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Recommended base flashing for an external wall

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choochoo_baloo20/03/2019 21:58:44
183 forum posts
25 photos

I'm completely new to general construction, and am quickly learning on the go...

for my new workshop build I want some type of base flashing to slip up under the corrugated steel wall cladding, to prevent rain and thee like pooling at the base of the wall.. [The wall is built from along the edge of the concrete floor and is edged by railway sleepers].

Trouble is that I don't really know what to look for. Hopefully the usual members who have helped me along so far can chip in?

Thanks in advance.

JasonB21/03/2019 06:56:00
16051 forum posts
1687 photos
1 articles


AdrianR21/03/2019 07:03:57
272 forum posts
20 photos

+1 for lead

I dont understand where the sleepers are, are they a path?

Best thing to do is make sure the finished ground level is lower than the floor and bottom of corrugated iron.

To reduce splash and improve drainage put a 12" wide, 2" deep layer of gravel next to the wall.

Sam Longley 121/03/2019 07:17:05
719 forum posts
26 photos

A cheap option is to stick Flashband down - but use the proper primer first & apply it properly on a warm day. ( or warm it a bit with a heat gun helps to make it sticky) Obtainable from any builders merchant & miles cheaper than lead.

Otherwise there are loads of firms that will pre form coated steel flashings if you send them a sketch of what you want

But provided the floor is higher than the surrounding ground & the cladding comes down just past the floor level so that the water is shed down past the concrete floor, you should not need anything. Just laying a flashing on the ground will not stop water coming under it . It has to follow the principle of water flowing down hill all the time..

If the cladding stops above the floor level & you want to shed the water over the edge of the slab( & may be behind the face of the slab) then you would need a flashing to direct the water over the edge, presumably a "Z" shape. That could be lead, zinc, flashband, or a preformed steel flashing. But the slab must be higher than the immediate  surrounding ground level otherwise you are wasting your time

Edited By Sam Longley 1 on 21/03/2019 07:33:47

John McNamara21/03/2019 08:00:34
1306 forum posts
113 photos

A couple of rows of brick set in a high strength mortar will keep your steel sheeting out of the dirt. Then use a formed metal flashing the slips up behind the sheeting across the brickwork then down over the outside brick edge say 40mm it will keep the water out. Lead will also work however it may cost more than the formed sheet and it can look a little rough. You will l also need to apply a waterproof membrane inside against the concrete floor and brick corner (Use masking tape to keep the edge on the floor neat). If you are painting the outside I would also apply membrane to the bricks and the edge of the slab before painting.

Steel will rust quite quickly if it is covered by soil.


JohnF21/03/2019 08:40:59
853 forum posts
102 photos

Hi CCB I looked at the OP late last night and still cannot picture exactly what you are trying to do? Is it a frame building with steel cladding, a brick wall with cladding above? Still several possibilities!

maybe a simple hand drawn cross section sketch showing the base and the bottom part of the construction would make it clear and thus you will get correct advice


ps where are you?

Speedy Builder521/03/2019 08:47:32
1801 forum posts
127 photos

We use zinc sheet over here as lead is fast becoming a non preferred metal to have around.

JasonB21/03/2019 09:18:24
16051 forum posts
1687 photos
1 articles

This from CCb's album may help, assume studwork is ontop of the sleeper?

Most of the profile sheeting makers do a range of profiles for eaves, corner and also a drip profile for the bottom

KWIL21/03/2019 10:26:19
3111 forum posts
56 photos


Whilst lead is not a "nice" material, you are only going to handle it once when fitting and hopefully will wash your hands properly before eating lunch.

RMA21/03/2019 10:32:44
162 forum posts

I can't work out what you're doing. Is the sleeper a sole plate for the wall? Why use a sleeper, it won't last forever. If you're going to the trouble of laying a concrete floor, why not do it properly and put a shallow footing with 2 or 3 courses of bricks and run the DPC through them. Your exterior cladding would then run outside of the brick course and finish well above ground level.

John Paton 121/03/2019 10:47:38
170 forum posts
6 photos

There are three issues:

1. You will probably need a horizontal damp proof course (DPC) between anything in direct contact with the ground and any 'non durable' (timber) structure above to stop moisture wicking upwards. The DPC could usefully be turned down 25mm on the outer side of the kerb to act as a bit of a weather check against splashing / wind driven rain.

Taking Jason's sketch above, this would go over the top of the sleeper and tie in with the DPM. This also assumes a 'proper' sleeper which is made durable by pressure treatment with 'proper' creosote. Poorly treated timber will rot rather quickly in that environment and modern 'sleepers' sold in garden centres are not sufficiently well treated for use in that location. You need the real salvaged sleepers which you can tell by being really hard and heavy and impregnated deep brown colour. Modern fencing type material even though sold as pressure treated has been found to rot in 5 years when in contact with the ground - speak to owners of modern housing estates about their fence posts! If you cannot get proper sleepers consider dense concrete blockwork which is easy to lay. The hollow blocks are good as they knit together very strongly if you overlap the joints of each course and fill the voids with mortar or concrete as you go. Two courses gives a very firm kerb using a detail similar to Jason's and a nice accurate edge to pour concrete into.

2. If the kerb (sleeper in Jasons sketch) rises at least 150mm above ground level you can safely run the sheet metal cladding down to within 50mm of ground level so it creates its own 'flashing' in front of the kerb. General rule of thumb is for the f'lashing' to extend a minimum of 75 mm below the timber structure. This could be a bit less, maybe 50mm, in very sheltered locations or somewhat more in extremely exposed locations such as hilltops and coastlines.

3. With profiled metal sheeting be aware that condensation will form on its inner face at night and this needs to be able to dribble down within the void of the corrugation to 'somewhere it will cause no problem'. Also that vermin will be able to get up through the corrugations and nest in the nice dry wall cavity and in time gnaw through into your workshop. Easiest fix is to infill the corrugations with cement mortar but that will lie damp with condensation drips in some conditions so make sure it is below the dpc level.

JohnF21/03/2019 15:14:05
853 forum posts
102 photos

RMA & John P have pretty much said everything -- I'm not a builder but have done fair bit of DIY so my thoughts are as per sketch below added to the comments above relating to wood [sleepers] for what you are doing these are definite no! Use bricks or blocks, blocks are quicker but bricks are stronger -- your choice ?

One thing in particular is a layer of sand over the compacted hardcore is a must to protect the DP membrane from the hardcore

One error on the sketch --- let the metal cladding go BELOW the DPC in the brickwork by say 2" [50mm] or a brick !

Good luck John


RMA21/03/2019 16:30:41
162 forum posts

Just to add a tip you might not be aware of. The above diagram is fine and I agree with bringing the cladding down to below DPC. The hardcore needs to be good stuff, Ideally small chunks that fit together well. I have a wacker to compact this sort of thing, if you can get your hands on one it will make life easier. However I've found small hardcore will settle quite well by moving it around as you put it in. Whatever you have it needs to be compact.

Sand (or blinding as it's called) needs to fill all the gaps in the hardcore, it's no good just laying it on the top. Obviously the wacker can be used for this operation, but if you can't get one, a little tip is to wash the sand into the hardcore using a hosepipe. When you're happy with that, leave at least an inch on top for the membrane to lay on. Try and get the membrane over the wall and lay the DPC on top of that, bedding it in with some mortar. and then build on top until you have the right thickness for your concrete floor.

I hope that has helped. I'm not sure what you're putting into the workshop, but most machines benefit from solid foundations. Start off right and you'll end up right!!

Dave Halford21/03/2019 19:03:01
441 forum posts
4 photos

If doing brickwork is a concern dense concrete blocks can do a 215mm high wall in one row, my shed has been stood on those for years. Do not get the light ones as the frost will smash them back to dust.

You must insulate the inside of a metal shed else it will literally rain inside with the condensation on the inside of the roof and you'll bake in summer.

JohnF21/03/2019 19:08:36
853 forum posts
102 photos

I believe you can get pre-insulated profile metal cladding now -- check it out locally


RMA21/03/2019 20:05:11
162 forum posts

Unless you've already bought the materials, I would seriously consider a timber construction which you can dry line and insulate. Easily fixed to your base and can be made in sections. Comfortable in both winter and summer. Hopefully you'll only build it once, so think about it before you start. As others have said, a metal box will need a lot of insulation. Have adequate ventilation as well.

Solid concrete blocks are very good, but the dense ones are heavy. Built a large pit in a garage once with them, never again! Good luck with the project and let us know how you get on.

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