The Self Build Self Help Site Questions answered

A one-stop shop of information for people interested in self build - whether self building a complete home or undertaking an extension, renovation or modification.

Questions answered
How do I stop a cut roof from spreading?
If the cut timber roof is just an alternative to a trussed roof, and joists run between the wall plates, then the joists will stop the roof from spreading.

Quite often a cut timber roof is used for a room where the ceiling height will be restricted, for example over a garage.

In this case the joists won't run between the wall plates, but will have to be collar ties. Purlins are also usually used to support the rafters, and this will tend to stop them spreading. The final part in this is where the ends of the rafters are skew nailed to the wall plates or fixed to the wall plates with truss clips. And, before that, the wall plates are tied down to the walls with galvanised straps and, optionally, fixed to the walls with frame fixings or hammer fixings.

How do I mix vermiculite cement mix for chimney insulation?
Its not easy to find the mix you need, its not made clear on the packaging and is hard to find if you browse the Internet. The mix is 6:1 of Vermiculite to ordinary Portland cement. Make up quite a dry mix. You can use a cement mixer and often it is luck whether the mix goes well or whether you end up with a load of marbles in the mixer. If you do get marbles then try adding the materials in a different way. Try adding the cement to the water and letting it go to a slurry before adding the vermiculite.

Although light and bulky, you'll find that you will need many bags for the average chimney stack. One bag of vermiculite when made up will roughly fill a volume of 0.1 cubic metre.
What is the method of attaching a wall plate to the inner leaf of a cavity wall?
In the first instance, bed the wall plate onto the blocks / bricks with a normal cement mix. Make it wet enough to be able to tap the wall plate down as necessary to get it level. Once it has set, we usually leave it for 2 days to set hard, fix it down to the wall with frame fixings. Some builders may leave this step out as it also gets held down with steel straps, but it doesn't take long, provides a stronger fix and stops the wall plate becoming dislodged when the trusses are put on.

See the Hints & Tips section for more information.

What is the cause of brown water running down my chimney breast?
First of all, check for a leak from a water tank or pipe in the roof space. If you have an old metal tank then it may well have developed a small leak and the remedy is obviously to replace with a new plastic tank - this goes for a main water tank and for for the header tank for a central heating system.

Next, check for a leak in the roof - this may require a professional roofer to check the slates / tiles, ridge tiles and valleys, flashings etc. We have seen a roof "suddenly" spring a leak in the lead valley - how did the valley suddenly get punctured? Well it didn't. It was punctured several years ago by some hamfisted roofer who was replacing some slates, dropping some onto the lead valley. The underlying old bitumen based roof felt eventually cracked through the effects of heat, as does all such roof felt and, bingo, the considerable amount of water coming down the valley then drip drip dripped through the hole and roof felt.

The worst scenario could be that the water is penetrating the chimney stack. A badly built stack, or a stack on an old house, will allow rain to pentrate directly through the roof and into the supporting chimney breast. If so, the stack must be rebuilt - see our page in the Hints & Tips section on Building a chimney stack.
How do I join DPM together?
Generally, DPM is used to provide a waterproof barrier beneath a concrete slab. In this case, the DPM sheets should have a lap of at least 150mm. When putting sheets together, you normally find that you have enough to provide more than 150mm lap - 300mm is best. Having said that, the construction of the sub base to go under the slab means that water will drain away very well and the material is doing what it says in the tin - its a DAMP proof membrane. (If you want to "tank" a structure then DPM is not recommended and you will need to ask your architect and or the builder or plasterer).

You will need to stick the sheets together otherwise the concrete will get between the sheets and bang goes the barrier against water ingress.

The DPM is brought up and over the blocks of the inner leaf. The DPC (100mm wide) is then laid over the top. The two membranes do not need sticking together here - the weight of the blocks will ensure a good contact and prevent water from seeping above the Damp Proof Course.

Surfaces must be clean and dry. Adjacent sheets must be overlapped by a minimum of 150mm. Visqueen Double Sided Jointing Tape should be used to join adjacent DPM sheets. The sheets should then be sealed by using 100mm wide Visqueen Girth Jointing Tape. Where the sheets have been perforated they should be patched with sheets of identical thickness lapped at least 150mm beyond the limits of the puncture and sealed with Visqueen Double Sided Jointing Tape and Visqueen Girth Jointing Tape as per standard lapping instructions above.
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