How do you build a concrete base for your new log cabin?
Once you’ve decided where the log cabin will sit in the garden (and be prepared to change your mind several times at this stage before you start digging!) you will need to clear the area of debris, obstructions, vegetation etc.
It’s then time to start marking out exactly where the cabin will go with string and pegs. Bear in mind here that you need to leave enough space around the cabin to be able to paint it, and this may well require putting a step ladder up too to paint the top of the cabin. Bear in mind also that many cabins have a significant overhang of say 350mm or more and that you may want to fit guttering to the sides in addition. The last thing you want is the overhang protruding into the neighbour’s garden and rainwater shedding onto his patio or wherever. So leave enough space around the cabin for such eventualities; once you’ve laid the slab base it’s not so easy to shift it!
Peg out the footprint of the log cabin and make sure you orient the cabin facing exactly the direction you want; don’t rely on the fence or hedge being straight, square or at perfect right angles.
The next stage is to excavate the marked out area. This will produce a far greater amount of spoil than you can imagine as the soil really bulks up in volume when its dug out. If you can, re-use the soil nearby (raised bed, rockery?) that is the easiest solution, otherwise hire a skip and barrow the spoil into it.
From this point on a lot depends on how big the cabin is going to be and how much weight the concrete base should be designed to cope with, which in turn depends on what you are going to use it for. For example a log cabin that will be used as a garage will plainly require a stronger base than one designed as a home office.
You can, in fact build a wooden form and lay the concrete pad directly onto well compacted soil with no sub base; chances are if you get a log cabin contractor on the job that’s what they will do. But if you are going to all this trouble, and you are going to do it yourself, why not do it right and lay a proper sub base?
Your local builders merchant is a very good place to go for advice here. Call them and explain the project, giving measurements for the cabin footprint (base size); they will be able to tell you how much sub base to order and how much sand and cement will be needed for the base for the depth and size of concrete pad you require. Alas, there is no simple answer about exact depth of sub base and thickness of concrete pad here; every project will be different.
Much depends on the ground as well. If the site is well drained and level that makes life easier. If the soil is undisturbed over many years and therefore naturally well compacted that’s also a big bonus. But you may well need to level the site to some degree and then think a bit about achieving good drainage around the sides of the cabin; a layer of gravel should suffice. You should consider where electrical cables and drainage channels are needed at this stage, especially if you plan to run the electrics through the base and into the log cabin floor.
So, recapping, mark out with pegs, skim off top layer of turf or vegetation, remove all roots and large rocks etc then you are ready for compaction. Hire a compactor or vibrating plate from your local hire shop for the day – tell them what you need it for to get the right size for the job. A few (depending on how well compacted the soil already is) runs over will compact the top soil sufficiently. Now it’s time to tip in the sub base material; order this from your local builders merchant who will deliver it in half tonne sacks ready for you to barrow and tip it into position. Various granular sub base material are available from the diy sheds or’MOT Type 1′ is a commonly used one at the builders merchant (its the sub base used in raod building for the Ministry of Transport).
A word of caution here. Do not be tempted to make up your own subbase material eg old broken bricks and rubble etc. What is required is a nice level compacted sub base surface and you just will not get this with any old rubble; in fact you risk making inflection points beneath the concrete base over which stress cracks could occur. It’s not worth saving a few pennies here, and besides the granular sub base is relatively cheap and far easier to work with. So barrow in your sub base material, rake it level with a strong metal tined rake and compact again with the vibrating plate. If necessary add more sub base until the required thickness is achieved.
Now at this point it is more or less essential to incorporate a damp proof membrane. Couldn’t be simpler. Go to your local diy shed and find a Damp Proof Membrane (basically a big polythene sheet) bigger than the size you require and cut it to size with scissors. The DPM should go between the sub base and the concrete mix to prevent water rising up from the ground, saturating the concrete pad and reaching the cabin bearers. The DPM should be lapped up the sides of the form and can be trimmed later on. Fact: most log cabin ‘professional installers’ wouldn’t bother with this stage, but then, it’s not their cabin is it? Better to do it right.
If you are lucky enough to have very good access to the cabin site you might consider getting ready mixed concrete in from a cement truck instead of mixing and laying your own. Call your local cement mixer company for a quote to see if its viable. If you start talking about hiring concrete pumps to get the mix to the back garden it will be cost prohibitive for sure. But if it can be done for a reasonable price you’ll have an excellent pad laid in one go so well bonded and strong. A team of mates with barrows could be deployed to assist here! Much depends on the size of the log cabin; a big cabin needs a big base and that means an awful lot of concrete.
For the vast majority, hiring a portable cement mixer for the day will suffice. Now if you’ve never used one of these don’t panic! No disrespect to builders, but basically all you do is tip the right quantities of sand, cement and water in, press the large green button and the drum mixes the concrete for you. Press the large red button to stop the mixer, tip the concrete into a barrow and pour it where it’s needed; you can even pour straight from the drum into the form. The exact mix will again depend on the strength and depth of the pad required for the cabin you have chosen and the use to which it will be put. It is far better, though, to err on the side of caution and ‘over engineer’ with a deeper and stronger pad on a deeper sub base than to skimp at this stage. You might regret just putting in the minimum base required, but you will never regret putting in a deep, solid base
Before mixing you will have needed to construct your ‘form’; scrap timbers nailed or screwed together to enclose the size/depth of the concrete base. You need to take care when constructing the form that it is absolutely bang on the right size of the cabin footprint and exactly level. It does have to be level so that you can tamp down the wet concrete to get the top of the concrete mix to the top of the form; this is the way to ensure it is perfectly level. Note: if the base is not level you will have problems with the build; logs may be difficult to interlock the higher up you go and you could experience ill-fitting cabin windows and doors. The form has to be good and level; once it is it should be secured with battens at the corners and sides, though bear in mind that you will need to be able to take the forms away once the concrete has gone off (hardened).
Mixing the concrete should not be done on a hot sunny day as the mix will dry out too quickly and the result will be weak, crumbly concrete. Much better to do it on an overcast day; at least wait till the sun goes down before mixing. You will need two people for this job. One can mix while the other pours, tamps and rakes. Mix the concrete all in one go because it all needs to bond and set in one go. It is vital to use the correct proportions of sand, cement and aggregates. Use a 5 gallon bucket levelled at the top to get the proportions. Use only recommended aggregates from the builders merchant. Use fresh water. As a rule, the less water in the mix the stronger the resulting concrete will be (though it has to be wet enough to pour and be workable).
When you have filled the form it needs to be tamped down; get a long piece of wood and move it back and forth across the form with a sawing action. Fill any depressions with more concrete and repeat. Tap the sides of the form with a heavy object to help remove air bubbles. To prevent the concrete base from drying out cover it with damp sacking or failing that a polythene sheet to keep the moisture in and slow down the chemical process which slowly cures the concrete. Stay off it for several days. Keep wetting the sacks. It will go on hardening as long as there is moisture in the mix. The longer you leave it the better. Give it at least a fortnight and preferably several weeks more.
Hopefully, you will have built a good, solid, level concrete base on which to start the cabin build. But if not … you’ll have to break it all out and start again as you cannot build a good cabin on a poor base. All the more reason to do it right in the first place!
Note: As there’s only so much you can take in online, if you’ve never built a log cabin base before get yourself a comprehensive DIY book with a detailed section on formwork and concreting.