Foul Drainage Basics

[MortarThePoint]

Brett Martin's B2803 is a 90 degree 280mm inspection chamber, but the exits have a sharp 45 bend with the channel being straight within the inspection region. Osma's 4D918, on the other hand, has the bend in the inspection region. Osma have a useful selector, a copy attached. Slightly bigger though, 315mm vs 280mm. Here's some information about their Multi-Base IC range.

 

The Osma 4D918 looks a bit silly from above, but there is a straight alternative (4D910) that could be used with 45 elbows.

 

Even the smaller (250mm) Osma 4D960 could be used with two 45 elbows.

 

Brett Martin B2803:

 

Osma 4D918:

 

Osma 4D960:

F0026752_0001.pdf

[Mr Punter]

1 hour ago, MortarThePoint said:

Brett Martin's B2803 is a 90 degree 280mm inspection chamber, but the exits have a sharp 45 bend with the channel being straight within the inspection region.

 

They look very useful.  I have used straight ones with a 45 either end, but that meant extra fittings, more cost and more space.

[Russell griffiths]

Have you priced any of this stuff up, the last time I looked at an osma chamber base I thought it should come with a free Rolex. 

[saveasteading]

Osma is always expensive, even discounted. Of course it is high quality and gets specified on big jobs. But for 1/3 the price everything  on the cheaper stuff still works OK.

The only advantage of the big names that I have perceived in real life is the ease of joining. Down a wet trench on a frosty day that £10 may be worth it, but not normally.

 

And are the pipes equally strong? That doesn't matter if properly surrounded.

[Oz07]

I've done a good workaround for that osma 90 base above. I have a load of long rest bends and if you look at these its marked where to cut out to use on flat. Then I set on lean mix and benched around the opening with compo and just used standard risers around it. Worked a treat. Not sure how much cheaper it is if you have to buy the bends I already had them. 

[Oz07]

Why don't you rip the existing ic out, put a 5 inlet ic inline with your run and extaend the existing pipes to meet to new ic. 

[MortarThePoint]

For the two turns, I think I'll use Osma 4D960 plus two bends as I've identified a cheap source and didn't like the look of their 90 bend chamber (4D918) and the total cost of that was looking like £100+ each. The first corner can be achieved with a total turn of 2x22.5=45 if I bring the pipe out from the house at 45 degrees. Despite all being level invert, it says to always use the main channel rather than make the turn with the chamber's 45 inlet.

 

I loved the look of the Polypipe SFA7 option but it's too expensive.

[MortarThePoint]

Neither Floplast nor Osma make a 22.5 degree bend in 110mm, but Brett Martin does:

 

Single Socket:

https://direct-drainage.co.uk/products/110mm-single-socket-22-5-bend-b5071-underground-drainage-pipe-fitting

 

Double Socket:

https://direct-drainage.co.uk/products/110mm-double-socket-22-5-bend-b4051-underground-drainage-pipe-fitting

[MortarThePoint]

Estimating how much gravel is needed. For 110mm pipe, the trench needs to be at least 410mm wide and the depth of the gravel at least 210mm including the height of the pipe or 160mm if ground suitable. That works out as 0.41m*0.21m = 0.086 m3/m or 0.41m*0.16m = 0.66 m3/m (ignoring pipe for contingency). Round that up to 0.09m3/m or 0.7m3/m. A bulk bag of pea gravel seems to be about 2/3 m3 so that provides enough for 7m or 10m of pipe.

 

7m or 10m per bulk bag. Does that sound right?

 

10mm pea gravel links:

   https://www.travisperkins.co.uk/bagged-aggregates/travis-perkins-gravel-and-shingle-bulk-bag-10mm/p/938182

   https://www.jewson.co.uk/p/pea-gravelpipe-bedding-bulk-bag-AGSTB016

   https://www.wickes.co.uk/Tarmac-10mm-Gravel-Pea-Shingle---Jumbo-Bag/p/131897

 

Osma:

PolyPipe:

[saveasteading]

Haven't checked the sums but seems rightish.  Beyond about 5 bags it might be worth buying a small truck load, but wastage can be worse.

Bags can be offloaded and spread around near the points of use to reduce barrowing. 

 

[Oz07]

trench width seems big I've seen plenty of drain runs excavated with a 12" bucket. Is that a reg or just a guide?

[MortarThePoint]

5 hours ago, Oz07 said:

trench width seems big I've seen plenty of drain runs excavated with a 12" bucket. Is that a reg or just a guide?

 

I was surprised too. Can't see it in part H, but the manufacturers seem to agree as I think Floplast also has that. Seems a tad excessive and a 12" bucket feels sensible and would provide 100mm on three sides of the pipe 

[saveasteading]

21 hours ago, Oz07 said:

excavated with a 12" bucket

It's fine as long as you can get in the trench and do a thorough job in bedding and getting the gradient... and joining the pipes.  Unlikely.

But you could do most of the depth in 400 or more and the bottom in 12".

[MortarThePoint]

Osma only seem to make plain ended pipes in their Underground range (OsmaDrain), so no socket ended pipes. Any idea why that is? Socket ended pipes would appear to be a lot easier to work with on longer runs. Also, for Floplast the cost uplift from P/E to S/S is minimal and certainly less than the cost of a coupler.

Edited November 26 by MortarThePoint

[MortarThePoint]

I have Osma rest bends which have an invert 290+50=340mm below top of socket. Clearance to house makes this run about 20m, call it 21m which would be a 350mm drop at 1:60. If I have the 250mm chamber 50mm below the 450mm chamber's invert (600mm bgl), that would be a depth of 650mm. It's not a compliant depth for that 250mm chamber, but BCO was OK with a junction so I can just bury the chamber under a removable slab.

 

The invert at the rest bend would be 650 - 350 = 300mm bgl so the rest bend's socket will stick out of the ground. I'll paint that black. Should work, but have to check fall to STP to confirm.

 

Top of pipe at the rest bend will be 190mm bgl, falling to 190 + (8,000/60) = 323mm at the turn. The rainwater pipe it needs to cross will have an invert depth of about 175 + (5,000/100) = 225mm or 175 + (5,000/80) = 238mm at this point, so 100mm or 85mm clearance between pipes. Rainwater pipe invert after 20m needs to be 425mm and can start at 175mm, so may be 1:80.

 

Osma rest bend:

[saveasteading]

On 26/11/2025 at 08:15, MortarThePoint said:

Osma only seem to make plain ended pipes

To my surprise that seems to be the case. Why? I can only assume that cut pipes need joining fittings anyway so it becomes a standard detail.

[MortarThePoint]

14 hours ago, saveasteading said:

To my surprise that seems to be the case. Why? I can only assume that cut pipes need joining fittings anyway so it becomes a standard detail.

 

For long straight runs, socketed pipes are useful though and have fewer seals. I've gone with Osma fittings and pipes. A pipe and coupler works out about £3 more than a socketed Floplast pipe. I asked and Floplast didn't recommend using the socket of a pipe as one of the sockets of a rocker pipe.