do here is to create a two span continuous beam. The bad news is when you do this the central support attracts about 60% of the overall load and rather than 50% if the timber was formed in two pieces..simply supported.. things can start to protest if at a lower floor level the load does not go straight down to the founds for example.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

7 hours ago, Gus Potter said:

This is important. You need to look at the end grain of the existing timber and see which way the circular pattern of the grain runs. The side of the old joist will be "cupped" Turn your new timber so the end grain pattern runs the same way so that when it dries it tends to cup the same way as the old timber, if not it will pull the joint apart or split the timber.

Very useful.

Thanks.

[saveasteading]

17 hours ago, Gus Potter said:

4 bits out of a standard 4.8m length of timber

Red pen.

3 lengths of 1.6m or   4  of 1.2m.

 

Interesting about the glue. I had never heard of that and would have bolted. Your way much easier.

[Gus Potter]

On 02/12/2022 at 16:56, saveasteading said:

Red pen.

3 lengths of 1.6m or   4  of 1.2m.

 

Interesting about the glue. I had never heard of that and would have bolted. Your way much easier.

It can be as often you find that you get your dog washers, mark out the timber and drill.. all looks good but if a drill hole is near a knot they can be ferociously hard and the dog washer won't bed properly.. then it all goes downhill as it's tempting keep tighening the bolt and hoping for the best.. you can wreak the old and new timber that way.

 

The main thing as I said earlier is to look at the shape of the old timber and the grain.. it's often not hard to do and it you get the enjoyment of practicing a craft .. for all on BH.. just take a few minutes to get it right. The principles will stand you in good stead. But if the old timber is too cupped the glue won't cover all the surface area.. let common sense prevail.

 

I mentioned earlier a good long lap length. Two reasons.

 

1/ A long lap length creates a stiffer beam so you reduce deflections. That works in your favour.

 

2/ You may have someone ask.. but you are creating a laminated beam here, like a bit of ply or certain types of Glulam beam so what about rolling shear. Rolling shear occurs in a bit of plywood for example where the different plys are glued together and at the glue joint the grains of the timber want to roll over each other. The glue may be good but the cellular structure of the timber and capilliary tubes roll over each other.. hence rolling shear which for example you need to sometimes check in timber portal frame joints and the like.

 

The long lap length allows you to head off these hard and time consuming calculations / nasty SE questions at the pass.

 

Glued joints can be great at times but they are not the silver bullet for all your dilemmas.

 

Lastly if you are considering glueing things then make sure you use a structural glue and remember that it only works if the mating surfaces are clean, dry, not frozen (working temperature) and kept dry while the glue cures.