Richard Greenland

Last week I blogged about durable outside décor. This week it’s about dealing with rising and penetrating damp in floors.

Many older buildings suffer with floors rotting as the timber joists bear directly onto damp ground below. Tell-tale signs are larger than normal gaps appearing between skirting boards and floors. This should not be ignored, the floor needs lifting, and if damp, the timber bearers should be placed on plastic (DPM or Damp Proof Membrane). Obviously any rotten timbers need replacing with treated timber. Also cut the ends from any floorboards or joists touching potentially damp walls. Leave a ½” to 1” gap on all sides. The skirting will cover it.

Joists on DPM on damp sub-floor

Similar problems may be encountered on upper floors of older buildings, because before the advent of joist hangers, joists were built into exterior walls. If the wall becomes damp the joists will rot. Last year I spent a week replacing a bedroom floor of a student HMOs. The joist ends all down one wall had rotted because the wall was damp. The solution was to splice or replace the joists (not a job for amateur builders) and support them on metal joist-hangers cemented into the wall. I also wrapped the ends of the joists in thick plastic to prevent further penetration.

The source of the damp then had to be removed. It frequently amazes me how many landlords cut corners to save a little money short-term. There was a gutter leaking water down the wall, and a copper pipe built into the masonry without the protection of a plastic sleeve. Cement is alkaline and rapidly causes copper to corrode and leak. Rather than investigate the damp, he’d simply boarded over the wet patch with plywood. The work easily cost him a thousand pounds, but could have been prevented with very little expense.

For concrete ground-floors a built-in DPM stops damp rising. A hardcore base is laid and vibrated to bed it down firmly. Then it is ‘blinded’ with sand to cover sharp edges. A special heavy-duty polythene is laid over this, lapping right up at the sides, and the concrete cast on top.

If you don’t want to dig up existing concrete floors to re-lay them on polythene, so long as they aren’t very wet, a quicker solution is to paint with two coats of bitumen emulsion. This will remain slightly sticky, but if covered with newspaper, you can overlay carpet and underlay on top. As it is rather pungent when wet, your tenants may have to be moved out overnight. For very wet concrete on which bitumen emulsion won’t set, it’s possible to lay DPM directly over the surface and carpet on top of that. It’s not something I’ve ever done myself as it’s a very poor solution leading to little damp voids and condensation under the DPM, but it might provide a temporary solution for some.

This leads me to penetrating damp next week, and probably condensation after that, things any landlord with older properties will have encountered.

Photo to follow.