Loggia Ledger

[MortarThePoint]

I'm trying to finalise my thoughts around this loggia we are having on the back of the house. The main two open questions are the attachment to masonry at eave level and the post bases to use.

 

Masonry attachment: It's straightforward at ridge level, using a 50 x 125 ledger which will be bolted with anchor screws every 330mm (Flemish bond repeat). At that point in the structure there is mainly a shear load. Less clear is what to do for the timbers at eave level. I am planning to have rafters on 600m c/c coming down onto the timber beam, but fewer lateral joists. I definitely have to have them at the post locations. What about in between the posts that is a 3m gap? I had thought to use face hangers, but a ledger would be easier though the fixings would be subject to mainly tensile loading. A ledger going full length gets close to the tops of windows and doors (circled red). I could have sectional ledger if few 'joists'.

 

Post Bases: I'm thinking to go with Simpson Strong Tie PPT post bases which would raise the timber 100mm up and avoid rot. That would require the concrete to come to the surface (or near). There is going to be a slab patio around this area.

 

 

 

Edited November 14, 2022 by MortarThePoint

[MortarThePoint]

Attached is a useful document that includes allowed loads for these types of fixings that are made by a range of suppliers (including Rawlplug LX). In brickwork it looks like M10 allows around 0.6kN for both shear and tensile loading. @Gus Potter would I be right in thinking the loading at the location circled in the previous post would only be tensile? I don't think there would be much and wouldn't it tend to be compression anyway?

 

 

 

At the upper ledger, as a ball park calculation:

• Tile load: (1.375m / COS(46)) * 77kg/m2 = 152kg/m [1.375m lateral distance between beam and wall)
• That's carried by the ledger at the wall and by the beam at the posts, probably slightly more by the beam due to eave overhang (not factored in), assume 50%
• Double to cover timber, snow load, someone standing on it and safety factor 152kg * 50% * 2 -> 1.5kN per meter of ledger.
• Masonry anchor screw every 330mm: 1.5kN * 0.330m = 0.5kN of shear.

That's a bit close, so maybe consider going up to M12 screws.

Ankerbolt_Masonry.pdf

Edited November 14, 2022 by MortarThePoint

[MortarThePoint]

Rough weight on the beam above posts:

• area of tile per metre of beam: 2.5m - 0.5*(1.375/COS(46)) = 1.5m
• weight of tiles per metre of beam: 1.5m2/m * 77kg/m2 = 116kg/m
• Double to cover timber, snow load, someone standing on it and safety factor: 116kg/m * 2 -> 2.3kN/m

The two middle posts each carry about 3.1m so vertical load on post is 3.1m * 2.3kN/m = 7.1kN

Post Base: has plenty of capacity 85.7kN (extract at bottom of this post).

Post: I don't know about the post (timber column) itself as I haven't found any load tables. The closest I have come is:

"According the diagram above - the safe loads for 4x4 Douglas Fir-Larch Columns with lengths 7.5 feet (2.5 m) are approximately 8 kips (35.6 kN, 3624 kgf)." https://www.engineeringtoolbox.com/wood-columns-safe-loads-d_1834.html  (typo not comforting there as should be 2.25m not 2.5m.)

6x6 looks to be over 25 kips at the height proposed, so that's over 100kN so more than the base. However, the webpage doesn't specify constraint so that could greatly reduce the capacity though not by more than the factor of 14 margin I've got (or 5 for a 100mm x 100mm post if I wanted to downgrade).

 

Beam: uniformly loaded at 2.3kN/m. Proposed 2-ply 50x200.  I calculate using https://skyciv.com/free-beam-calculator/ about 4mm of deflection based on that and with freely rotating ends. (UDL of 1.2kN on a 50x200: Y=10800MPa, Iyy=33.3E6 mm4). That's span divided by 772 so OK. Constrained ends (by symmetry about the post) would reduce this deflection further.

 

Edited November 14, 2022 by MortarThePoint

[Gus Potter]

On 14/11/2022 at 14:00, MortarThePoint said:

I'm trying to finalise my thoughts around this loggia we are having on the back of the house.

Wow! You have put a lot of thinking into this..

 

Bit late here but will try and answer some of your points. I'll put my "BC /checking Engineer's" hat on to be pedantic.. the following is a bit of an outline on how I may do it and some of the things I look for.

 

On 14/11/2022 at 14:48, MortarThePoint said:

• Tile load: (1.375m / COS(46)) * 77kg/m2 = 152kg/m [1.375m lateral distance between beam and wall)
• That's carried by the ledger at the wall and by the beam at the posts, probably slightly more by the beam due to eave overhang (not factored in), assume 50%
• Double to cover timber, snow load, someone standing on it and safety factor 152kg * 50% * 2 -> 1.5kN per meter of ledger.
• Masonry anchor screw every 330mm: 1.5kN * 0.330m = 0.5kN of shear.

 

Well done on picking up on the snow and access load. You also have a snow drift load and "snow dumping coming off the higher roof, but due to the pitch and size of canopy that can be effectively ignored especially as it is what we call and "accidental" snow load case. You do also have wind load but on something this size it I think this can be neglected as you have good heavy tiles mitigating roof uplift. If this was a big commercial building, lightweight roof covering with the roof sitting just below the window cills I would check it to make sure it does not lift the cills.. could be embarrasing!

 

Oh.. please excuse any typos.

 

Have repeated your calcs in a slightly different way but made an allowance for the rafter self weight just to let you see how I may do it. I use kg as it's pretty close to kN that we use as normally units. Your tiles look pretty heavy, normal Marley concrete tiles are about 50 kg/ m ^2.. you must be using some nice clay ones, jealous here.. like a nice tile.

 

I'm going to use Eurocode Design here as later this is compatible with the fixings then jump back to using the permissible stress codes of BS 5268.. as it's easier. I'll explain as I go along.

 

Tile load on slope = 77 kg/ m^2.. for all this is the declared weight by the manufacture of the tiles.. it varies depending on how much head lap and that changes depending on the weather exposure and roof pitch.. who said life was a breeze?

 

On plan load for @MortarThePoint= 77 / cos(46) = 110 kg/m^2 on plan.

 

Add 10% for timber rafters, sarking etc. 1.1 * 110 = 121 kg... this is the "Permenant" load, using the BS codes this is called the Dead load.

 

Now we have a roof access load. In both codes this is ~60 kg / m^2 for domestic applications but this is a load that is applied on plan. A slope load is the load running up the rafter.. like tiles. A plan load is more easily understood if you think about snow falling straight down or a few folk standing on a stair or a roof. You can only fit so many folk standing vertically and only so much snow can lie on a roof.

 

This 60 kg/m^2 is the imposed load.

 

Now we have two figures for the plan load.

 

Permenant load = 110 kg.. lets convert now to kN roughly divide by 10 thus 10 / 10 = 1.1 kN

 

Imposed load =  60 kg/m^2 lets convert now to kN roughly divide by 10 thus 60 / 10 = 0.6 kN

 

Now we apply safety factors to these loads, the different kinds of loads get different safety factors. Using the simplistic Eurocode formula safety factors are 1.35 (Permenant loads) and 1.5 ( Imposed loads) the calculation follows.

 

1.35 * 1.1 + 1.5 *0.6 = 2.39 kN/m^2 on plan.

 

Now as above take the effective span on plan of the rafter as being 1.375m (conservative) and as the rafter is simply supported at the ends the loads at the supports are calculated as follows:

 

2.39 * 1.375 / 2 = 1.64 kN/m. In other words the "DESIGN" load on the ledger is 1.64 kN per metre run of the ledger. The design load at the bottom of the rafter is the same neglecting the slight overhang.

 

If you were checking this you would check the rafters, the hangers before checking the ledger. Just like you follow the money you follow the loads and check each bit as you go. That is really pedantic. To make it fly take the rafter span on slope in feet divide by two m  and add one inch.. that will probably be fine as it is a canopy. Thus 2.4m = about 8 feet.. 8 / 2 = 4 + 1  = 5 inches. I would adopt a 120 x 45 C16 rafter @ 600 centres max for the rafters. Standard Simpson joist hangers will do fine at the top end of the rafter.. can show they will work but it will take all night to finish this post if I do.

 

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Lets now look at the capacity of the fixings and work out how many we need.

 

 

If you follow the method I have used above to calculate the loading. I start with the loads (characteristic loads) and apply safety factors to get the design load. We now check to make sure that the DESIGN load is less the design RESISTANCE. Not normally a shouty bloke (use of capitals) but for all it is really important so please forgive. Things like masonry fixings are very tempremental not least as we are often fixing into masonry that is variable.. brickies can have off days and so on.

 

In the table that @MortarThePointhas uploaded you can see two columns. One characteristic and one design. Take the M12 fixing.. the number we are looking for is the design resistance of 1.62 kN in shear. Now here it can get really complicated as we are using the Eurocodes. You'll see there is also a figure for the approved resistance. This is the value that you would use if you did not use the safety factors on the loading calculation... permissible stress design codes. Lastly always take note of spacing and edge distances.

 

Lets now work out how many fixings we need per metre based on an M12 bolt and see if we can practically install these.

 

Design load is 1.64 kN/m. Design resistance is 1.62 kN per fixing. Suddenly you can see the loads are pretty small and that you only need one roughly one fixing per metre run of ledger. To be exact we would need: 1.62 / 1.64 = one fixing per 987 mm which is a funny number to deal with on site.

 

Practically I would say one M12 fixing per rafter bay if they are at 600 centres.

 

Now all looks great above but lots of BH folk start using the same idea for 3 - 4 m spanning floors and ledgers for trussed rafters.. you can quickly see how these fixing will stop working!

 

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The ceiling tie serves to restrain the top of the posts. Just use the same fixings and spacing as the rafter ledger.. line them up so they look good. 70 x 45 C16 may be what you want.. if hanging plant pots  / growing a vine of something go for 95 x 45 C16 / maybe C24 as it has generally less knows and looks less cheep. You coaul calculate the wind load in the roof but using the same fixing as for the ledger will be fine.

 

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The transfer beam along the top of the posts. Have run out of gas here a bit and it takes a bit of explanation.. you use the design loads to check the strength and the unfactored loads to check the deflection. With timber you need to calculate both the bending and hear deflection.. bit more work. Suffice to say that on that roof you could I think show that a 200 x 45 C24 timber would work or two 170 x 45 C16's. The single timber may look a bit "weedy" and not sturdy to the eye.. it's also about it looking balanced and visually pleasing.

 

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A 100 x 100 post around 2.4 m long will take a lot of vertical load, again can show another day how you check it.

 

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Simpson post bases.. they are good, last a while, just a little more expensive.

 

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[saveasteading]

Gus, you are doing so much detail here which is wonderful. 

I think I would add a standard sign off saying that this information is without full knowledge of the project, and unchecked, and as suggestions only, and an SE should be engaged.

[MortarThePoint]

@Gus Potter So kind of you to share some of your knowledge on this. It's interesting to see how it is all worked out the right way. Also, great rafter sizing tip! Are you happy with the ledger size (120 x 45)?

 

On 17/11/2022 at 00:27, Gus Potter said:

Standard Simpson joist hangers will do fine at the top end of the rafter

So you'd definitely recommend hangers rather than a birds-mouth at the top?

 

I was a bit low with my design load then (1.5kN/m vs 1.64kN/m). I compared the Design Load with the table's Approved Resistance (shear) as I wasn't so sure and felt better to us a lower capacity. Better to know what you're doing and use the correct figure though 🙂 You've picked the drill diameter 12mm fixing which is M14 I think (14mm thread). I'd think I prefer a greater number of small fixings so I think I would need either of:

• M10 at 0.8 / 1.64 = 487mm, could go with 300mm to match rafter pitch or 330mm to match brick bond
• M12 at 1.14 / 1.64 = 695mm, could go with 600mm but perhaps that's too aggressive

Is that valid?

 

On 17/11/2022 at 00:27, Gus Potter said:

The ceiling tie serves to restrain the top of the posts. Just use the same fixings and spacing as the rafter ledger.. line them up so they look good. 70 x 45 C16 may be what you want.. if hanging plant pots  / growing a vine of something go for 95 x 45 C16 / maybe C24

So do you think the ceiling ties should be at the same c/c as the rafters or could they be every other rafter or even just at every post? Just at every post feels a bit weedy, but every other rafter (1200mm c/c) could make the ceiling area feel a bit more open. I'd go with 95 x 45 C24 for these. Matching and lining up the ledger fixings sounds good. If using fewer ceiling ties, particularly if only at posts, I was wondering about using short sections of ledger, perhaps 900mm long with 4 fixings.

 

Would you screw (2no. 6.0mm x 100mm) through the beam / ledger to the ceiling tie or use mini truss hangers? I guess a nice feature of a hanger is it has a published load.

 

On 17/11/2022 at 00:27, Gus Potter said:

Suffice to say that on that roof you could I think show that a 200 x 45 C24 timber would work or two 170 x 45 C16's. The single timber may look a bit "weedy" and not sturdy to the eye.. it's also about it looking balanced and visually pleasing.

Agreed, 2no. 200x 45 C16 or C24 should look nice and sturdy and exceed those two in strength. The architect has used 150 x 150 posts which obviously are stronger than 100 x 100. Sitting a 90mm wide beam on top of a 10mm column could look awkward though.

[ProDave]

Re the posts, I use 6" Larch for my balcony.  My neighbour made the post bases and these are set onto concrete blocks cemented to the concrete slabs that end below ground level.  My posts are 3.3 metres long.

 

 

Post foot bracket

 

[Gus Potter]

On 17/11/2022 at 09:33, saveasteading said:

Gus, you are doing so much detail here which is wonderful. 

 

On 17/11/2022 at 09:33, saveasteading said:

I think I would add a standard sign off saying that this information is without full knowledge of the project, and unchecked, and as suggestions only, and an SE should be engaged.

 

You make a good point here and the advice you give is sound.

 

To expand a bit on my own personal approach.

 

When I joined BH one of the thoughts I had was.. do I use my own name or not. I concluded that if you mean what you say then why not use your own name? I run a business with a website (it's not slick as I wrote it myself and it needs updating but have plenty work)  and sell my wares using my own name. Why not just use my own name BH?

 

I can see where you are coming from. If you are writing on any public forum or even giving free advice to members of the public then potentially you can be liable for it.

 

I'm an SE so carry insurance. Part of my policy covers me to give advice even if not paid for. That allows me to go into detail that others may not be able to do in public. That said if you look at my posts you'll see that I often introduce a caveat. Above I used the words "outline advice" .. you would have to be pretty determined to raise an action against me.

 

In the round though I would hope that in the first instance if someone on BH felt that I was wrong they would invite me to explain or just say.. Gus.. your numbers etc don't add up and give me an opportunity to correct the record. We all make mistakes from time to time.

 

 

 

 

 

 

 

 

 

 

 

 

[Gus Potter]

16 hours ago, MortarThePoint said:

So you'd definitely recommend hangers rather than a birds-mouth at the top?

Don't think we talked about birds mouths? They are great but you need to tie back the posts and roof so they don't move horizontally. You can either use the rafters or the ceiling ties to do this.

 

You can tie the posts back to the wall using the ceiling ties. But rather than using hangers for the ceiling tie you would use say Simpson ABR 9020 brackets. But this give a very much an "exposed look" often not pleasing to the eye.

 

Have not got time tonight but will try and post a detail on how you make it all work while pleasing to the eye over the weekend.

 

 

16 hours ago, MortarThePoint said:

I was a bit low with my design load

Still.. well done you.

 

16 hours ago, MortarThePoint said:

you've picked the drill diameter 12mm fixing which is M14

The nominal fixing diameter is M12 so you need practically a 14mm diamter drill hole and blow / brush it out clean for the resin.

 

16 hours ago, MortarThePoint said:

So do you think the ceiling ties should be at the same c/c as the rafters or could they be every other rafter or even just at every post?

You could do hit and miss but much depends on how you want it to look.

 

 

[PeterW]

11 hours ago, Gus Potter said:

 

I can see where you are coming from. If you are writing on any public forum or even giving free advice to members of the public then potentially you can be liable for it.

@Gus Potter @saveasteading @ETC We are aware there are many professionals on here and generously give their time and advice on the forum. To that end there is a clear statement in the terms and conditions 

6.3   We do not verify, confirm or accept any responsibility for any of the information, advice or material posted in any section of the website. If you use any of the information, advice or material posted you do so solely at your own risk.

 

I regularly put on my posts that members should take professional advice - an Internet forum is neither regulated or guaranteed !! 

[Iceverge]

On 17/11/2022 at 09:33, saveasteading said:

I think I would add a standard sign off saying that this information is without full knowledge of the project, and unchecked, and as suggestions only, and an SE should be engaged.

 

There's an Irish forum with a building section and it's almost useless because there's a few die hards on there ( including mods) who constantly reply "that's a question for your architect/engineer"etc. 

 

Unfortunately this fear of litigation stifles debate and imagination and drives both professionals and DIYers off the site. 

 

 

 

 

 

 

 

 

[saveasteading]

35 minutes ago, Iceverge said:

this fear of litigation stifles debate and imagination and drives both professionals and DIYers off the site. 

I wouldn't think so. I was more worried that some people will run with the free info, whereas there might be more to it than they have said.

Also when doing sizes there is a risk of making errors. Perhaps no risk of being successfully sued, but any hassle wouldn't be fun.

[ETC]

3 hours ago, PeterW said:

@Gus Potter @saveasteading @ETC We are aware there are many professionals on here and generously give their time and advice on the forum. To that end there is a clear statement in the terms and conditions 

6.3   We do not verify, confirm or accept any responsibility for any of the information, advice or material posted in any section of the website. If you use any of the information, advice or material posted you do so solely at your own risk.

 

I regularly put on my posts that members should take professional advice - an Internet forum is neither regulated or guaranteed !! 

A very good point but I think this should be given “as read” - I certainly do. The information posted on this forum is - largely - personal opinion and should not replace relevant professional advice.

 

For my own part I only offer to help with preliminary design proposals and anything that I post is my personal opinion. Anything beyond that is beyond what I am prepared to do on a public forum. 

 

I hope to continue to offer design advice to any OP who is “struggling” to design their home or who is just looking to get a second - personal - opinion on what their designer has prepared for them.

 

Thank you.

 

ETC.

[PeterW]

2 hours ago, ETC said:

 

I hope to continue to offer design advice to any OP who is “struggling” to design their home or who is just looking to get a second - personal - opinion on what their designer has prepared for them

My general view on architects is well known on the forum … you’ve proved there are some very good and personably ones who do listen .. thank you !

[SteamyTea]

On 18/11/2022 at 23:31, Gus Potter said:

To expand a bit on my own personal approach.

I still think you should write a book, you explain things so well.

[saveasteading]

On 19/11/2022 at 00:59, Gus Potter said:

M12 so you need practically a 14mm diamter drill hole

Ahh. My first ever disagreement with Gus.

Most studs are undersized. I suspect a 12mm stud was once a 12mm rod before threading. So they usually, just about, push into a hole of the same nominal diameter.

 

That doesn't leave enough space for the resin, so a drill of half or a mm bigger is plenty.

It maybe doesn't matter too much for one or 2 small holes, but for lots of deep  holes it adds up to many a tube of resin. 2mm oversize is double the quantity of resin as for 1mm. Lesson learnt when I provided 6 tubes of resin for foundations, but the groundworker used a much bigger bit....off to the BM for a dozen more.

 

Thicker resin is not aiding strength either.

 

As Gus says  cleaning the hole is important.

Also winding the stud in rather than pushing, until resin screws up to the surface.

 

 

[Gus Potter]

10 hours ago, saveasteading said:

Ahh. My first ever disagreement with Gus.

Call it a clarification, any discussion, different view point is always helpful and healthy.

 

I agree with you that you could end up using excess resin. But to keep say BC and the SE's happy best to follow the manufacture's installation instructions to the letter where possible, unless you know a lot about fixings and their design. Hilti and Fischer both make mention of nominal drill hole size for their threaded rod applications in solid brick. For an M10 rod nominal drill hole size is 12mm and for an M12 nominal drill hole size is 14mm and so on up to and M16 which needs a nominal drill hole size of 18mm.

 

You can also (with careful checking) install rods into bricks that have voids. Here you get a sieve sleeve that goes in the hole first. You need quite a lot more resin for this as you need to flood the sleeve andsome of that disappears into the voids.

[saveasteading]

15 hours ago, Gus Potter said:

best to follow the manufacture's installation instructions to the letter where possible

Absolutely.

I was very keen on tubed resin when it was fairly new, thinking that the mixer nozzle made it idiot proof.  Wrong.

I had personally tightened a nut onto a 30mm foundation bolt and seen the bolt lift out of the foubdation. The groundworker (not ours) had not mixed the hardener in, and i found it nearby. Column down again and report to very nasty main contractor. Moral don't use 2 parts that need manual mixing.

 

But then I found how the first few mm, cm, foot? of stuff isnt always mixed in the nozzle and has to be thrown away. And how pushing the rod into the filled hole  just expels the mixture. Etc

I do worry that this process is not supervised closely enough.

 

For general interest, the pullout failure of a bolt in concrete  is a cone shape.

I saw this in action when a vehicle hoist failed due to insufficient anchor depth by the specialist installer.

Thus the failure plane area increases dramatically with depth.

The top 25mm of concrete should be discounted because it has often been overworked.

 

 

 

[markc]

Agree with the above, having set and tested literally thousands of chem anchors (plus had hundreds checked and signed off by Hilti) I’ve learned a lot.

embedment is very important, hole cleaning and effective mixing are musts.

I always spin the rods in backwards (drill running anti-clockwise) so the threads are pushing and mixing rather than pulling the resin out - this is especially important with glass tube systems because the tube forms part of the anchor - I have seen ground workers breaking the tubes and pouring the contents into holes 🙈.

but chem anchors are convenient and when set correctly (in a sound and suitable concrete/stone/block etc) are only surpassed by cast in caged Holding down bolts

[MortarThePoint]

The internet is full of people who know too little, but are happy to give their opinion and those who know a lot and are too scared to share. I take pointers as to aid my understanding in ultimately making a call on it. If I've paid for time, it's different, but someone being kind enough to share their knowledge can't be expected to have 'checked their working' as they would if one the clock.

[SteamyTea]

4 minutes ago, MortarThePoint said:

The internet is full of people who know too little, but are happy to give their opinion

Just the internet, you never been to a Weatherspoons?

Edited November 21, 2022 by SteamyTea

[MortarThePoint]

On 18/11/2022 at 23:59, Gus Potter said:

Don't think we talked about birds mouths? They are great but you need to tie back the posts and roof so they don't move horizontally. You can either use the rafters or the ceiling ties to do this.

I was thinking at the top of the rafter where it meets the top ledger.

 

I assumed the ceiling ties constrained the posts and ring beam laterally. Are you saying that the rafters themselves could do that by using a bracket on the rafter. Interesting!

 

On 18/11/2022 at 23:59, Gus Potter said:

But rather than using hangers for the ceiling tie you would use say Simpson ABR 9020 brackets. But this give a very much an "exposed look" often not pleasing to the eye.

Would that be one bracket either side of the ceiling tie then? as you say, starts to look a bit mechanical.

 

I had been thinking along the lines of the image below, but as the datasheet extract shows there is no information about lateral resistance.

 

[MortarThePoint]

Would it be possible to use hidden woodscrews like below? The fist one would require making a huge ladder shape before then raising it into place. The second could be added in situ and the 'blocks' could be angled to look more pleasing to the eye (?). I'm not artist 🙂  

 

[Gus Potter]

On 21/11/2022 at 19:00, MortarThePoint said:

Are you saying that the rafters themselves could do that by using a bracket on the rafter. Interesting!

Give me a bit of time and I'll post something.. need to finish something on the day job first.

 

On 21/11/2022 at 19:00, MortarThePoint said:

starts to look a bit mechanical.

Yes they are not pleasing to the eye right enough.

[saveasteading]

On 21/11/2022 at 20:09, MortarThePoint said:

possible to use hidden woodscrews

That screw into the web is not strong in rotation.

The angles in the prev drawing look heavy but won't be. You can get figures on screw shear strengths, but does this look as if it might fail sideways?