Can the d-5 admixture help solve the problem of a leaking roof in the house?
Yes, it will help. You should pour a layer of concrete the existing floor with reinforcement mesh using the D-5 admixture (3% of the weight of cement).
The thickness of the concrete slab should not be less than 20 cm. It is not recommended to use cement-sand grout as the only layer of waterproofing in flat roofs, if reinforced concrete structures are made without it. In thin cement-sand grouts located under the open air permanent changes of temperature (day-night, winter-summer) lead to through microcracks.
This leads to damage in their waterproofing. In such cases it is necessary to provide additional reliable waterproofing. It is recommended to use a flat roof slab from monolithic reinforced concrete with the D-5 admixture at least 200 mm thick.
The slab will have its own high waterproofness, so the microcracks will not develop along the entire thickness of the slab, and no additional waterproofing will be required.