Log cabin bearers

Discussion in 'Carpenters' Talk' started by Colin426, Dec 3, 2012.

  1. Colin426

    Colin426 New Member


    I recently constructed a log cabin from Dunster House.  Its 4.5m by 3.5m sitting on 70mm pressure treated bearers, sitting on top of a 150mm thick concrete base, damp proof membrane, then 150mm crushed concrete.  The concrete base is 4.7m by 3.7m, providing 100mm concreate border on all sides, as per the manufactures intructions.  The attached picture show this.  I'm having paving laid at the front and stone chippings around the other three sides.

    My worry is the rain is hitting the side on the cabing and probably going underneaf the cabin, though gaps between the bearers and the concrete.  I really want the cabin and bearers to last as long as possible and so dont want the bearers to rot.  A friend suggested wrapping the bearers in a damp proof membrane, but think its too late now as its built.

    Is it going to be a problem that rain could potentially and probably is getting underneaf the bearers?  There's bit on soil gathering in the gaps also, which I'm trying to minimise until the paving and chipping are down.

    Would it be a good idea to either:

    • Silicon sealant around the botton of the bearers where they meet the concrete?
    • Should I perhaps use a bitumen paint (Like Black Jack) to paint the bearers and the 100mm concrete to try to seal it all?
    Any help or suggestions would really be appreciated.

    Many thanks

    Attached Files:

    • 001.JPG
      File size:
      1.8 MB
  2. Aron Searle

    Aron Searle New Member

    Ouch, poor detail.

    Better to have the timber slightly oversail the concrete rather than the other way round.

    It's the best of a bad job, but yes silicon seal around the bottom, also look at where the horizontal butt each other, moisture may wick up into the members their due to the lack of a clear fall.

    Splashback will also be an issue, so thouroughly seal the end grain, but I would leave the rest of the timber with a breathable finish rather than an impermable one like bitumen, as some moisture is bound to get past the mastic due to gaps/capillary action and/or inevitable failure.
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    The WORST thing to do now, is silicone seal the bearers. Any water that does get in, will not get out, making situations worse.

    Skirt all round with a plastic dpc(150mm) so that the concrete is completely inaccessible to rain and splashing.

    Water/rain can then run down the walls, onto the dpc and off into 'safe' ground.

    If you don't like the look of a 6" dpc black plastic skirt, maybe fix some thin 'log' timber all round, again positioned to give run-off over the edge and away from the bearers. Should this need replacing, it would be an easier job than replacing the bearers(which if allowed to remain wet, will seep to the flooring also).

    Mr. HandyAndy - Really
  4. was dunc before

    was dunc before New Member

    The kind of damage you are thinking about occurs when timbers are in contact with permanently damp surfaces. particularly ground rising damp. the concrete and dpm you laid will hold back the damp. that's why you did all that hard work.   things will get wet in a rainfall, but they also dry out just as quick. What you will see in the next 25 years is the timber performing as it is expected to do. that includes weathering and seasoning, small amounts of decay, but not wholesale disintegration.
  5. Colin426

    Colin426 New Member

    Hi Aron

    The thought did occur to me at the time that the base should not be larger than the structure, but I was following the guidance of Dunster house who state on their site ...

    "If laying a concrete footing we would suggest you add 100mm all the way round the sizes given for a cabin to help spread the weight of the building..."

    Looking at the gallery * it seems that just about every customer has followed this advice, so if its bad, there going to be alot of unhappy people out there.

    Link removed by screwfix peter
    Last edited by a moderator: Mar 3, 2014
  6. Colin426

    Colin426 New Member

    Hi Andy

    Makes sense to me what your saying about not to trying to seal the bearers.  Will have a think about maybe making some sort of timber trim.

  7. Colin426

    Colin426 New Member


    Good to hear you talking in terms of 25 years :) I hope you're right.  I've raised my concern with Dunster House and their reply was...

    "With regards to the water surrounding the bearers, this will not be an issue as the bearers are pressure treated and are designed to be in contact with wet ground. The bearers are guaranteed against rot and insect infestation for 10 years to reflect this."

    I hope they last alot longer than 10 years.

  8. Aron Searle

    Aron Searle New Member

    Having the slab protrude like that is a bad detail, that they have so many of these buildings like that concerns me.

    A building like that should easily last 60 years with the right detail and maintenence.

    "With regards to the water surrounding the bearers, this will not be an  issue as the bearers are pressure treated and are designed to be in  contact with wet ground. The bearers are guaranteed against rot and  insect infestation for 10 years to reflect this."

    Which is as good as admitting it's a bad detail and will promote damp in the timber.  The timber should be treated with a copper organic preservative to use class 4 to meet the above criteria, which seems a bit unlikely to me.

    Handy andy is right, a skirt detail will be better, tucking in the top of the skirt into the groove, but it's still not ideal as if water gets behind it, it will "sweat".
  9. Aron Searle

    Aron Searle New Member

  10. Colin426

    Colin426 New Member

    Thanks for the reply.  Really not what I wanted to hear but thanks for being so honest.  I'm pretty depressed now as I put alot of effort into building this cabin to work from home and has wiped out my families savings.  I also feel let down by Dunster Houses advice, hope anyone buying a log cabin finds this post.

    Really don't know what to do now.  Will have to think about what skirting I can fit...
  11. Aron Searle

    Aron Searle New Member

    Can you just cut the slab so it's flush?

    Then the problem laregly goes away, as the above link the water just goes down, and you can stick on a small kick to aid it.
  12. Sealants are no good, neither is the wood touching the ground, I cant see it lasting 10years.

    Pity you didnt ask the question before erecting the cabin, the best method is a course of engineering bricks all round, and where the bearing is needed, with a dpc on top then sit the cabin on it.

    How heavy is the cabin, can you jack it up, lay a course of bricks ( one side at a time ) then drop it back down.
  13. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    The link from Aron is good, as it supports Cotswold builder's theory of the engineering bricks, and disagrees with Dunster house method of adding 100mm of base all round, the info in the link seems pretty good.

    But with the 'jacking and stacking' theory, I would assume this wouldn't be possible as there are likely to be intermediate bearers, unreachable, which would also need a line of bricks under them.

    Possibly and simply, a full dpc skirt AND feather edge timber all round.

    Mr. HandyAndy - Really
  14. Colin426

    Colin426 New Member

    I've found this 100mm advice on a few log cabin provider sites now and also some blogs, but agree that its bad advice.  Wish I'd gone with my instinct as I felt it was wrong...

    The cabin does have intermediate bearers and is probably pretty heavy as its double walled with an insulating layer in between.

    Anyway, as I dont want to make any further mistakes and I've no experience of dpc, can you suggest and products and adhesives I might need?

  15. Aron Searle

    Aron Searle New Member

    If you go with the option of building a skirt, I would use a breathable membrane rather than a DPC, as I am concerned moisture may get behind the DPC and be trapped.

    You could attach the skirt with a timber bead, sealing the top of the bead to stop water getting behind.

    The other risk is water just pooling on the flat slab surface and then just puddling and getting underneath the sole plate, you could drap the membrane over the slab, and maybe cover it with loose stone, might look a little silly however, would be much better to cut the slab flush with the timber, more work but eliminates the problem anyway.
  16. Colin426

    Colin426 New Member

    Do you know of any breathable membrane which may be suitable?

    Agree with you that cutting the slab would be best.  Thing is I've only ever used a small angle grinder to cut a few paving slabs in the past.  To cut through 150mm of concrete, I'm guessing I'd need to hire something much bigger?  Big petrol angle grinders I've seen look a bit to bulky to get close to the cabin, or are there different types?
  17. GrahamTaylor

    GrahamTaylor Member

    A few years ago I helped a customer deal with almost exactly this problem - his was a wooden garage/workshop structure and the slab had been set too low so that water pooled on it after he had landscape gardening done in the area.

    First task was to lift the entire structure .... actually much easier than it sounds because we hired in some inflatable lifting bags (aka air jacks). These were long flat bags that we could pull though the gap between bearers by using draw lines - pretty fiddly but worked fine. We then inflated the bags (CAREFULLY) and lifted the entire structure by about 120mm.  Then slid in concrete fence posts from each side to sit under the bearers (he had to search around a bit to find posts that where accurately square and parallel along their entire length. The posts almost met in the middle. Then let the air out of the bags (VERY SLOWLY) checking that the posts were stable all the time.
    Once the structure was resting on the posts we banged in wooden noggins to keep everything sqaure and secure. These noggins were deep enough that they braced both the concrete and wooden bearers . To avoid these resting in the wet they had 10mm plactic spacers underneath to ensure that air could circulate and dry out the void when water levels fell again.

    We used concrete posts but I am sure you could use any other long impermeable load bearing material depending on what height you are trying to achieve. We did consider using a 20 mm extruded uPVC molding (easy to get from glazing manufacturers) but eventually chose not to go that way.  We could also have used simple breeze blocks and pushed them in using long battens - would have been fiddly to work in a small gap.

    Finally used scrunched up chicken wire pushed into the gaps to prevent mice making it a home.

    Took two of us (the customer plus me) two days to do though he had done all the running about to get air jacks and concrete bearers once I'd come up with the method.

    In retrospect we decided that we probably should have run the concrete posts at right angles to the wooden bearers - would have meant more point load but probably more stable. Doing it this way would have made it more difficult to use the airjacks and would have required use of temporary props between the lifting stage and lowering the building.

  18. ngn

    ngn New Member

    Following Dunster advice on base, I?ve done the same thing, and laid a base 10cm bigger than the shed. Luckily, the shed has not arrived yet, and I'd rather not cut the concrete (damp proof and rebar complications). 

    So how about:

    1) lay lines of engineering bricks in rows under the bearers, missing every other one for air movement
    2) glue DPC to the underside of the bearers and lay on top of the bricks
    3) attach a loose felt skirt around the base of the shed to catch any splash back from the concrete
    4) paint the exposed concrete with a flexible PVA sealer


    Many thanks
  19. dpg6

    dpg6 New Member

    I know this thread is a little old but I have a similar/related issue and rather than starting a new thread...

    I have erected a cabin on a larger concrete base - the base is 1m wider than the cabin to the sides and back.

    I have a damp proof membrane over the concrete, extending 50-100mm beyond the cabin in all directions.

    The cabin bearers are 45mm pressure treated timber and the cabin log walls are resting directly on the bearers.

    The roof overhangs the walls by around 250mm and I have installed guttering. The cabin walls themselves do not get very wet in rain (so far) due to the overhang.

    The concrete base extends well beyond the roof overhang and is getting wet in rain. The damp proof membrane sitting on the concrete is getting wet and consequently the bearers are sitting in fairly damp conditions.

    I had thought of folding up the damp proof membrane against the bearers (with a pressure treated timber batten to keep it in place) to keep any rain water on the surface of the concrete away from the bearers.

    As there is minimal water running down the walls I don't think I have to worry too much about run-off over the batten, but I was thinking of putting a bead of silicone where the batten and cabin walls meet.

    My question is: should I wait for several consecutive days of good weather to allow the bearers to dry out before wrapping their outer face in the damp proof membrane?

    Would the membrane prevent the bearers from drying out sufficiently?

    The interior face of the bearers is also covered with damp proof membrane, BTW: I installed a second membrane inside the cabin before laying insulation boards and floor boards above them. The inner membrane is folded up and held against the bearers by the insulation board - so if/when I fold the outer membrane against the bearers it will mean that 3 sides (front, back and base) are covered by damp proof material with the top being under the cabin logs. Is it realistic to think that any trapped moisture in the bearers would wick up into the logs and dissipate that way?

    The problem with waiting for several dry days is that this could be some time given the time of year.

    Thanks in advance for any advice.
    Last edited: Oct 13, 2014
  20. rtudor

    rtudor New Member

    Finding this very interesting as I have a 7.5m x 4.5m cabin in my garage waiting to be assembled. I have laid half of the concrete base and the rest of it I will finish next week. I intend covering the concrete with a damp proof membrane (300 gauge Visqueen). I have gone through the same thought process as the original author. I would have liked my base to have been a little higher but I didn't want to upset the neighbours by having the apex any higher than needs be. My base is only 20mm above the lawn height. I will be cutting the front away to create a wooden veranda and digging the other sides down a little and adding some decorative stone. I did read in the blurb from Dunster House to create a run-off at the concrete edge. I guess this means bevelling the edge of the concrete downwards at the last 100mm around the edge. How to do this is beyond me. I think I will be bringing the edge of the DPM up at the edges and stapling it around the outer bearers maybe. Or I may try to create some form of bevelled run off around the edge. I much prefer the idea of keeping wood surfaces exposed to air as covering them may trap in moisture. If anyone has any ideas how to create the wedge shaped run offs around the edge I would welcome suggestions. I am thinking about 2cm high at the cabin running away to nothing at the edge of the slab. There may be a pvc profile that would do the job.

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