Increasing attic joist depth

[mark8par]

Evening all, new to the forum. 

 

I want to increase the insulation in my attic but the current joists are only 100mm, I also want to re board it as its used for storage.  So my thoughts are to run a length of 4x2 along the current joists (hanger to hanger) and then add another 4 x 2 the same and this would increase the height to 300mm, I could then run 250mm of insulation, under building regs but its not being used as a living space.  I also thought about running the second 4x2 at 90 degrees to allow me to cover the joists (thermal bridging). I have looked into something like a loft leg but im not sure about the weight they can take and we store a 50kg tent up there. 

Any help would be appreciated. 

Thanks 

Mark

 

[TonyT]

What about the plastic feet you can buy From the sheds, I bought a box, it’s a job once I get the Xmas decorations out and make a bit of space 

[mark8par]

3 minutes ago, TonyT said:

What about the plastic feet you can buy From the sheds, I bought a box, it’s a job once I get the Xmas decorations out and make a bit of space 

They look ok but apparently can only take 25kg per sqm

[Ferdinand]

If you care concerned about floor loadings then I would do some general load and point load calcs, as the weight will more be in your supplementary wood and the boarding than in a 50kg tent.

 

I used loft legs when a T needed storage space, and they soon add up once your need a decent area.

 

Depending how you intend to load it, the loft legs may allow you to dispense with your extra lengths of wood.

 

Wickes say 50kg per sqm, and have a short vid hjere:

 

[Nick Thomas]

Interesting product. I'm looking at boarding out my loft this month or next for better access to the solar inverter, and maybe adding some DC batteries.

 

https://www.loftleg.com/faq has a bit on the 25kg/m² number:

 

Quote

 

5) What is the maximum load I can apply to the Loft Legs?

Loft Legs are strong and can support weights of 500kg however, they do not change the load bearing characteristics of a loft.

 

British Standards BS6399-1:1996 for new build homes installed with a loft hatch require trusses to be designed to carry a 25Kg per square metre storage load plus the temporary load of a person moving around in the loft. Some homes may have been designed to exceed this but if you want to exceed 25Kg per square metre we would recommend consulting a structural engineer.

 

 

 

Perhaps the batteries can be fixed to the wall instead...

Edited October 13, 2022 by Nick Thomas
.

[mark8par]

37 minutes ago, Nick Thomas said:

Interesting product. I'm looking at boarding out my loft this month or next for better access to the solar inverter, and maybe adding some DC batteries.

 

https://www.loftleg.com/faq has a bit on the 25kg/m² number:

 

 

 

Perhaps the batteries can be fixed to the wall instead...

We are also having the solar inverter and batteries in the loft, I have asked for them to be fixed to the wall. 

 

I am not sure about the loft legs, the weight limit does bother me.

[Nick Thomas]

The way I read it, the limit is not the loft legs, it's the loft itself, and would apply to literally anything you do up there?

[Gus Potter]

12 hours ago, Nick Thomas said:

The way I read it,

Yes Nick good point. Below is a screenshot from a typical truss manufacture. The loadings they use have been in common use for a long time so you can take these as a guide to an old cut timber roof.

 

For ease of reading 100 N (Newtons) = ~ 10 kg so 250N = ~ 25 kg.

 

The load we are looking at here is in the second column in the ceiling tie loads.. called imposed load (loft storage) .. that also includes anything you put on top of the original timbers @Ferdinand.. any legs, secondary flooring and so on.

 

Confusion often arises as there is a difference between long, medium and short term loads in timber design. A bit of wood will carry quite a bit more weight if it is just subject to a load (short term) like a person of "large stature" walking about in the loft checking the plumbing.. but like glass it is less able to carry long term loading such as storage and the bits and bobs ( eg, extra flooring and insulation) folk put up there long term. Loft storage is considered a long term load for design purposes.

 

But if you put a 50 kg tent and can be sure that after you have put in a "secondary floor" that spreads the load about and it will not overload the roof structure then you are good to go. Roughly you often find that if you put in an extra layer of

flooring you can add another 10kg per sq m of say xmas decorations. Now to be on the safe side you need a lot of secondary unloaded floor!

 

What folk often do is to say, can I put the tent over a load bearing wall below? Now you need to use common  sense and make sure that the wall is truly load bearing all the way down the building. If so then the tent will have little impact compared to the weight of say a load bearing masonry wall.

 

 

 

 

[Ferdinand]

Yes - that all seems well expressed, @Gus Potter.

 

This is the comment from the linked FAQ page, which a - specifies that a person accessing the loft shall temporarily be supportable, and b - is 1996 building regs, so earlier lofts need to be considered separately (and Wickes quotes 50kg). I'd do a couple of joist dimension and spacing measurements and span calculations and compare with load and span tables making conservativer assumptions eg *:

https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C24

 

(Notes: 1 - a sensible assumption will need to be made about timber grade. 2 - a 1kg load imposes about 0.01 kN ) 

 

British Standards BS6399-1:1996 for new build homes installed with a loft hatch require trusses to be designed to carry a 25Kg per square metre storage load plus the temporary load of a person moving around in the loft. Some homes may have been designed to exceed this but if you want to exceed 25Kg per square metre we would recommend consulting a structural engineer.

 

One tactic is deliberately to board only near the hatch so no large people climb into the loft.

 

As for putting things like boilers and batteries in lofts - not something I like. I have one in a loft in a rental, and it is a bit of a sod. I boarded out a walkway to the boiler in 2ft wide OSB over the bottom layer of insulation, then put the top layer of insulation in the same direction so that removal of one stripe would clear the walkway, and put a sign next to the loft hatch.

 

I would not put a potential fire hazard or weight like a battery anywhere near a loft. They go in the garage or in a fireproof booth.

 

F

[SteamyTea]

Any help.

 

 

 

[Temp]

The best way is to insert new deeper joists. These should rest on packing placed on walls each end.  The packing should raise the new joists, 10-15mm or enough so that when they flex the don't touch the plasterboard in the middle. That reduces cracking of the ceiling below. The new joists should only be fixed to existing rafters at the ends where both are supported. The main problem with this approach is getting them into place in the loft.

[Nick Thomas]

Thanks for the extra detail! Anything that goes near structural calculations is way over my head at the moment, but it's good to get chapter and verse nonetheless.

 

Couple of other suggested options come up in https://www.theloftboys.co.uk/lofts/loft-boarding/ - they also suggest counter battens, and (presumably their proprietary) "LoftZone StoreFloor"™®© system, which is like the legs, but maybe a bit less work to install.

 

I hadn't really thought about the safety aspect of batteries in the loft, but fixing them to the wall at least avoids the loading issues - I just need to improve the access and get some boarding down that they can use while installing. The batteries I'm going for are LiFePO4, so not incredibly high-risk from a fire point of view. They're DC-side wth the solar, so putting them anywhere *but* the loft would imply a ridiculous DC cable run. They and the inverter rely on convection for cooling, so boxing them in would probably do more harm than good.

 

Worst-case, at least the fire wouldn't be between me and the exit,  right?

 

Edited October 15, 2022 by Nick Thomas
dc-side clarif

[Nick Thomas]

With a bit more research I've decided to go with the "LoftZone StoreFloor". With the boards, it works out at £40-50/M² for the kit; on a supply+fit basis, it's ~£100/M².

 

I don't have a huge acreage to do so it hardly matters either way, but it seems like an easy enough project. Famous last words.

 

 

 

 

[Nick Thomas]

Just an update on this - the stuff came last week and I've fitted the initial platform in front of the inverter in the loft.

 

 

The alu rails go crosswise to the joists, and there are two types of legs - triangular and stick. The idea is to use the triangular legs every other joist, then come back and reinforce with the straight legs under half of the missed ones, in a staggered pattern.

 

I seem to have picked the hardest bit to start with though - the rails start flush against the gable end and come down past the hatch, which is > 600mm long so it interrupts one of the rails. Gable to hatch is just over 1.2M, and no room to use the triangular legs at either end so it's straight, triangular, straight, which also meant cutting the alu rails (they're slightly shorter than 1.2M) for that section in half.  I'm not missing any joists out for this section, so I'm sure it's still plenty strong enough, it was just.. awkward.

 

Next is the section where the loft ladder will be attached, so I have an L-shaped boarded area around the hatch. They suggest using some of the boards to cover the legs around the hatch (shown in the second picture), I guess I can just add some wood behind that to give the ladder something to screw into.